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Mudhivarthi, Subrahmanya R.
Process optimization and consumable development for Chemical Mechanical Planarization (CMP) processes
h [electronic resource] /
by Subrahmanya R. Mudhivarthi.
[Tampa, Fla.] :
b University of South Florida,
ABSTRACT: Chemical Mechanical Planarization (CMP) is one of the most critical processing steps that enables fabrication of multilevel interconnects. The success of CMP process is limited by the implementation of an optimized process and reduction of process generated defects along with post CMP surface characteristics such as dishing and erosion. This thesis investigates to identify various sources of defects and studies the effect of factors that can be used to optimize the process. The major contributions of this work are: Understanding the effect of temperature rise on surface tribology, electrochemistry and post CMP pattern effects during the CMP process; investigating the effect of pad conditioning temperature and slurry flow rate on tribology and post CMP characteristics; development of novel slurries using polymer hybrid particles and improvement in slurry metrology to reduce surface damage during CMP.From the current research, it was shown that the effect of temperature on CMP tribology is predominantly affected by the polishing parameters and the polishing pad characteristics more than the chemical nature of the slurry. The effect of temperature is minimal on the resulting surface roughness but the with-in die non-uniformity is significantly affected by the temperature at the interface. Secondly, in this research it was shown that the effectiveness and aggressiveness of the pad conditioning process is highly influenced by the conditioning temperature. This aspect can be utilized to optimize the parameters for the pad conditioning process. Further, post CMP characteristics such as dishing, erosion and metal loss on patterned samples were shown to decrease with increase in slurry flow rate. This research then concentrates on the development of novel low defect slurry using polymer hybrid abrasive particles.Several varieties of surface functionalized polymer particles were employed to make oxide CMP slurries. These novel slurries proved to be potential candidates to reduce surface damage during CMP as they resulted in low coefficient of friction and much less surface scratches as compared to conventional abrasives. Thus, this research helps to reduce defects and non-planarity issues during CMP process thereby improving yield and reducing the cost of ownership.
Dissertation (Ph.D.)--University of South Florida, 2007.
Includes bibliographical references.
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Advisor: Ashok Kumar, Ph.D.
Coefficient of friction.
x Mechanical Engineering
t USF Electronic Theses and Dissertations.
Process Optimization and Consumable Development for Chemical Mechanical Planarization (CMP) Processes by Subrahmanya R. Mudhivarthi A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Mechanical Engineering College of Engineering University of South Florida Major Professor: Ashok Kumar, Ph.D. Mohammad Rahman, Ph.D. Daniel Hess, Ph.D. Vinay Gupta, Ph.D. Qiang Huang, Ph.D. Pritish Mukherjee, Ph.D. Date of Approval: November 2, 2007 Keywords: temperature, tribology, slurry, electro chemistry, abrasive particles, scratches, pad conditioning, coefficient of friction Copyright 2007 Subrahmanya R. Mudhivarthi
DEDICATION This dissertation is dedicated to Lord Sri Shirdi Saibaba, Guru Sri Ammula Sambasiva Rao and my Parents, Mr. Subba Rao and Mrs. Parvatha Vardhani
ACKNOWLEDGMENTS Firstly I would like to thank my advi sor Prof. Ashok Kumar for giving me an opportunity to pursue research under his guidance and supp ort throughout my doctoral studies. I would like to thank Dr. Vinay G upta for his constant encouragement and guidance in several aspects of my dissert aion. I would like to Dr. Yaw Obeng for his invaluable help in sharing t echnical know how all along the c ourse of my research during his tenure earlier at Texas Instruments, Inc and presently at NIST. I would like to acknowledge the help of Dr. Mohammed Rahman whose guidance in the areas of heat transfer and thermal modeling was crucial for the completion of this dissertation. I thank Dr. Daniel Hess, Dr. Qiang Huang and Dr. Pritish Mukherjee for being a part of my dissertation committee and for their valuable suggestions to improve this work. I would like to acknowledge the financial support fo r this research from NSF GOALI grant #DMII 0218141. I would like to thank Dr. Norm Gitis of CETR Inc. and Dr. Anthony Kim of Intel Corporation for providing me w ith Internship opportunities which helped me to gain industry experience and be tter perspective of my research. I owe every achievement of my life to my parents and all the members of my family. I am extremely grateful for their continuous encouragement and love throughout my life. Special thanks go to all my friends for their love and affection which helped me survive the most difficult times of my life during this work. I would also like to thank the former and present colleagues of Dr. Ashok Kumars research group who have been very helpful during the course of this work.
i TABLE OF CONTENTS LIST OF TABLESÂ…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â….. vi LIST OF FIGURESÂ…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…... vii ABSTRACTÂ…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…. xiii CHAPTER 1: INTRODUCTIONÂ…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â….......1 1.1 Semiconductor manufacturing trends.................................................................1 1.2 Need for planarization.........................................................................................4 1.3 Damascene architecture for MLM using metal CMP.........................................5 1.4 Other CMP applications......................................................................................7 1.5 Outline of the thesis............................................................................................8 CHAPTER 2: BACKGROUND OF CMP PROCESSÂ…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â…Â….....11 2.1 CMP process.....................................................................................................11 2.1.1 Material removal mechanism....................................................................12 220.127.116.11 Mechanical aspects of CMP..............................................................13 18.104.22.168 Chemical aspects of CMP process....................................................14 2.1.2 Governing factors of CMP process...........................................................15 22.214.171.124 Process parameters............................................................................17 126.96.36.199 Consumable characteristics...............................................................17 2.2 Tribology of CMP process................................................................................19 2.2.1 Tribology during CMP process.................................................................19
ii 2.2.2 Tribo-metrology of CMP..........................................................................20 2.3 Thermal studies during CMP process...............................................................25 2.4 Challenges during CMP process.......................................................................27 2.4.1 Non-planarity............................................................................................28 2.4.2 Surface scratches.......................................................................................30 2.4.3 Delamination during cu/low-k CMP.........................................................31 2.5 Research objectives...........................................................................................32 2.5.1 Process optimization: e ffect of temperature.............................................33 2.5.2 Thermal model..........................................................................................33 2.5.3 Low defect slurry development................................................................34 CHAPTER 3: EFFECT OF TEMPERATURE ON COPPER CMP PROCESSÂ…..........35 3.1 Introduction.......................................................................................................35 3.2 Materials and techniques...................................................................................36 3.2.1 CMP bench top tester................................................................................36 3.2.2 Nanoindentation........................................................................................39 3.2.3 Electrochemistry and x-ray photoelectron spectroscopy..........................40 3.3 Results and discussion......................................................................................42 3.3.1 Effect of temperature on CMP tribology..................................................42 3.3.2 Effect of temperature on electrochemistry................................................44 3.3.3 Effect of temperature on surface chemical compounds............................45 3.3.4 Effect of temperature on polished copper surface....................................51 188.8.131.52 Surface roughness.............................................................................51 184.108.40.206 Non-uniformity.................................................................................52
iii 220.127.116.11 Mechanical properties.......................................................................54 3.5 Conclusions.......................................................................................................59 CHAPTER 4: EFFECT OF TEMPERATU RE ON TRIBOLOGY DURING CMP .......60 4.1 Introduction.............................................................................................................60 4.1.1 Effect of polishing pad char acteristics on CMP tribology........................60 4.1.2 Effect of slurry characteristics on CMP tribology....................................61 4.1.3 Effect of wafer contour ch aracteristics on CMP tribology.......................62 4.2 Experimental methods and materials................................................................64 4.3 Results and discussion......................................................................................66 4.3.1 Effect of process paramete rs and slurry temperature................................66 4.3.2 Effect of oxidizer on CMP tribology........................................................71 4.3.3 Effect of slurry chemistry on CMP tribology...........................................73 4.3.4 Effect of temperature on pad conditioning process..................................76 18.104.22.168 Pad conditioning process..................................................................76 22.214.171.124 Pad conditioning experiments...........................................................78 126.96.36.199 Results and discussion......................................................................79 4.4 Summary...........................................................................................................87 CHAPTER 5: THERMAL MODEL Â– STE ADY STATE HEAT CONDUCTION ........89 5.1 Introduction.......................................................................................................89 5.2 Thermal model development............................................................................91 5.3 Temperature contours on wafer surface............................................................94 5.4 Slurry flow rate experiments.............................................................................97 5.4.1 Materials and techniques.........................................................................100
iv 5.4.2 Experimental procedures and samples....................................................101 5.4.3 Results and discussion for flow rate experiments...................................104 5.5 Summary.........................................................................................................108 CHAPTER 6: NOVEL SLURRY DEVELOPMENT TO REDUCE CMP DEFECTS.. .....110 6.1 Introduction.....................................................................................................110 6.2 Silica hybrid particles.....................................................................................112 6.2.1 Hybrid particle synthesis.........................................................................112 6.2.2 Particle characterization..........................................................................113 6.2.3 Post CMP surface characterization.........................................................113 6.2.4 Experimental conditions for silica hy brid particle slurry testing............114 6.2.5 Results and discussion silica hybrid particles......................................115 6.3 Ceria composite particles................................................................................120 6.3.1 Composite particle synthesis...................................................................121 6.3.2 Experimental conditions for com posite particle slurry testing...............122 6.3.3 Results and discussion composite particles.........................................122 6.3.4 Post CMP surface characterization.........................................................126 6.4 Summary.........................................................................................................129 CHAPTER 7: SUMMARY AND FUTURE WORK.......130 7.1 Effect of temperature on CMP process...........................................................130 7.2 Effect of slurry flow rate on CMP process.....................................................132 7.3 Novel slurry development to reduce CMP defects.........................................132 7.4 Future work.....................................................................................................133
v 7.4.1 Thermal model........................................................................................133 7.4.2 Electrochemistry and impedance spectroscopy......................................134 7.4.3 Low defect slurry development..............................................................134 REFERENCES 135 ABOUT THE AUTHOR. 143
vi LIST OF TABLES Table 2.1 Factors that govern the output of CMP process.16 Table 3.1 Mechanical properties of copper thin films before and after polishing....................................................................................................58 Table 4.1 Process parameter conditions for the study of effect of temperature........64 Table 4.2 Process parameter conditions for slurry chemical experiments................65 Table 4.3 Corrosion potential and current density from polzarization experiments...............................................................................................75 Table 4.4 Consumables and process para meters in polishing experiments... 78 Table 4.5 Process parameter set fo r patterned CMP experiments..84 Table 5.1 Consumables and process parameters in slurry flow rate experiments.............................................................................................102 Table 6.1 Details of the slurry samples made out of hybrid and silica particles...................................................................................................115 Table 6.2 Experimental conditi ons for slurry testing..............................................115 Table 6.3 Surface roughness of wafer after CMP using various abrasive particles ..................................................................................................119 Table 6.4 Numerical values for coefficient of friction, surface roughness and removal rate............................................................................................125
vii LIST OF FIGURES Figure 1.1 Increase in transi stors per die with years of technology advancement.......1 Figure 1.2 Schematic of a basic MOS cap acitor with contact metallization................2 Figure 1.3 Cross section SEM images of the multilevel metallization stack...............4 Figure 1.4 Roughness resulting out of metallization without CMP.............................5 Figure 1.5 Conventional and damascene appr oaches to fabricate MLM structure......7 Figure 2.1 Schematic of CMP process as simulated on the bench top tester.............12 Figure 2.2 A close up view of the polishing interface................................................14 Figure 2.3 Stribeck Curves generated using coefficient of friction data....................22 Figure 2.4 Pattern dependent non-planarit y resulting due to CMP process...............28 Figure 2.5 Post polish surface anomalies due to pattern deviations resulting in wafer surface non-planarity......................................................................29 Figure 2.6 Optical microscopy of scra tches and pitting on wafer surface.................30 Figure 2.7 Delaminated interface during CMP process.............................................31 Figure 2.8 Delamination at via layer and a) copper dielectric cap inte rface b) the trench layer stress point..................................................................32 Figure 3.1 Bench top CMP tester that can simulate real time CMP process.............37 Figure 3.2 MIT 854 mask lay out with varyi ng pattern line widths and densities.....38 Figure 3.3 Nanoindenter XP used for the mechan ical characterization of thin films..........................................................................................................39
viii Figure 3.4 Schematic of the electrochemical cell for the polarization experiments...............................................................................................41 Figure 3.5 Increase in removal rate and coefficient of friction with change in temperature...............................................................................................43 Figure3.6 Change in corrosion current density and corrosion potential with change in slurry temperature.....................................................................44 Figure 3.7 Comparision of cu (2P3/2) spectra for as-received and slurry treated samples at different temperatures.............................................................46 Figure 3.8 Increase in dissolu tion rate with an increase in slurry temperature..........47 Figure 3.9 Curve fitted cu (2P3/2) spectra for as-received and slurry treated samples at different temperatures.............................................................48 Figure 3.10 Comparison of oxygen (O1(s)) spectr a for as-received and slurry treated samples at different temperatures.................................................49 Figure 3.11 Concentration ratio of oxide/met al on copper surface treated at different temperatures...............................................................................50 Figure 3.12 AFM images of the blanke t copper sample after CMP at (a) 18.5oC (b) 21.5oC (c) 27oC (d) 31oC.....................................................................51 Figure 3.13 (a) AFM images and schematic of the pattern describing the analysis for non-uniformity measurement (b) non-uniformity with-in die versus the temperature of polishing........................................51 Figure 3.14 Load versus displacement curve...............................................................55 Figure 3.15 Load versus displacement for po lished copper samples at different slurry temperatures....................................................................................57
ix Figure 3.16 Hardness versus displacement for unpolished and polished copper samples......................................................................................................57 Figure 3.17 Modulus versus displacement for polished and unpolished copper samples......................................................................................................58 Figure 4.1 Effect of pressure and velocity on coefficient of friction during copper CMP..............................................................................................67 Figure 4.2 COF versus p*v duri ng copper CMP using commercial copper slurry.........................................................................................................69 Figure 4.3 COF versus p*v during glass polishing using de-ionized water...............69 Figure 4.4 Dependence of coefficient of fric tion with slurry temperature at different process conditions......................................................................70 Figure 4.5 Effect of % concen tration of hydrogen peroxide on coefficient of friction at different p*v values..................................................................71 Figure 4.6 Effect of slurry chemistry on the coefficient of friction during CMP......73 Figure 4.7 Potentiodynamic polarization scans for copper-slurry systems using various slurry chemistries.........................................................................74 Figure 4.8 Effect of substr ate (polished with 5 % NH4OH + 5 % H2O2 + 5 mM BTA) on COF during CMP............................................................75 Figure 4.9 Pad surface temperature during cond itioning at different temperatures..............................................................................................79 Figure 4.10 Pad coefficient of friction measured in-situ during pad conditioning at different temperatures...........................................................................80
x Figure 4.11 Coefficient of friction curv es during pad conditioning at different temperatures..............................................................................................81 Figure 4.12 Carriage position during conditi oning tests plotted versus time...............82 Figure 4.13 Pad wear rate versus temperature during conditioning.............................83 Figure 4.14 Removal Rate and Coefficien t of friction measured during copper polishing after pad conditioning at different temperatures.......................85 Figure 4.15 Dishing depth measured at 50 m feat ures for samples polished after pad conditioning at different temperatures.......................................86 Figure 5.1 Local polishing interface temperature for a silicon wafer at different flow rates of alumina as the slurry (b=0.075mm, =100 rpm, q=455W/m2).........................................................................95 Figure 5.2 Polishing interface temperature fo r a silicon wafer at different coefficient of friction (q = 455 to 593W/m2, b = 0.075mm, = 100 rpm)............................................................................................95 Figure 5.3 Local polishing interface temperature for a silicon wafer at different ( ) spinning rates and alumina as the slurry (Q=65 mL/min, b= 0.075mm, q =1364W/m2)..........................................96 Figure 5.4 Cross sectional temp erature within the wafer during CMP at a rotation speed of =200 rpm (Q = 65 mL/min, b = 0.075 mm, q=1049W/m2)............................................................................................97 Figure 5.5 Bench top CMP tester mod. CP-4...........................................................100 Figure 5.6 Coefficient of friction real-time graphs for three slurry flow rates.......103
xi Figure 5.7 Coefficient of friction and copper removal rate versus slurry flow rate...........................................................................................................104 Figure 5.8 Effects of slurry flow rate on (a) dishing, (b) erosion and (c) metal loss...........................................................................................105 Figure 5.9 AFM images of dishing prof iles of a 50 m wafer feature at a) 20 ml/min b) 30 ml/min c) 45 ml/min d) 55 ml/min and e) 75 ml/min slurry flow rates.................................................................107 Figure 5.10 Measured temperature rise on the pa d surface during copper polish at different slurry flow rates....................................................................108 Figure 6.1 TEM images of the Hybrid particles and silica particles........................116 Figure 6.2 COF data measured in -situ during CMP of thermal oxide wafers using various kinds of abrasive particles................................................116 Figure 6.3 Removal rate measurements during CMP of thermal oxide wafers using various kinds of abrasive particles................................................117 Figure 6.4 FTIR spectroscopy on thermal oxi de wafer before and after CMP........118 Figure 6.5 AFM images of the thermal oxide wafer su rfaces polished with (a) 300 nm hybrid (b) 500 nm hybrid (c) 50 nm silica and (d) 150 nm silica ....................................................................................119 Figure 6.6 TEM image of the composite particle dark spots indicate CeO2..........123 Figure 6.7 DLS of microgels where the par ticle phase separates at nearly 32C....123 Figure 6.8 FTIR characterization of sili ca removal from the wafer surface............124
xii Figure 6.9 Optical microscopy (5X magnification) images of wafers polished with (a) ceria composite particles, (b) 0.5wt% CeO2, (c) 0.25wt% CeO2 nanoparticles.............................................................127 Figure 6.10 AFM images of wafers polished with (a) 0.25wt% CeO2, (b) 0.5wt% CeO2 nanoparticles and (c) composite particles.................128
xiii PROCESS OPTIMIZATION AND CONSUMABLE DEVELOPMENT FOR CHEMICAL MECHANICAL PLANA RIZATION (CMP) PROCESS Subrahmanya R. Mudhivarthi ABSTRACT Chemical Mechanical Planarization (CMP) is one of the most critical processing steps that enables fabrication of multilevel interconnects. The success of CMP process is limited by the implementation of an optimized process and reduction of process generated defects along with post CM P surface characteristics such as dishing and erosion. This thesis investigates to iden tify various sources of defects and studies the effect of factors that can be used to optimi ze the process. The major contributions of this work are: Understanding the effect of temperature rise on surface tribology, electrochemistry and post CMP pattern effect s during the CMP process; investigating the effect of pad conditioning temperature and slurry flow rate on tribology and post CMP characteristics; development of novel sl urries using polymer hybrid particles and improvement in slurry metrology to reduce surface damage during CMP. From the current research, it was shown that the effect of temperature on CMP tribology is predominantly affected by the po lishing parameters and the polishing pad characteristics more than the chemical nature of the slurry. The effect of temperature is
xiv minimal on the resulting surface roughness but the with-in die non-uniformity is significantly affected by the temperature at the interface. Secondly, in this research it was shown that the effectiveness and aggressivene ss of the pad conditioning process is highly influenced by the conditioning temperature. Th is aspect can be utilized to optimize the parameters for the pad conditioning process. Further, post CMP characteristics such as dishing, erosion and metal loss on patterned samples were shown to decrease with increase in slurry flow rate. This research then concentrates on the development of novel low defect slurry using polymer hybrid abrasive particles. Se veral varieties of su rface functionalized polymer particles were employed to make oxide CMP slurries. These novel slurries proved to be potential candidates to reduce su rface damage during CMP as they resulted in low coefficient of friction and much less surface scratches as compared to conventional abrasives. Thus, this research helps to reduce defects and non-planarity issues during CMP process thereby improving yield and reducing the cost of ownership.
1 CHAPTER 1: INTRODUCTION 1.1 Semiconductor manufacturing trends According to Â“MooreÂ’s LawÂ”  the dema nd in the semiconductor industry with respect to the number of transistors per chip will be doubl ed every 1.5 to 2 years. Another observation as per Moore is that the micropro cessor performance in terms of millions of instructions per second (MIPS) will also doubl e in the same 1.5 to 2 years . Figure 1.1 below compares the trend in microprocessors according to the se miconductor industry performance in the past . Figure 1.1 Increase in transi stors per die with years of technology advancement  Entire research in semiconductor device manufacturing is focused to implement such an increase in device density, which marks the advancement in technology to the
2 next level. A transistor, one of the millions of devices that constitute an Integrated Chip (IC), is typically a MOSFET (metal-oxide-sem iconductor Field Effect Transistor), which consists of a source, gate and a drain. Thes e devices can be made to operate faster by reducing the size of the devices and to have the devices pack ed densely onto a given chip area thereby decreasing the distance the carri ers have to travel [4, 5]. The minimum feature size decreases with the size of the device itself and this also translates into reduction in intermediate pitch or spacing between the features. Figure 1.2 below shows the schematic of a MOS capacitor wi th first-level of metallization. Figure 1.2 Schematic of a basic MOS capac itor with contact me tallization [4, 5]. Shrinking of device dimensions is crucial for the IC performance because the transit time (Tr) of the electrons with a velo city (V) in a device is directly proportional to the length of the gate (Lg) (transit time is the ratio of gate length to velocity of the electron), and the transit times in turn dictate the frequency of operation [5, 6] Thus, we can understand the efforts of the semiconductor industry to reduc e the gate length of the devices. As the
3 devices become smaller and smaller, their fabrication (otherwise called semiconductor device processing) faces several design, manuf acturing and process control challenges. Once the devices are fabricated on the silicon s ubstrate, they need to be connected to form a device network and at the same time need to be connected to the outside world using interconnect materials. The metallizati on connecting the devices at the silicon level is called contact or first-level metallization and the metallization that connects the devices to the outer world is the second-level meta llization [4, 5]. The materials required for metallization need to be selected based on their electrical, chemical and mechanical characteristics since the properties of these materials dictate the frequency of flow of charge. The time delay (TRC) in seconds or the frequency of charge flow associated with the interconnect materials is computed as a product of resistance (R) of the metal lines and the capacitance (C) of the insulating in terconnects. Substituting for resistance in terms of wiring dimensions and material propert ies, RC time delay can be written as : TRC = 2 2 2 01 4 2 T P L C R where is the resistivity of the metallic interconnect, k is the dielectric constant of the insulator, 0 is the permittivity of vacuum, L is the length of the interconnect line, P is the pitch between interconnect lines, and T is th e thickness of the line . The RC delay has to be reduced in order to achieve better pe rformance of the Integrated Chip. To reduce RC delay from the dimension point of view, th e length of interconnect wiring needs to be reduced, which explains the trend of increasing the number of metallization layers. As per the International Technology Roadmap for Semiconductors (ITRS) the minimum and maximum number of metallization layers in 2007 and beyond need to be 13 and 17
4 respectively . Figures 1.3 below show s the cross section SEM micrograph of multilevel metallization scheme, which uses 7 layers of metallization. Figure 1.3 Cross section SEM images of the multilevel metallization stack  1.2 Need for planarization In order to successfully manufacture such an MLM structure as shown in the figures above, the topmost layer of the previous metallization level has to be optically flat with minimal surface roughness. This is because if there exists any residual roughness at the previous layer, it will get compounded as the layers increase and soon after a couple of metallization layers, the r oughness will be so signific antly high that lithography (patterning) experiences issues with the dept h of focus and any further processing is not possible. It has been proven [7, 8] that Chemical Mechanical Planarization (CMP) process is the method of choice among other pl anarization techniques to achieve local
5 and global planarization across the wafer su rface. The figure 1.4 below shows an SEM image of an interconnect structur e without planarization . Figure 1.4 Roughness resulting out of metallization without CMP  It can be seen from the above figure that it is not possible to proceed with further processing steps as depth of focus issues come up during photol ithography and several other subsequent processing challenges, such as voids with in interc onnect layers due to compounded roughness, occur. Thus, CMP process becomes a crucial processing step in device fabrication in order to achieve su ccessful fabrication of MLM structure. 1.3 Damascene architecture for MLM using metal CMP Besides increasing the number of metallizat ion layers, reducing the resistance of the metal interconnects and the cap acitance of the dielectric layer reduces the RC delay. Hence, it can be said that choosing the inte rconnect materials with optimum electrical
6 properties would reduce RC time delay. The de sired optimum properties of the materials that can find application as interconnect materials are listed below . Low Resistivity Easy to form Easy to etch for pattern generations Should be stable in oxidizing ambients, oxidizeable Mechanical Stability, good adherence, low stress Surface smoothness Stability throughout processi ng, including high temperat ure sinter, dry or wet oxidation, gettering, phosphorous glass (or any other material) passivation, metallization. No reaction with final metal, aluminum/copper Should not contaminate devices, wafers, or working apparatus. Good device characteri stics and lifetimes. For window contacts Â– low contact resistan ce, minimal junction penetration, low electromigration. Following the above properties as guide-lin es, copper has been chosen to replace aluminum as the material for metallic interconnects to reduce RC delay [10, 11]. However, choosing electrically superior c opper over aluminum comes with an inherent drawback that an etchant to precisely pattern copper is not available and an alternate or a Â‘damasceneÂ’ approach to make the metalliza tion layers was eventually developed .
7 Figure 1.5 below compares the processing step variation between conventional and damascene approaches. Figure 1.5 Conventional and damascene appr oaches to fabricate MLM structure . In the damascene approach, the dielectric is patterned instead of the metal layer and then copper is deposited onto the patterned dielectric material. The excess copper or the copper over burden over the patterned dielectric is removed using CMP process. 1.4 Other CMP applications Besides traditional CMP of dielectrics a nd copper or tungsten (metal) CMP, there are many other avenues and applications for CMP process. CMP finds its application not
8 only in CMOS applications but also in Micro Electro Mechanical Systems (MEMS) applications . CMP is also being devel oped for Integrated Optical elements, phase change memory materials, magnetic materials, and advanced substr ates [9, 13]. Typical MEMS devices that involve CMP in their fabr ication are accelerometers, torque sensors, and microfluidic processors; and typical materials involve d are oxides, polymers, doped oxides, oxynitrides, polysilicon and certain metals for specific reflective surface applications. Of course, signifi cant process understa nding still needs to be developed for these new processes, which have many ch allenges like process stability, enhanced removal rates on the order of microns as co mpared to the nanometers in conventional CMP to achieve acceptable throughput, and novel chemistries for several different layers that do not usually find place in CMOS fabrication. 1.5 Outline of the thesis This dissertation is broadly divided into two parts, the first half dealing with the effect of temperature and slurry flow rate on CMP, pad conditioning, and post CMP results. The second half deals with the deve lopment of novel soft abrasive particles, which offer a superior surface finish by re ducing surface damage and scratches as compared to conventional abrasive particles. Chapter 2 explains the basic mechanis m of CMP process in both mechanical and chemical aspects and presents in de tail the surface tribology during CMP and the various factors affecting the CMP tribology. Further, the chapter provides background into the thermal aspects of CMP, thermal mo deling efforts in the recent past, and the
9 challenges involved with the pro cess. Objectives of the present research are then listed towards the end of the chapter. Chapter 3 presents the experimental deta ils and analytical techniques to study the effect of temperature on CMP process with respect to the coefficient of friction, removal rate, post CMP surface quality, and non-uniformity. Further in the chapter, results from electrochemical and surface chem ical studies are presented and discussed. Chapter 4 provides details about the tribologic al studies with varyi ng temperature and the effect of process parameters on the role of temperature on tribology. It also presents the research on the effect of temperature on pad conditioning proces s and the subsequent CMP process on patterned samples. This part of the research is particularly important for the process optimization of CMP. Further, Chapter 5 elucidates the development of thermal model for estimation of temperature on the surface of the wafer as a function of radius and thickness. It also presents the slurry flow effects on CMP. In this research a viable solution to reduce post CMP surface anomalies such as dishing, eros ion, and metal loss has been proposed. It was shown that usage of higher flow rate is highly beneficial for CMP. However, it is suggested that cost of production should be ta ken into consideration in relation to the defect severity. Chapter 6 pres ents the development of slurri es containing hybrid abrasive particles, which are softer as compared to the conventional particles, to reduce scratching and prevent severe surface damage during CM P. Polymer abrasives with silica and silicaceria composite as the surface oxide are developed and used for oxide CMP, and
10 resulting friction and surface finish are comp ared to the polishing done by conventional particles. This provides anothe r solution to reduce defects dur ing CMP, which is the main goal of this dissertation. Finally, the major fi ndings of this dissertat ion are summarized in chapter 7 of this report.
11 CHAPTER 2: BACKGROUND OF CMP PROCESS 2.1 CMP process CMP is the process of reducing the non-plan arity both with in the die and as well as across the wafer surface . In a CMP process, the wafer being planarized is pressed (facing down) onto a polymer polishing pad and is abraded in the presence of slurry, which contains active chemicals and abrasive particles. Thus, CMP can be described as a process which uses the combin ation of mechanical energy fr om the pad and abrasives and chemical energy from the slurry chemicals to polish and remove material from the wafer surface. The schematic of the CMP process that is simulated in this research on a bench top tester is presente d in the figure 2.1. The application of CMP in electronic device fabrication is significant in both memory and logic (Microprocessor) device fabri cation . It is being further introduced into many other modern applications such as MEMS  and ot her electronic device fabrication. As the device and interconnect wiring dimensions cont inuously shrink with the technology advancement in device manufac turing, the output specifications of the CMP process have become and are becoming much more stringent. For example, the allowable dish depth has come down to less than 15 nm and many such process specifications change as the industry progres ses into integrating much weaker low K dielectric materials with copper [2, 14].
12 Figure 2.1 Schematic of CMP process as simulated on the bench top tester The process induced defects such as micro-scratches and particle residue significantly hamper the device yield. Thes e defects along with post CMP characteristics such as dishing and erosion need to be mi nimized if a successful implementation of the process is to be achieved. Yield and thr oughput of the polishing process is highly dependent on the process parameters and consumable characteristics. 2.1.1 Material removal mechanism As mentioned above, CMP process invol ves both chemical and mechanical components acting in synergy to bring about removal of overburden and at the same time achieve planarization. In this context, it is important to understand the mechanism of material removal during CMP. The following sections provide an insight into the
13 mechanical and chemical aspects of CMP, wh ich are responsible for effective material removal from the wafer surface. 188.8.131.52 Mechanical aspects of CMP CMP is inherently an abrasion proce ss with two body and three body abrasion occurring simultaneously at the interface. Two body abrasion occurs when the abrasive particles from the slurry interact with th e wafer surface, and also when the pad surface asperities slide against the wafer surface. As the roughness of the pad is in the order of microns and the size of the abrasive particles is in the order of na nometers, a significant amount of two body abrasion takes place across the wafer involving the interaction between only the wafer and pad surface aspe rities. However, the contact is more complicated at times when the wafer, pad, and abrasive surfaces come in contact, constituting a three-body abrasi on. In this case, the contact involves abrasive particles that come in contact with the wafer, which are held in place under a given pressure by the pad asperities. As the abrasive particle s are dragged across the wafer surface under pressure applied using the polishing pa d, ploughing and cutting processes occur simultaneously, resulting in the material removal from the wafer surface. Figure 2.2 below shows the close up view of the interface , which presents the details of various mechanisms at the interface.
14 Figure 2.2 A close up view of the polishing interface . The lubrication at the polishing interf ace is provided by the fluid (water based chemicals) in the slurry. The type of c ontact between the pad-slurry-wafer and the intensity of abrasion during polishing is dictat ed by the thickness of the fluid film at the interface. Thus, the dynamics at the contact involves several mechanical interactions. 184.108.40.206 Chemical aspects of CMP process CMP process, as the name suggests, has an active chemical component along with the mechanical component, where the wafer su rface is modified by the chemicals in the slurry before getting abraded. Another important feature of the slurry is to dissolve the abraded material, thereby avoiding re-deposi tion of the removed material onto the wafer surface. In case of oxide CMP, the oxide su rface is first hydrolyzed by various chemicals of the slurry and the abrasive particles abrade the surface. Besides providing the mechanical component, the abrasive partic les while in contact with the wafer bond themselves chemically and remove the material as they separate fr om the surface [7, 16]. Thus, along with the chemicals in the slurry, the chemical natu re of the abra sive particles
15 also has a significant role to play in oxide CMP process. In case of copper CMP, the slurries can be both acidic a nd alkaline in nature [15-20]. The metallic copper surface is modified by the active ingredients and the pH of the slurry, following the above mentioned reac tions, to form dissolvable copper oxides and hydroxides [8, 15, 21]. The rate of oxida tion of copper depends on the particular formulation of slurry and the concentration of oxidizers and comp lexing agents of the slurry. The formed surface copper compounds will then be abraded off the surface by the abrasive particles and the pad asperities. In the case of copper CMP, abrasive particles only provide mechanical action and the chemical nature of the particle s does not play any role. The abraded copper compounds are then di ssolved into the slurry and are carried away along with the dispensed slurry. This dissolution is very crucial to the removal process, as it avoids the re-deposition of the material onto the wafer surface. Thus, the material removal rate depends equally on the mechanical as well as chemical aspects, justifying the name given to the process as chemical mechanical planarization. 2.1.2 Governing factors of CMP process CMP is a process involving many disciplines with several factors that affect the final output. These governing factors can be categorized broadly into a) process parameters and b) consumable characteristics. Table 2.1 below give s an overall list of aspects that govern the output of CMP process.
16 Table 2.1 Factors that govern the output of CMP process Process parameters Cons umable characteristics 1. Load (psi) 2. Angular velocity (RPM) of polishing pad and wafer carrier 3. Slurry flow rate (ml/min) 1. Wafer : Contour and size Pattern density Pattern dimensions Characteristics of inter-level dielectric layer Chemical compatibility of underlying layers to the slurry components 2. Pad: Bulk characteristics Surface characteristics Groove design Groove dimensions 3. Slurry pH of the slurry Particle size distribution Zeta potential Additives Oxidizer and its concentration
220.127.116.11 Process parameters Pressure and relative velocity at the polishing interface are the most crucial process input variables which impact the CMP performance. As per PrestonÂ’s Law  it can be easily seen that the pressure and th e velocity during polishi ng dictate the removal rate during the process. Beside s removal rate, pressure and velocity also determine the friction characteristics at the interface and also determine the regime of lubrication as per Stribeck Curve . High pressures and veloci ties result in high shear forces applied on the wafer surface, which might induce dela mination at the poorly adhering interfaces mainly involving dielectric materials . Besides pressure and velocity, many other factors such as slurry flow rate and pad surface temperature significantly affect the removal rate and also have a pronounced effect on the overall process output. 18.104.22.168 Consumable characteristics The characteristics of the wafer being po lished have a significant effect on CMP process. The size and contour of the wafer de termines the contact area and the polishing uniformity across the wafer surface during polis hing. The wafer contour also changes the interaction between the pad and the wafer at different pressure settings, resulting in a change in contact dynamics at the interface . Looking at a die level, the pattern density and dimensions along with the layout of pattern in a die affects the uniformity with in the die, resulting in change in uniformity of polish and the generation of post CMP characteristics such as di shing and erosion [26, 27].
18 The polishing pad is another consumable of CMP, which provides a major part of the mechanical component to the polishing. Typically, a polishing pad is constituted of two sections: the top section of the polishing pad, which cons ists of grooves and surface asperities; and the bottom, or bulk portion of the pad, which supports the upper portion and helps in achieving polishing uniformity [8 28]. These two secti ons of the pad can either be engineered separately, which is put together at the point of use or the bulk portion of the pad can be surface treated a nd grooved to suit a spec ific application of either metal or dielectric CMP. Another impor tant characteristic of the polishing pad is the ability of the pad to transport slurry efficiently to the polis hing interface . The dimensions, such as width and depth of the pad groove, along with the groove pattern are also important to have a uniform slurry distribution on the pad surface. Along with the pad grooves, pad surface asperities or the te xture are very crucial for CMP performance [30, 31]. A primary function of the pad asperiti es is to prevent the abrasive particles of the slurry from sliding off the pad due to cen trifugal forces of rotation and to have an efficient pad-wafer contact . The abrasive particles, which are held at the contact by the pad asperities, are the only particles available to provide active mechanical component during CMP. Slurry constituted of oxidizers, complexing agents, abrasive particles, and dispersants is one of the major factors that affect CMP. Slurry plays a critical role in modifying the surface being planarized, abrading the modified surface, and also dissolving the abraded debris. The concentrations of its various constituents significantly influence the output of CMP. Th e characteristics of the abrasi ve particles, such as size
19 distribution, zeta potential, unifo rm dispersion, etc., need to be maintained and monitored continuously to avoid formation of agglomerat ed particles or chippe d of particles with sharp edges. Failing to do so will result in the wafer surface being severely scratched, hampering the device yield and impacting the overall production lin e. Slurry also provides the chemical selectivity for different underlying layers (b arrier and dielectric layers in case of Copper CMP) that are not supposed to be polished using suitable additives and pH conditions. Manufacturing th e slurry so that it does not damage the underlying barrier and dielectric layers is cr itical to avoid yield a nd reliability issues. Successful implementation of CMP process significantly depends on optimizing the above mentioned factors for each process. Optimizing the process factors, selecting appropriate consumables, and ensuring consis tent process performance are governed in turn by the cost involved, risk involved and time lines for each technology cycle. 2.2 Tribology of CMP process 2.2.1 Tribology during CMP process Tribology is the science deali ng with friction, wear and lubrication during sliding of two surfaces. It is a Greek term which literally translates to Â“science of rubbingÂ” in English. Studying various aspects of tribol ogy helps in understanding various process mechanisms that happen at the rubbing inte rface. As mentioned previously in this chapter, CMP is an abrasion process which involves rubbing of wafe r and pad surfaces in presence of chemical slurry and abrasi ve particles. Like many other industry specifications, during CMP lower friction and effi cient lubrications are desirable. But at
20 the same time CMP process is aimed at ob taining higher wear (e ven though of the order of atomic layers) of the polished materi al to achieve consid erable throughput. The performance and the output of th e CMP process depend on the nature of polishing, which in turn depends on the contact area and the frictional forces associated with it. Thus studying the fr ictional characteristics during CMP proves significant to understand and improve the process. There are numerous theoretical and analytical models that explain the frictional characterist ics during a lubricating sliding contact [33, 34]. The interface of CMP is much more complicated as compared to conventional interfaces considered in these models and i nvolves several additional factors such as the abrasive particles, chemical component of th e slurry etc. Thus, these models find minimal application with respect to the CMP process. A better way of unders tanding the interface dynamics and mechanisms with respect to tribology is to study the frictional characteristics in-situ or during the process. The parame ters that could prove useful to study the tribology at interface are coefficien t of friction and acoustic emission signal . 2.2.2 Tribo-metrology of CMP The above mentioned coefficient of fric tion and acoustic emi ssion signal (AE) are crucial to characterize the friction charact eristics of a system consisting of sliding surfaces irrespective of the natu re of contact or the lubrica ting medium. These parameters along with other parameters such as wear rate of wafer surface and pad wear are termed as Â‘Tribo-metro logyÂ’ .
21 Coefficient of friction, as defi ned by Leonardo Da Vinci in 14th century , is the ratio of tangential force of friction (resisting motion) to the normal load. Such a coefficient representing the friction at th e polishing interface reflects the nature of interaction of the wafer-abrasive-pad material s. Coefficient of fric tion is influenced by several factors including the material prope rties of the interac ting surfaces. The shear force at the interface and the friction coeffi cient depend on many aspects, such as the padÂ’s mechanical properties, the kinematic pa rameters of the polishing process, slurry viscosity, and chemical properties. A detaile d description about th e effect of various factors on coefficient of friction is presented in the imminent sections and chapters of this dissertation. Taking the advantage of this aspect, changes in the wafer surface can be analyzed by monitoring the coefficient of friction data either in-situ or ex-situ. Another parameter that can be monitored during the CMP process is the acoustic emission or the AE signal. The AE signal is an estimate of the acoustic energy dissipated at the interface due to mechanic al interactions of sliding surfaces and abrasive particles at the interface. Higher AE signal indicates intens e mechanical interactions or aggressive abrasion at the interface and lower signal indi cates a smooth, mild polishing resulting in lower shear forces and less damaged wafer surface. A noisy signal could indicate the presence of agglomerates or occurance of delamination at the interface . The wafer surface wear rate, pad conditione r friction and pad wear are the other parameters that need to be monitored to effectively conduc t CMP performance. The wear rate of the surface being polished is aimed to be consider ably high to maintain process throughput whereas minimum possible pad wear is desired. Pad surface needs to be conditioned in
22 order to maintain the surface roughness and to f acilitate uniform slurry flow on the pad. Conditioning process helps to achieve mi nimal wafer to wafer non-uniformity and improves process optimization. Pad conditione r coefficient of friction during such a conditioning process should be monitored, which upon stabilizing indicates the end point of conditioning process . The following sections in the context of tribo-metrology, the original contribution of this author, were recen tly published in the tribometrology chapter of text book called Â‘Microelectronic applications of Chemical Mechanical PlanarizationÂ’ edited by Dr. Yuzhuo Li . Coefficient of friction used to generate Stribeck cu rves  offers an efficient means to monitor a tribological process. Stribeck curves as shown in Figure 2.3 are generated using coefficien t of friction data and the Sommerfeld number. These are greatly useful in determining the lubri cation regime at the polishing interface. Figure 2.3 Stribeck curves generated us ing coefficient of friction data 
23 The Sommerfeld number is defined as Sf = effp U where is the viscosity of the lubricant, U is the relative veloc ity, p is the applied pressure, and eff is the effective lubricant film thickness. Applying the above formula to the CMP environment, (viscosity of the slu rry) can be easily found, since pressure and velocity are known as they are the input process para meters. However, the fluid film thickness is the toughest to estimate. In recent research  the flui d film thickness was a pproximately estimated using the pad surface roughness. To account fo r the deviations of the slurry film thickness on different grooved pads, a dimensi onless factor has also been suggested. There are three main regimes of lubrica tion, namely a) boundary lubrication b) mixed lubrication, and c) hydrodynamic lubric ation at a lubricated frictional interface; even though there are other minor regimes called hydrostatic and elastohydrodynamic lubrication regimes [33, 34]. During CMP, th ere exists solid-solid contact between the wafer and pad during boundary lubrication, wh ere the removal process is dominated by surface abrasion. In this regime, polishing results in severe surface damage due to aggressive abrasion by slurry particles and the polishing pad. Also, the thermal energy dissipated in this case must be very high, resulting in a non uniform and inconsistent material removal rate. In the mixed lubrication regime there is a thin film of slurry which supports the applied pressure to an extent, a nd thus prevents aggres sive abrasion. It is beneficial to ensure that the CMP process is conducted in this lubrication regime, as it would reduce the surface damage to a great extent. Hydrodyna mic lubrication regime or hydroplaning mode of polishing results when the applied pre ssure is totally supported by
24 the slurry film present at th e interface. This may result in a very low coefficient of friction, but at the same time dramatically reduc es the removal rate as there is practically no abrasion. Knowledge of the lubrication regime of polishing is thus highly beneficial to understand the polishing pro cess in greater detail. As mentioned earlier, the coefficient of friction (COF) a nd contact acoustic emission (AE) can be constantly monitored an d recorded to determine the end point of the polishing process . A change in coe fficient of friction a nd AE signals can be observed as soon as the end point of the pr ocess occurs. This happens as the underlying thin film constituting the new polishing interface has different properties compared to the film that has been removed. Either in-situ or post CMP analysis of the friction coefficient data allows for calculations of the time to remove a particular layer, and thus the rate of material removal. These calcula tions can take place either dur ing the process or after the entire layer has been removed. Such monitoring of coeffici ent of friction prevents overpolishing, thus avoiding defects su ch as dishing and erosion. Das et al.,  studied the coefficient of friction signal in order to determine the end-point detection more effectively. The raw COF data was analyzed and the noise levels were filtered out to more precisely determine the end point of the process. Variance sequential probability ratio test (SPRT) method was adopted to anal yze and filter the raw data. Coefficient of friction signal can thus be processed and analyzed to detect the end point more effectively. Not only the removal rate or the end point detection of the process but also the uniformity of polish can be estimated from the coefficient of friction data. This can be done by monitoring the time taken for the coeffi cient of friction signal to change from
25 one level to another as the end point of the pr ocess is reached. This time for the change of signal can be termed as the transition tim e during end point. The longer the transition time, higher is the non-uniformity . Sim ilar analysis on AE si gnal can also be conducted to additionally determine process induced defects such as delamination and generation of micro-scratches. From the above discussions it can be s een that tribology during CMP process significantly influences the mechanism of material removal and surface polishing. Moreover, the coefficient of friction and ac oustic emission signal can provide valuable information regarding the nature of tribolog ical interaction at th e interface. In this research, investigation of coefficient of friction and factors influencing friction are studied in great detail to develop funda mental understanding of the CMP process mechanism and governing factor s. Such an understanding is aimed at improving the process control and optimization 2.3 Thermal studies during CMP process The thermal aspect of CMP, even though a significant factor affecting the process output as noted above, has not been researched as extensively as parameters like pressure, velocity, slurry flow rate, and other chemical aspects. Heat dissipation due to friction can result in a temperature rise at the interface a nd a rise of about 10 Kelvin at the polishing interface is high enough to double the removal rate during copper polishing . Also, it has been noted that a change of 1 Kelvin can affect the proce ss removal rate during polishing by 7 % . This is attributed to the low activation energy of the copper
26 oxidation reaction in th e slurry [41, 42]. Fractions of heat genera ted at the interface are either conducted to the wafer and pad, or c onvected away by the slurry which acts as a coolant at the interface. Research work on temperature rise on the surface of the polishing pad during interlayer dielectric and metal polishing, re moval rate dependence on temperature and its modeling, effect of slurry flow rate on pad temperature rise etc., has been carried out moderately in the recent past to understand the role of temperature on CMP performance [40, 43-45]. Borucki et al., [43, 44] have devel oped a thermal model for ILD polishing and then modified it slightly to get a model for copper (Metal) CMP which was validated by comparing with temperature measuremen ts on the pad during metal CMP. They developed a theoretical understanding of th e thermal aspects in their research and predicted temperature on the pad for the ini tial stages (first 60 sec) of polishing by evaluating the model based on transient heat transfer mechanism. White et al.,  have modeled dynami c thermal behavior which explains the energy exchange between the pad and slur ry. Heat accumulation in the pad and the convection of heat to the slur ry were explained in their re search work. Also, a transient thermal model was proposed to explain the initial transient ther mal behavior observed during CMP. Some other research works on thermal aspects such as using temperature change as end-point detecti on, experimental work involv ing the temperature rise on polishing pad [46-48], can also be found in the literature. These research works had modeling and experimental evidence of temp erature rise only on the polishing pad. The
27 first attempt to measure the temperature on the silicon wafer was done by Sampurno et al. . A direct temperature measurement set up was developed wherein a novel wafer carrier was designed such that the temper ature on the back side of the wafer was measurable using a thermal imaging IR camera. However, in all these research works the reported temperature rise is either the average temperature on the pad surface, a predicted average temperature on the wafer surface, or temperature rise at three isolat ed locations on the wafer. These works report the overall temperature rise but do not pr ovide the information about the radial temperature distribution on the wa fer surface. An analysis of the temperature distribution on the wafer surface after the polishing proc ess reaches a steady state has not been researched to date, which forms the basis of another research objective of this research work. The temperature profile on the wafer su rface as a function of radius and thickness will be developed for various input process variables. Since the material removal rate during copper CMP is highly sensitive to te mperature, understanding the temperature profile will help in reducing the with-in-wa fer non-uniformity, and thus improves yield by minimizing the number of faulty dies. 2.4 Challenges during CMP process Besides being the unanimous choice for local and global planarization, CMP process has inherent challenges particularly with the wide variety of materials being incorporated for Copper/Low K integration. So me of the critical chal lenges that are dealt with in this research are a) With in Die Non-Uniformity (WIDNU), b) dishing and
28 erosion due to non-uniformity in polish c) micro scratches on the surface and d) high friction resulting in delamination. 2.4.1 Non-planarity Post polish non-planarity could be a resu lt of several factors such as a) ,onselective slurry, b) pad asperi ties reaching deep into the wi de lines, both isolated and array, c) slurry with a high etch and di ssolution rate, and d) non-uniform pressure distribution on patterns with varying line width and patte rn densities [ 50-52]. Figure 2.4 Pattern dependent non-planarit y resulting due to CMP process 
29 Figure 2.4 above shows various post CMP surf ace characteristics  that result due to the above mentioned factors at different pattern densities and line widths. It can be seen that step 2 (copper clear) and step 3 (barri er removal) are more critical as compared to bulk removal in terms of pa ttern dependencies. Di shing, erosion, oxide and metal loss are the four types of post CM P pattern effects that result in deviation from achieving a planar surface. Dishing is the loss of metal from the wide metal lines, whereas erosion is the loss of oxide along with the narrow array of thin metal lines. Ox ide loss is the loss of field oxide next to an array of thin meta l lines separated by a wide oxide pattern and metal loss is the total loss of thickness of the metal lines separated by thin oxide pattern. Figure 2.5 below shows  the schematic of these pattern deviations. These post CMP characteristics affect the electrical properties of the interconnect structure and at the same time induce non-planarity over the wafer surface, causing lithography issues, which nullifies the primary purpose of CMP process . Governing factors that would cause the deviation from non-planarity after CMP pr ocess need to be understood in detail to avoid reduction in device yiel d due to these characteristics. Figure 2.5 Post polish surface anomalies due to pattern deviations resulting in wafer surface non-planarity .
302.4.2 Surface scratches Another source of defect that is genera ted due to the polishing process is the surface damage due to the mechanical interact ion of abrasive particles with the wafer surface. Scratches, both macro and micro, fo rm due to the deep indentation and dragging of the abrasive particles . Many factors such as particle size distribution, presence of large particles in the slurry, formation of aggl omerates due to slurry pH, which affects the surface charge on the particles causing them to bind to each other etc., could be the root cause of the surface scratches. Figure 2.6 shows some typical scratches that are formed on wafer surface during CMP process. Although some of these scratches can be removed during the final step of polishing called buffing (which eliminates shallow scratches), the more intense scratches are permanent and cannot be removed from the surface. Such a damaged surface will directly impact the el ectrical performance of the interconnect scheme and thus hampers the device yield. Thus, countering this ki nd of defect becomes a critical aspect of CMP process yield im provement. The third objective of developing new slurries to achieve reduced surface dama ge during CMP finds application in this context. Figure 2.6 Optical microscopy of scratc hes and pitting on wafer surface .
312.4.3 Delamination during cu/low-k CMP Besides replacing aluminum with copper, another way to reduce RC delay is the introduction of materials that have lower dielectric constant than SiO2 as the inter-level dielectrics. The drawback of introducing such materials with low dielectric constant (low K) is that they are mechanically weak materials [54-60]. These mechanically weak materials cannot withstand the shear forces applied during CM P process. Moreover, their interfacial adhesion energies ar e so low [7, 61] that even m oderate frictional forces can induce the failure of these interfaces. This wi ll impact the reliability of the multilevel metallization stack. Several studies  have been conducted to determine the causes of delamination and how it could be prevented. Some of the major conclusions are that process development for CMP using low pressu res and velocity is necessary to avoid such failures. They also suggested that sl urries and polishing pads that result in less friction at the interface need to be developed. Less friction at the interface results in lower shear forces, thereby decreasing the occurren ce of delamination. Figures 2.7 and 2.8 (a) and 2.8 (b) show the SEM images of the delaminated interfaces [7, 24]. Figure 2.7 Delaminated interface during CMP process 
32 Figure 2.8 Delamination at via layer and a) copper dielectric cap interface b) the trench layer stress point  These defects directly destr oy the device, resulting in reduced yield and increased cost of production. The above discussion of generation of micro-scratches and the occurrence of delamination forms the basis for the final objective of this thesis, which is to develop slurries that reduce surface dama ge during CMP and at the same time reduce the friction at the polishing interface in order to avoid delamination. 2.5 Research objectives There are mainly three objectives of this research work: a) Study the effect of temperature on CMP process including pad conditioning process, b) Develop a thermal model and accordingly study the effect of slu rry flow rate on CMP process and post CMP surface characteristics, and c) Develop a slurry with novel Â‘softÂ’ particles to achieve defect-free post CMP wafer surface. The objectiv es are explained in more detail in the following sections below.
332.5.1 Process optimization: effect of temperature Every process needs to be optimized to mi nimize the defects and at the same time maximize the removal rate in order to achieve both yield and throughput. CMP has several variables and factors that affect its Â‘outputÂ’, which in itself is constituted by several specifications with regard to removal rate, post polish surface quality, planarity, etc. One of the several factors affecting CMP that is least researched is the temperature, either be it operating or the temperature of the consumable s. Even though there are few research works in the recent past, the entire effect of temperature on various facets of CMP is far from being understood in its entirety. The first objective of th is research is to study the effect of temperature on the CMP process comprehensively and to conduct an in depth investigation into the causes of in creased removal rate with an increase in temperature. This investigation is aimed at providing an insight into the various electrochemical and surface chemical aspects of copper-slurry interaction at different temperatures. An extension of this study will be to study the effect of temperature on pad conditioning process. Chapter 3 presents the result from temperature study and elucidates various mechanisms occurring at the interface. Since temperature affects the pad surface asperities, the contact area and the copper-slurry interaction, the eff ect of temperature on the coefficient of friction at various process parameters is studied in detail. Chapter 4 elucidates the results of the temp erature effect on COF during CMP. 2.5.2 Thermal model Another objective of this research is to model the conduction heat transfer mechanism at the interface after a steady state has been achieved by the polishing
34 process. By solving the model, this research presents the temperature profile on the wafer surface as a function of wafer radius and th ickness. The analytical modeling effort is supported with the finite elem ent analysis using FIDAP package which can handle both conduction and convection heat transfer enviro nments effectively. The results from the model indicate a change in surface temperatur e due to a change in incoming slurry flow rate. Based on this result, a further study on th e effect of slurry flow rate on the CMP process is conducted not only in the aspect of tribology but also with regard to post CMP surface asperities and pattern re lated characteristics such as dishing and erosion are studied. 2.5.3 Low defect slurry development Another aspect of CMP process that gained a huge market in the recent past is the consumable market. The characteristics of th e process consumables such as polishing pad material, slurry additives, slurry abrasive part icles, dispersants, etc. influence the process output control. As observed in the previous section concerned with challenges during CMP, several defects such as micro-scra tches and delamination occur during CMP. These defects can be countered effectively if a Â‘gentleÂ’ CMP process is developed, which results in fewer number of scratches and lower friction forces. Slurry with nonconventional soft abrasive particles could be a possible solution. This forms the third objective of this dissertation.
35 CHAPTER 3: EFFECT OF TEMPERA TURE ON COPPER CMP PROCESS 3.1 Introduction Even though copper CMP process has been researched in the past [15, 62-66], certain factors of the process having signi ficant effect on overall copper CMP process output need further investigation. Temperatur e rise at the polishing interface constituted by pad, film, and slurry abrasi ves is a significant factor th at affects CMP, which has not been researched extensively nor been underst ood in its entirety. Ther mal effects need to be thoroughly researched in order to bette r understand the resulting modifications in mechanisms during the process of copper CMP. Research based on thermal effects during Interlayer dielectric (ILD) polishing, friction induced heating, temperature increase on the polishing pad, removal rate dependence on temperature, and its modeling has been carried out in the recent past to understand the role of temperature at interface on CMP performance [40, 41, 45 ,67]. Present research deals with the changes in electrochemical aspects and surface modification mechanisms of copper with a va riation in slurry temperature. Along with the electrochemical measurements, XPS studies dissolution rate and pH variation with slurry temperature are monitored and their c ontribution towards the increase in removal rate are discussed. The effect of temperat ure during CMP on patterned samples is also conducted to understand the effect of temper ature on with-in die non-uniformity. It is highly beneficial to understand the effect of temperature on CMP performance, in order
36 to achieve better removal rates without compromising any other planarization specifications. 3.2 Materials and techniques 3.2.1 CMP benchtop tester Electroplated Copper blanket films of 15000 thick were polished using the CETRTM bench top CMP tester (see figure 3.1 below). Programmable forces, speeds and slurry flow rates allow one to closely imitate fabrication plant (fab) CMP processes on any production polisher and to understand the pr ocesses in detail. The tester holds a 6 inch polishing pad and can hold up to a 2 inch diameter silicon wafer. Real time coefficient of friction can be measured usi ng a dual force sensor which measures lateral and normal forces. Lateral and normal forces are continuously monitored and recorded in-situ at a total sampling rate of 20 KHz. Feat ures of the bench top tester are provided in detail in previous publication [68, 69]. A 6Â” diameter polishing pad coupon attached to the bottom platen was used to polish a 1Â” X 1Â” sample coupon placed face down onto the pad. Cabot iCue 5001 slurry and IC1000/Suba IV polyurethane perforated pads were used to polish the copper samples. The slurry temperature was varied from 18.5o C to 30o C. The slurry flowing at the interface is th e only source to convect heat away from the interface. Hence, the variati on of the slurry feed temperature directly decreases the amount of heat taken away from the interface, thus increasing the interfacial temperature. The slurry temperature was controlled for each experiment during the whole project by monitoring and maintaining the temperat ure at a specific value within a 0.5oC variation. The experimental procedures dealing with the contact of wafer and the slurry were designed to be short in order to minimize ther mal losses. Also, the slurry containers were
37 insulated with casing made of thermocol material to further reduce thermal losses. The coefficient of friction (COF) was monitored in-situ from the bench-top tester data acquisition system. This is made possible by us ing a dual load sensor attached above the wafer carrier which can measure the normal a nd lateral forces during the process, the ratio of which gives the coefficient of fric tion. The variation in COF upon removal of the copper film was used to determine removal rate. Figure 3.1 Bench top tester that can simulate real time CMP process Polishing conditions were maintained at 3 PSI, 100 RPM bottom platen rotation, 95 RPM carrier rotation, slurry flow rate 50 ml/min. Patterned 0.8 (2 cm X 2 cm) square wafer coupons with 10 k electroplated copper layers an d an MIT 854 pattern were used AE Sensor Slurry Flow Slider Assembly Force Sensor Upper Carriage Pad with LowerPlaten Slurry Outlet
38 for non uniformity experiments . The pattern consists of different line widths ranging from 0.18 m to 100 m and pattern densitie s ranging from 1% to 100%. The layout of the pattern is presented in Figure 3.2 below. Patterned wafer coupons were polished at 4 psi, 150 RPM bottom platen rotation, 145 RPM car rier rotation, and sl urry flow rate was maintained at 75 ml/min. Figure 3.2 MIT 854 mask lay out with vary ing pattern line widths and densities To understand the effect of temperatur e on copper surface due to polishing at different temperatures in terms of mechanic al and surface properties, nanoindentation and atomic force microscopy (AFM) were conducted.
393.2.2 Nanoindentation The evaluation of the mechanical prope rties of the Cu layer deposited on the top of the stack was pe rformed using nanoindentati on. The experiments were performed on a standard MTS nanoindentor (see figure 3.3) us ing a three sided Berkovich diamond tip. The hardness and YoungÂ’ s modulus of the candidate thin films were calculated. Experiments were performe d in the continuous stiffness mode (CSM). Continuous stiffness mode enables one to conduct experiments while measuring the contact stiffness at each depth through out the indentation. Thus, a dynamic measurement is made possible and mechanical properties along the penetration depth are studied. Figure 3.3 Nanoindenter used for the mech anical characterization of thin films A Berkovich diamond indenter of radius 20 nm has been used as the indenter. The indentation depth was limited to the 150 nm (15 % of the total cu thickness) to avoid the effect of underlying thin film layers and the substrate. The calculation of hardness and modulus was performed from 70 to 140 nm of penetration depth. This range has been
40 chosen to minimize the substrate effects in calculation of mechanical properties of thin films and also to avoid getting data from the initial part where the indenter might not be totally stable and the minor variations pres ent due to system vibrations and machine stiffness might influence the data. The valu es of hardness and YoungÂ’s modulus as a function of indenter depth were plotted, a nd from these output parameters the effective hardness and modulus of the thin films was estimated. The effect of polishing temperature on the copper thin film has been elucidated from thes e illustrations. Post CMP Atomic Force Microscopy (AFM) was carri ed out to characteri ze the samples for surface roughness achieved after polish at di fferent temperatures using a Digital Instruments DimensionTM 3100 AFM operated in tapping mode at 256 Hz frequency of the cantilever. 3.2.3 Electrochemistry and x-ray photoelectron spectroscopy For electrochemical experiments, the constructed cell consisted of an Ag/AgCl/KCl saturated reference electrode with platinum stri p as counter electrode and a diced strip of copper thin film coated sili con wafer as working electrode. The schematic of the electrochemical cell is presented in figure 3.4. The backside of the wafer was isolated from electrical and chemical contac t by coating it with acrylate and polyester copolymer material. The electrodes were pl aced at least 10 mm apart from each other in the electrolyte. PARSTAT 2263 model advanced electrochemical system manufactured by Princeton Applied Research was used for polarizing experiments of the copper sample. Current density data was collected for an applied potential range of -0.9 Volts to 0.9 Volts scanned at a speed of 5mV/sec. The temperature of the electrolyte for each
41 experiment and the electrode-electrolyte contact area were maintained constant through out the experimentation. Figure 3.4 Schematic of the electrochemi cal cell for the polarization experiments. Surface modifications were investigated by X-ray Photoelectron Spectroscopy (XPS) measurements performed using Al K radiation on an as received copper film and a slurry treated copper film at four diffe rent temperatures. Ultr aviolet Photoelectron Spectroscopy (UPS) was performed on the samp les to estimate the work function to be used for XPS data. The samples were immersed in the slurry for 2 minutes, dried in an inert atmosphere, and then immediately transferred into the vacuum chamber for the collection of spectra. The peaks obtained were deconvoluted and were curve-fitted using
42 a combination of both Gaussian and Lorentzi an profiles . The copper samples were treated for 5 minutes in a temperature cont rolled environment, and gravimetric studies were performed to investigate the variation in dissolution rate with temperature. A precision balance (Sartorius R 200D research model) was used for gravimetric studies with a sensitivity of 0.01 mg. 3.3 Results and discussion 3.3.1 Effect of temperature on CMP tribology The temperature of the slurry was the only varying parameter during CMP of copper blanket samples performed on the CETRTM bench-top tester The slurry was maintained at the predetermined temperatur e (both above and below room temperature) for the duration of the polishing experiment. Removal rates were calculated from the insitu endpoint detection ability of the machin e, which shows a change of the COF at the complete removal of the thin film. The time for removal was noted, which gives the removal rate information. The variation of the removal rate along with COF with change in slurry temperature is plotte d in figure 3.5. An overall incr ease in the removal rate with temperature can be observed from the figure.
43 100 120 140 160 180 200 220 240 15 20 25 30Temperature oCRemoval Rate in nm/min0 0.1 0.2 0.3 0.4 0.5 0.6151719212325272931COF (no units) Removal Rate COF Linear (Removal Rate) Li(COF) Figure 3.5 Increase in removal rate and coefficient of friction with change in temperature This increase of the removal rate can be attributed to changes in both the mechanical and chemical nature of the polishing process. Th e change of the mechanical component of the process has two reasons. Firs tly, as a result of polishing pad softening (change in mechanical propertie s of the pad due to the increas e in temperature) the area of contact increases , thus increasing th e number of abrasive particles coming in contact with the wafer surface. This is s upported by the COF increase with increase in slurry temperature as shown in figure 3.5. Th ese results are in agreement with the results from previous investigations by other author s . Secondly, a decr ease of the viscosity of the slurry occurs with increasing temperature, which increases the friction at the interface and hence increases shear resu lting in higher removal rates .
443.3.2 Effect of temperature on electrochemistry To investigate changes in the nature of the surface chemical reactions depending on slurry temperature, electrochemical studies were carried out. The applied potential is plotted against output current density in yielding potentiodynamic curves for samples treated at different temperatures. The concentration of H2O2 in the slurry at different temperatures in the specified temperature range was verified to be constant using titration methods. The variation in pH was noted to be constant for all pract ical purposes (7.6 Â– 7.4) within the specified temperature range Figure 3.6 presents the corrosion potential and corrosion current density values de rived from the potentiodynamic curves. Figure3.6 Change in corrosion current density and corrosion potential with change in slurry temperature
45 Corrosion potential was directly obtained from the plotted E vs Log I curves. The potential at which both the a nodic and cathodic current rate s are equal is termed the corrosion potential by definition. The corrosion current was determined by extrapolating the cathodic and anodic curves, which approxim ately intersect on th e line of corrosion potential (Ecorr), x-co-ordinate for the inte rsecting point gives the corrosion current density. The variation in corrosion current density is in agreement with the removal rate data provided in figure 3.5. The corrosion po tential shifts towards more negative values with increasing temperature, indicating an enhanced anodi c reaction. Changes in the anodic profile of the graph can be seen from the potentiodynamic curves, which give an estimate of the corrosion or the consumption of the metal. A continuous increase in the anodic current densities for hi gher temperature system at hi gher potentials can be noted from the figure 3.6 above. This indicates mo re corrosion and more anodic (metallic) dissolution observed at higher values of temperature. 3.3.3 Effect of temperature on surface chemical compounds To further investigate the surface oxidat ion and modification, XPS was performed on as received and slurry treated samples at different temperatures. The work function as measured by UPS was found to be constant for samples treated wi th changing slurry temperature. Cu (2p) peaks were mainly anal yzed to investigate changes in the oxidation state of Cu surface and relative intensities of oxides on copper surface. Figure 3.7 shows a comparison of the copper peaks for copper sa mples as received and treated with slurry at different temperatures.
46 Figure 3.7 Comparision of cu (2P3/2) spectra for as-received and slurry treated samples at different temperatures From the spectra it can be seen that oxide peaks are relatively more intense, hence lowering the intensity of metallic copper for sample treated with slurry at 21oC (room temperature) as compared to the sample treated at 16.5oC. This indicates a higher oxidation rate. It can also be noted from the spectra that the shoulder of copper oxide is absent at higher temperatures. This is due to the increased dissolution rate of the surface oxides at higher temperatures. This hypothesi s is supported by the chemical dissolution
47 data presented in figure 3.8, which shows a gradual increase in material loss at higher temperatures. Figure 3.8 Increase in dissolution rate w ith an increase in slurry temperature. A low shoulder can be seen from the as received copper sample, which can be attributed to the native surface oxides present on the sample before treatment. Figure 3.9 shows the curve fits of the spectra shown in figure 3.7.
48 Figure 3.9 Curve fitted cu (2P3/2) spectra for as-received and slurry treated samples at different temperatures The oxide peaks (O (1s) spectra) studied for all the above samples are presented in figure 3.10. From the figure it can be noted that the copper oxide peaks are less intense than the alumina peaks (alumina particles are present in the slurry as abrasive). This is due to oxygen present in alumina deposited onto the sample surface being more sensitive to XPS as compared to the oxygen present in copper oxide. This is also the reason for
49 more intense copper oxide detection on the as-received sample as compared to the samples treated with the slurry as can be seen in figure 3.7. Figure 3.10 Comparison of oxygen (O1(s)) spec tra for as-received and slurry treated samples at different temperatures The relative concentrations of compounds on the surface were determined as a product of peak intensity and the full width at half maximum values. The ratio of copper oxide to copper metal depending on slurry temperature is shown in figure 3.11. The initial decrease in oxide to metal ratio from 16.5oC till room temperature is due to
50 enhanced dissolution rate of copper in the slur ry and not much increa se in oxidation rate. The minor increase in the oxide to metal ratio above room temperature is due to increased oxidation rate, which is greater than the increase in dissolution rate. From these observations it can be reasoned that along with an increase in oxidation rate there occurs simultaneous dissolution of surface oxides into the slurry at a higher rate at elevated temperatures, contributing to an overall incr ease of the material removal rate during copper CMP. 0 0.05 0.1 0.15 0.2 0.25 151821242730Temperature oCoxide/metal conc ratio Figure 3.11 Concentration ratio of oxide/met al on copper surface treated at different temperatures
51 3.3.4 Effect of temperature on polished copper surface 22.214.171.124 Surface roughness Surface imaging post CMP samples polished at various slurry temperatures was performed using Atomic Force Microscopy. Surface images of 10 m size at 200 nm vertical data scale were take n at five locations on the wafer surface and the value of the surface roughness was averaged. The three dimens ional images of the surface that has the roughness closer to the average are presented in figure 3.12 (a-d). (a) (b) (c) (d) Figure 3.12 AFM images of the blanke t copper sample after CMP at (a) 18.5oC (b) 21.5oC (c) 27oC (d) 31oC
52 It can be seen from the figures that the sample polished at 18.5oC has a relatively rougher surface. But for the sample polished at below room temperature, all the other samples had the similar roughness and no signi ficant change has been observed. There was a minor increase in the depth of sc ratches for samples polished at higher temperatures. This increase in scratch depth might be due to two reasons: a) Changes in particle size distributions at hi gher temperatures as discusse d by Kim et al , b) active dissolution of the modified surface layer exposing the underlying untreated copper surface to the particles. The ch ange in depth of scratch is not found to be significant to change the overall surface roughness. In summ ary, the effect of temperature on the surface roughness generated during CMP was found to be insignificant. 126.96.36.199 Non-uniformity Patterned samples following the CMP at diffe rent temperatures were analyzed for the step height differences on isolated wide lines and narrow arrays. As was mentioned at the beginning of the chapter, the wafer samples contained MIT 854 mask pattern with a wide variety of feature sizes and densities. 9 m wide isolated lines and 1 m arrays were analyzed for step height differences. With -in die non uniformity was then computed as the difference in step height re duction at the isolated wide (9 m) metal line and array of narrow (1 m) features. The AFM images and the sc hematic presented in the figure 3.13 (a) elucidates the features being analyzed. The dependence of with in die non-uniformity on the polishing temperature is presented in figu re 3.13 (b). From the figure it can be seen that the percentage difference in step he ight reduction decreases as the temperature
53 increases from below room temperatures un til it reaches room temperature and then increases as the temperature increas es above room temperature . Figure 3.13 (a) AFM images and schematic of the pattern describing the analysis for non-uniformity measurement (b) non-uniformity with-in die versus the temperature of polishing.
54 A high level of non-uniformity at 15 oC is attributed to the non-uniform chemical activity of the slurry and stiffened pad surf ace asperities, which result in predominant mechanical action. As the temperature increas ed the chemical activity increased, and a balance between the chemical and mechani cal components of CMP is achieved resulting in least nonÂ–uniformity. At elevated temperatures soft pad asperities reached deeper into the wider lines but could not reach deep into the narrow isolated arrays causing differences in removal rates, resulting in increased non-uniformity. From these results, it can be understood that even though an increase in temperature increases the removal rate, it also increases non-uniformity with-in die during polishing. Such non-uniformity needs to be monitored more cautiously to preven t damage to the underlying features towards the end of the polishing step. 188.8.131.52 Mechanical properties To further investigate the changes in ch aracteristics of th e copper film with polishing process, nanoindentation studies were conducted on the pre-polished and postpolished copper samples. Mechanical properties of the thin film being polished play an important role during CMP. The surface scratche s, being one of the critical aspects that determine the polishing performance, depend on the mechanical propert ies of the sliding surface. Mechanical properties of copper befo re and after polishing have been estimated as described below. The unloading curve of th e load-displacement curve gives stiffness, which is used in the calculati on of the modulus of elasticity. With the help of continuous stiffness measurement, hardness and elastic modulus can be measured along the depth of penetration [76, 77]. Figure 3.14 presents th e typical load versus displacement curves
55 obtained during indentation. Stiffness is meas ured as slope of unl oading curve as shown in equation 3.1 Figure 3.14 Load versus displacement curve (3.1) Where Â‘SÂ’ is the contact stiffness and Â‘AÂ’ the contact area The basic assumptions for the analysis are: Deformation upon unloading is purely elastic. The contact between a rigid indenter of defined shape and the sample is modeled using SneddonÂ’s equation. The deformation of the sample and of the i ndenter tip can be combined and given as reduced elastic modulus as shown in equation 3.2 as follows [76, 77]: (3.2) Where Er is the "reduced modulusÂ”, is the Poisson ratio and Â‘iÂ’ andÂ‘sÂ’ refer to the indenter and sample respectively. s 2 s i 2 i rE 1 E 1 E 1 rE A 2 dh dP S
56 Hardness of the thin film being indented can be determined as the ratio of maximum load and the area of contact (see equation 3.3). This area of contact is estimated as a function of indentation depth using area coefficients which depend on the shape of the indenter. H = A P (3.3) Where Â‘PÂ’ is load and Â‘AÂ’ is contact area = fn (hc 2) Figure 3.14 shows the load versus displacement graph for the copper samples polished at different temperat ures. The hardness and modulu s of the copper samples is calculated according to the above mentioned pr ocedure. Figure 3.15 presents the hardness of the unpolished film and for the films post CMP at different temperatures. From figure 3.16 it can be noted that th e hardness of the polished c opper surface in creased with increasing slurry temperature. The unpolis hed sample had significantly lower hardness than the polished ones. This change can be attributed to the work hardening phenomenon during CMP. The work hardening phenomenon app ears to be more influential at elevated temperatures. Similarly, the modulus of the polished and unpolished thin films along the penetration depth is presented in figure 3.17. It can be seen that the modulus of elasticity increases with increasing slurry temperature.
57 0 0.2 0.4 0.6 0.8 1 1.2 050100150 Displacement Into Surface (nm)Load On Sample (mN) 60 F 85 F 70 F Figure 3.15 Load versus displacement for po lished copper samples at different slurry temperatures 0 1 2 3 4 5 6 050100150Displacement Into Surface (nm)Hardness (GPa) 60 F 85 F 70 F unpolished Figure 3.16 Hardness versus displacement for unpolished and polished copper samples
58 Also, the modulus of elasticity of the unpolished film is significantly lower than that of the polished samples. The numerical data from the nanoindentation experiments is tabulated below in Table 3.1 which presents in detail the change in mechanical properties of thin films with a change in slurry temperature. Table 3.1 Mechanical properties of coppe r thin films before and after polishing. Sample Modulus (Gpa) Hardness (Gpa) Unpolished Cu 118.51 3.72 1.22 0.06 Polished at 60 F 136.38 5.45 1.57 0.087 Polished at 70 F 136.29 4.77 1.66 0.093 Polished at 85 F 143.45 2.98 1.72 0.068 50 100 150 200 250 050100150Displacement Into Surface (nm)Modulus (GPa) 60 F 85 F 70 F unpolished Figure 3.17 Modulus versus displacement for polished and unpolished copper samples
59 This confirms that the state of the copper surface does not change much during polishing at different temperatur es . This suggests that the changes in coefficient of friction are not concerned with the wafer surf ace characteristics, but mainly due to the mechanical properties of the pad and also some chemical interaction changes. 3.5 Conclusions The effect of the interfacial temperat ure during the proces s of copper CMP was studied to better understand the polishing proces s. The role of the temperature on various aspects of CMP like the electrochemical behavi or of the metal in the slurry, the surface modification of the metal during CMP, and the surface roughness has been investigated. An increase in the removal rate with an incr ease in interfacial temperature was observed. An increase in the coefficient of friction with rising interfacial temperature was attributed to larger area of contact at the interface due to pad softening. Electrochemical and XPS studies carried out at differe nt temperatures indicate hi gher anodic reaction rates and higher dissolution rates of the formed surface oxides. The eff ect of temperature was not significant on the roughness of the copper surface post CMP. However, the scratch depths were deeper suggesting an increase in area of contact between pad and wafer at elevated temperatures. The With in Die N on-Uniformity (WIDNU) initially decreased from below room temperature to room temp erature and then increased at elevated temperatures. The mechanical properties of the polished copper fi lm however did not change with a change in temperature, sugge sting no role of wafer surface in the observed change in tribology at the interface.
60 CHAPTER 4: EFFECT OF TEMPERATURE ON TRIBOLOGY DURING CMP 4.1 Introduction The tribology at the interface is dependent on a wide variety of aspects, such as the geometrical and material characteristics of the consumables involved in the polishing interface, the normal load, the sliding velocity, the interfacial temperature, etc. From the temperature study presented in the previous chap ter it was learned that the coefficient of friction increases linearly with temperature. However, this result was deduced from experiments at a particular process paramete r set. To deduce a comprehensive inference on overall dependence of friction on temperature, further experimental analysis at several process conditions is necessary. This forms th e basis for the experimental investigation presented in this chapter. The following sub-se ctions, original contri bution of this author, which were recently published as a section of a chapter in a text book , present a brief background on the effect of various consum able characteristics on CMP tribology. 4.1.1 Effect of polishing pad ch aracteristics on CMP tribology The grooves or perforations on the polishing pads have a significant impact on the polishing mechanism and outcome [79, 80]. Gr ooves or perforations on the pad allow for effective slurry flow under the wafer surface and thus are very crucial for an effective CMP process. Phillipossian et al.  carried out fundamental tribol ogical studies during dielectric CMP on pads with different gr oove types at various slurry abrasive concentrations. The COF data was fitted as a function of Sommerfeld number and a
61 tribological mechanism indicator Â‘ Â’ , an index to describe the change in coefficient of friction, was estimated. PrestonÂ’s coeffi cient was calculated for each combination of pad type and slurry combination and was found to be correlating well with the coefficient of friction data. Stribeck curves were genera ted using the friction data for a variety of groove and pad types. From the shapes of individual curves, the authors deduced that some of the pads polished in partial lubrica tion regime, and some in boundary lubrication at lower Sommerfeld numbers a nd transitioned to partial lubr ication regimes. Consistent removal rates and uniformity were observed as long as the polishing regime is in boundary lubrication regime. Bu t polishing is aggressive in the boundary lubrication regime (where the particles abrade the wafe r surface and a solid contact exists between pad and wafer) which might induce delamina tion during CMP for next generation ICs where mechanically weak low K dielectrics are integrated with copper. Analyses of Stribeck curves, PrestonÂ’s coefficient, CO F, and tribological mechanism indicator correlating each other helps to understand the polishing mechanisms. Such an analysis not only helps in the process development but also provides useful feedback to the pad development manufacturers. 4.1.2 Effect of slurry characteristics on CMP tribology Abrasive particles in the slurry provide a majority of the mechanical component during CMP, whereas the slurry chemicals modify the exposed copper surface. Thus, both the slurry chemicals and the abrasive pa rticles play a major ro le in the abrasion process and the interface tribology. Li et al  have studied the effects of slurry surfactant, abrasive size, a nd abrasive content on the tr ibology and kinetics of copper
62 CMP. They measured the friction coefficien t at the interface dur ing copper CMP using different slurry samples with varying ch emical and abrasive characteristics, and generated Stribeck curves. From their results, it was concluded that the effect of slurry abrasive weight percentage had no effect on the tribological mechanis ms of polishing, but the slurry particle size was shown to have a significant effect. They also concluded that the presence of surfactant significantly lowers the coefficient of fricti on. They stated that the removal rate during copper CMP correlate d more with the variation of frictional forces (stick-slip) rather than the COF valu e itself. Investigations in the past also emphasized the effect of various additives of the slurry on the tribology during CMP [82, 83]. It was observed that the presence of a flocculent reduces the surface frictional force. Also, the ionic strength of the slurry has a significant impact on the frictional force. It can be seen from these works that the slurry ch aracteristics in terms of abrasive content, particle characteristics, and the presence of surf actant used to provide better dispersion of particles, and ionic strengths of the slurries have an effect on friction. This is where the present research proves beneficial as it studies the effect of ox idizer and slurry pH (slurry buffer) on coefficient of friction, which deal s with the electrochem ical interaction of copper-slurry system. 4.1.3 Effect of wafer contour ch aracteristics on CMP tribology Besides process consumables like polishing pad and slurry characteristics, the geometrical shape of the wafer being polishe d also has an effect on the tribological interaction. The wafer contour determines the area of contact between the wafer and pad along with the abrasives. Thus, the amount of surface asperity interact ion and the particle
63 wafer interaction depends also on the wafer cont our. The fluid film that is in contact with the wafer surface also is dependent on the cont our. Scarfo et al.,  conducted polishing tests at different process c onditions on different wafer samples with concave, convex and intermediate surface contours. It was seen th at the change (due to process conditions) in coefficient of friction changes with the shape of the wafer. It was shown in their results that the pressure experienced by the wafer at different applied pressures and velocities changes with the shape of the surface. Pressure changes have been noted the most for concave shaped wafers. Also, it was shown that the coefficient of friction changes were quite significant with change in normal pr essure for concave shaped and near flat surfaces. This explains the importance of the nature of the contact in the context of tribology and polishing mechanism. Keeping in view the above mentioned eff ects of various process consumables, an in-depth study of the effect of the nature of the copper-slurry interacti on and the effect of temperature on pad conditioning process on the interface tribology is performed as a part of this research work. The impact of a cha nge in pad conditioning temperature on the subsequent copper CMP performance in term s of both interface tribology and post CMP surface characteristics is also presented. Thus, this chapter includes a detailed investigation on the influence of both proce ss parameters and consumable characteristics on the CMP tribology.
644.2 Experimental methods and materials Copper blanket thin films were polished at different slurry temperatures to determine the dependence of coefficient of friction on slurry temperature. Polishing experiments were carried out on the CETR be nch top CMP tester. Th e contact interface is constituted of a 6Â” (15.25 cm) polishing pad coupon on a revolving platen and a 1Â” wafer coupon as the upper specimen. Cabot 5001 copper polishing slurry was continuously fed into the interface and the slurry temperature was varied from 18.3 oC to 30 oC, which simulates the increase in temperature at the interface. The slurry temperature was controlled for each experiment during the w hole project by monitoring and maintaining the temperature at a specific value within a 0.1 oC variation using a hot plate and temperature controller from Corning Inc. Table 4.1 Process parameters for the study of effect of temperature. Further, various types of slurry combin ations were employed to investigate the contributions of mechanical and chemical components of the CMP process on the coefficient of friction and surface interactions The weight concentration of the abrasive Parameter Value Polishing Pressure 2, 3, and 4 psi Platen velocity 100 and 250 RPM Slurry flow rate 75 ml/min Polishing Pad IC 1000 K grove polishing pad Slurry Temperatures 18.3 oC, 21.1 oC, 23.8 oC, 26.6 oC, 29.4 oC
65 particles, peroxide percentage in the slu rry, the slurry buffer (ammonium hydroxide in this case) present in the slurries was ch anged during the polishing experiments. The average size of the silica abrasive particle used in these slurries was 40 nm in diameter manufactured by Fuso Chemical Co., Ltd. Co efficient of friction data during polishing with various slurries was estimated at three different pressures keeping constant platen rotational speed. Coefficient of friction was determined in-situ by taking the ratios of the normal and the lateral loads monitored con tinuously during the polishing experiments. The detailed description of the polishing m achine and the signals monitored during the process is provided elsewhere [69, 85]. The deta ils of the process parameters used for the coefficient of friction experiments are given in Table 4.2 below. Table 4.2 Process parameter conditions for slurry chemical experiments The electrochemical cell for potentiodynami c polarization experiments, to study the effect of slurry buffer on the copper su rface, consisted of an Ag/AgCl/KCl saturated Parameter Value Peroxide concentrations 0, 2. 5 %, 5 % and 7.5 % by weight Component for slurry pH NH4OH Â– 5 % by weight 10 mM Acetic acid /Sodium Acetate Pressures 1, 2, and 3 psi Platen velocity 200 RPM Slurry flow rate 75 ml/min Pad IC 1000 K grove polishing pad
66 reference electrode with platinum strip as counter electrode and a diced strip of copper thin film coated silicon wafer as working elect rode. The back and sides of the wafer were isolated from electrical and chemical cont act by coating it with an insulating epoxy material. The electrodes were placed at least 50 mm apart from each other in the electrolyte. A PARSTAT 2263 model advanced electrochemical system manufactured by Princeton Applied Research was used for pol arizing experiments of the copper sample. Current density data was collected for an applied potential range of -0.3 Volts to 0.9 Volts scanned at a speed of 0.166 mV/sec. 4.3 Results and discussion 4.3.1 Effect of process parame ters and slurry temperature Coefficient of friction (COF) is defined as a ratio of shear to normal force (COF= N sF F; where Fs is the shear force and FN is the normal force). COF data collected during polishing at different temp eratures and different proce ss conditions is plotted in the figures 4.1 and 4.2. It can be noted from the figures that the coefficient of friction decreases with increase in both pressure and th e platen velocity. The observed decrease in COF with increase in pad velocity can be a ttributed to a thicker slurry film at the interface at higher velocities, thus resulting in effective lubrication at the interface. However, the decrease in COF with pressure is against the popular theory that the COF increases with increase in down force in bounda ry and partial lubrication regimes. The observed decrease in COF with increase in polis hing pressure might be due to the elastic deformation of the pad surface asperities, in which case the coefficient of friction varies
67 as load-1/3 or load-1/4 according to the adhesion theory of friction [33, 86]. This behavior of coefficient of friction with down force is consistent with previous studies conducted on the bench-top tester [87, 88]. Figure 4.1 Effect of pressure and veloci ty on coefficient of friction during copper CMP Also, according to the study by Scarfo et al , the contour of the wafer surface, if it is concave or flat, result s in a decrease of COF with increase in normal load. This was attributed to the suction of concave contours and development of positive pressures under the wafer at high pressures, resulting in decrease of COF. Th is suggests that the nature of area of contact during the polishing process might have influenced the friction characteristics to behave agai nst the conventional trend. COF 0.3 0.4 0.5 0.6 0.7 0.8 0.9 100 250 100 250 100250 2 34 TS / RPM within DF / psi
68 Another plausible explanation for the d ecrease in COF with increase in down force is due to the viscoelastic nature of the polyurethane pad mate rial. The second order normal reaction from the pad increases upon in crease in applied dow n force. Also, the increase in shear force increases the second order normal reaction . This increase in second order normal reaction adds up to the normal reaction (see equation 4.1) of applied force decreasing the coefficient of friction. SN FN shearF F F COF [4.1] Where, FFN is the first order elastic normal react ion (recovery) component of the normal force; FSN is the second order normal reaction; Fshear is the shear force According to the work published by Maria Ronay , the second order normal reaction increases with increase in both dow n force and shear force, thus making the denominator bigger than the numerator. This expl ains an overall decrease in the values of COF with the increase in the down pressure. Upon plotting the coefficient of friction with P*V (see figure 4.2), a decreasing trend of COF with increasing P*V was noted. This was consistent at different slurry temperatures. This unconventiona l trend is attributed purely to the nature of pad surface asperity deformation without any role of slurry chemistry or abrasive particle characteristics. This was confirmed when the exact behavior wa s observed (see figure 4.3) when a glass piece was polished using de-ionized water using the same polishing pad and process conditions at the same flow rate This decreasing COF with P*V suggests an inverse relation between coefficien t of friction and removal rate.
69 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0500010000150002000025000p*V (pa*m/s)COF 18.5 oC 24 oC 30 oC Figure 4.2 COF versus p*v during copper CMP using commercial copper slurry 0.150 0.250 0.350 0.450 0.550 0.650 0.750 05000100001500020000P*V (pa*m/s)COF glass DIW Figure 4.3 COF versus p*v during gla ss polishing using de-ionized water. The coefficient of friction data was plotted as a variability chart to explain the role of process parameters more effectively. Figure 4.4 shows the dependence of COF on
70 slurry temperature at different process conditio ns. From the figure it can be seen that the effect of temperature on COF was observed only under certain process conditions. Figure 4.4 suggests there exis t three regimes Â– namely scat tered, monotonic increase, and ineffective . The effect of temperature on COF seems to be scattered (not correlated) at low pressure regime, monotonic increase during medium pressure, and absent during high pressure regime. However, from the statis tical (ANOVA) analysis of the data, it was found that platen velocity ha d the major effect as compar ed to down force and slurry temperature. This proves that process paramete rs like pressure and pl aten velocity dictate the effect of temperature on th e coefficient of friction. Figure 4.4 Dependence of coefficient of fric tion with slurry temperature at different process conditions. The increase in coefficient of friction with temperature in the monotonic regime can be attributed to various factors. Firstly, the change in mechanical component (pad Scattere d Insensitive Monotonic Increase
71 COF 0.4 0.5 0.6 0.7 0.8 0.9 0 2.5 5 7.5 0 2.5 5 7.5 0 2.5 57.5 4329.23 8658.44712987.67 %H2O2 within PV property) being one of the factors due to an increase in tan (delta) of the pad with temperature . Secondly, copper oxidation reaction is known to be sensitive to temperature due to its lower ac tivation energy [44, 91], which suggests that another factor that influenced the COF could be the incr ease in surface oxidation and dissolution due to increase in temperature . This hypothesi s leads to further study of dependence of coefficient of friction with varying oxidizer concentrations and slurry chemistries. 4.3.2 Effect of oxidizer on CMP tribology Further experimentation was carried out to study the effect of changes in slurry chemistry on COF during copper CMP process. In this regard, the concentration of hydrogen peroxide in the slurry was change d and its effect on COF was studied. COF values obtained from these experiments are presented in figure 4.5. Figure 4.5 Effect of % con centration of hydrogen peroxide on coefficient of friction at different p*v values.
72 The results indicate that COF increases w ith increasing peroxide concentration in the slurry. The surface oxidati on of copper is least in the absence of hydrogen peroxide. It has been reported in the past that the su rface oxide layer increases as the peroxide concentration in the slurry increases [63, 92]. The surface oxide layer is not totally disrupt ed by the abrasive particles at higher concentrations of peroxide, and dynamic re passivation of copper layer occurs. Thus, a surface oxide layer is always present at higher concentrations as compared to the exposed metallic copper surface at lower concentrations The increase in the coefficient of friction could thus be attributed to such a change in the nature of surface layer at the interface. Another reason for the increase in coefficient of friction, which is related more to the consumable development than the process mechanism itself is the surface chemical decomposition of polyurethane material as re ported by Obeng et al . According to their study, it was concluded th at the polyurethane material was decomposed at higher concentrations of peroxide and the change in pad properties were not ed from DMA of the as received and slurry soaked pads. From the above discussion it is conclusive that chemical interaction at the copperslurry interface plays a significant role in the magnitude of coefficient of friction at the interface. This conclusion leads the way for a further detailed study of the chemical component on the coefficient of friction. The e ffect of slurry pH on coefficient of friction using acidic, basic and neutral slurries is st udied and the results are correlated with the electrochemical interaction of the copper with slurry at different pH conditions.
734.3.3 Effect of slurry chemistry on CMP tribology Polishing experiments to study the effect of the slurry component that maintains pH and its interaction with the wafer surface on the CO F were conducted. Cu CMP employing slurries with and without the slu rry buffer was carried out. Thermally grown oxide (SiO2) film was also polished using the sl urry containing ammonium hydroxide as slurry component formulated for copper CMP. From the results of these experiments (see figure 4.6), it was observed that the trend of coefficient of friction was independent of the pH of the slurry, indicating th at the majority of the contribution is from the mechanical component of polishing. However, the indi vidual magnitudes differed significantly with the change in chemical nature of the slur ry, including presence and absence of a buffer and oxidizer and the acidic or basic nature of the slurry. Figure 4.6 Effect of slurry chemistry on the coefficient of friction during CMP COF 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1 2 3 1 2 3 1 2 3 1 23 Psi 5 % peroxide 5 % peroxide None5 % peroxide Oxidant 10 mM acetate buffer 5 % Ammonium hydroxide None Slurry Buffer Copper Substrate
74 These results were correlated with the potentiodynamic polarization measurements, which show the surface electroc hemical interactions of copper in various chemical environments (see figure 4.7). Th e corrosion potential and current densities from the potentiodynamic polarization curves are presented in Table 4.3. From the table it could be seen that the corrosion potentia l becomes nobler when acidic or no buffer is used as compared to the ammonium hydroxi de buffer. The corrosion potential data correlates well with the coefficient of friction data. Also, from the polarization experiments it was observed that the corrosi on current density is higher for ammonia based slurry. This suggests that formation a nd dissolution of oxide la yer is significantly higher for the ammonia based slurry, where s the passivated oxide la yer does not dissolve into the slurry in acidic or no buffer systems. Figure 4.7 Potentiodynamic polarization scans for copper-slurry systems using various slurry chemistries. -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.00E-091.00E-081.00E-071.00E-061.00E-051.00E-041.00E-031.00E-021.00E-011.00E+00 log I (Amp/cm2)potential (volts) acetic acid buffer ammonia buffer no buffer-just peroxide
75 Table 4.3 Corrosion potential and current density from polzarization experiments. Slurry chemical Ecorr (mV) Estimated Icorr (mA/cm2) 5% NH4OH 120 1.5 No buffer (just 5% H2O2)261 1.50E-03 Acetic acid buffer 351 2.50E-03 Thus, it could be seen that the change in surface oxidation phenomenon plays an important role on surface tribology. To further confirm this, we studied the effect of oxidation of the wafer surface on the coeffici ent of friction. Copper and thermal oxide wafers were polished with copper oxidizing slurry. Thermal oxide wafer polished using copper chemistry exhibited a higher coeffici ent of friction as compared to the copper wafer (see figure 4.8). COF 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1 2 3 1 23 CopperSiO2 Psi within Substrate Figure 4.8 Effect of subs trate (polished with 5 % NH4OH + 5 % H2O2 + 5 mM BTA) on COF during CMP
76 This is attributed to the absence of a ny surface modified layer in the case of silicon dioxide film and the formation of a soft er oxide layer in the case of copper wafer. 4.3.4 Effect of temperature on pad conditioning process From the above results, it was seen th at the pad surface asperities are highly sensitive to temperature. In such a case, si nce pad conditioning process is concerned with the pad surface asperities, it is hypothesized that the change in conditioning temperature will have a pronounced effect on the conditioning process end point (effectiveness of pad conditioning) and at the same time will impact the pad wear rate (aggressiveness of pad conditioning). Before going further with th e experimentation, the following discussion original contribution of this author, recently published as a section in a chapter of a text book  will provide a necessary introducti on to the pad conditioning process and its significance to CMP process. 184.108.40.206 Pad conditioning process As the surface of a new (unconditioned) polishing pad is in general smooth and wets poorly, it does not provide good slurry transport to the pad/wafer interface. Pad conditioning is therefore necessary to open up the closed cells in the polyurethane pad to provide a consistent polishi ng surface throughout the padÂ’ s lifetime. Under-conditioned pads are prone to have a glazing effect on th eir surfaces, resulting in reduction of surface roughness. Such a reduction in surface roughness reduces the removal rate and increases non-uniformity. Over-conditioned pads result in excessive loss of pad material, which dramatically reduces the pad lifetime. Pad replacement b ecomes necessary due to two
77 reasons: changes in physical properties of the pad, and changes in grove dimensions because of pad wear. Replacing the pad pr ematurely would increase the cost of consumables and the machine down time, wh ich affect the throughput of CMP process and overall process operational costs and thus the productivity inde x of a manufacturing process. Hence, it is crucial to understa nd the governing factor s of an effective conditioning process. Pad conditioning has been proven to have a signif icant effect on the removal rate during CMP processes. Pad conditioning process can be optimized by monitoring the tribological aspects during the pr ocess. Coefficient of friction between the conditioner and the pad surface and the pad mate rial removal rate (w ear) are two critical parameters, which upon monitoring will provide sufficient information regarding the process. Many aspects such as pad conditioner performance, optimization of the conditioning process variables, etc., can be achieved with the help of tribometrology. An important aspect of pad conditioning which has not been studied is the role of temperature. As both conditioning and polishing processes are by nature abrasion processes, heat energy is dissipated at the interface, which elevates the temperature at interface. CMP pads are highly sensitive to th e temperature changes as pad is a polymer material whose properties change significantl y with temperature, and chemical kinetics during polishing are well known to be affected by temperatur e. Understanding the effect of temperature on pad conditioni ng process is possible by m onitoring the tribological aspects of the process. In the present study, the effect of pad conditioning temperature on the effectiveness and aggressiveness of conditioning process and on dishing and erosion during subsequent CMP process was invest igated. Long pad conditioning experiments
78 using water at different temp eratures were carried out a nd subsequent copper CMP was performed. 220.127.116.11 Pad conditioning experiments The second series of experiments with pad conditioning at different temperatures was carried out with subs equent copper polishing. The temperatures during conditioning were maintained constant (~ 0.5oC) by constant monitoring of the water temperature in the beaker and subsequent adding of cold or hot water whenever necessary. Patterned copper polishing was conducted soon after th e conditioning to study the effects of changes in the conditioning process on CMP pe rformance in regard to dishing and nonuniformity. During the short period of tim e between the conditioning and polishing process, the polishing pad surface was maintained at the conditioning temperature by continuous dispense of water at that particul ar temperature. The process parameters for the pad conditioning experiments are tabulated in Table 4.4. Table 4.4 Consumables and process pa rameters in polishing experiments Description Value Conditioning pressure and Pad RPM 4 psi and 150 RPM Polishing Pad IC 1000 perfor ated/ Suba IV sub pad Water Flow Rate during conditioning 200 ml/min Pad conditioning temperatures 10oC, 20oC, 24oC, 28oC Hard conditioner (Outer dia Â–4Â” and inner dia 3Â”) for Ex-situ condition Ring Conditioner by TBW industries Â– 200 grit size.
718.104.22.168 Results and discussion The pad surface temperature transients associated with conditioning experiments are presented in Figure 4.9. From the figure, different levels of conditioning temperature can be noted. Coefficient of friction between conditioner and pad surface was continuously measured during the pad conditioning experiments. 0100200300400500600 Time sec 0 5 10 15 20 25 30 35 40 45 T1 C Figure 4.9 Pad surface temperature during conditioning at different temperatures. The mean values of the coefficient of fr iction, along with standard deviation, are plotted against the pad conditioning temperat ure in Figure 4.10. It can be seen that friction between the pad and conditioner incr eased with increase in temperature during conditioning. The observed increase in pad co efficient of friction might be due to increase in surface contact ar ea between the conditioner and the pad. This increase in contact area results in reduction of overall load experienced by the pad. It is the nature of 38oC conditioning 24oC conditioning 1 0 o C co n d iti o nin g 20oC conditioning Pad Surface Tem p erature
80 polymer materials that the coefficient of fric tion when in contact with a hard inelastic material increases with reduction in applied load. This justifies the increase in coefficient of friction with increase in temperature. 0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29 0.31 0.33 0510152025303540Pad conditioning temperaturePad-conditioner COF Figure 4.10 Pad coefficient of friction m easured in-situ during pad conditioning at different temperatures The pad-conditioner coefficient of friction curves plotted versus time obtained during conditioning are presented in Figure 4.11. The coefficient of friction stabilized faster during conditioning at lower temperatures than at higher temperatures. The pad wear and resulting exposure fresh pad surface during conditioning was easy at lower temperatures. This is the reason for faster stabilization of COF at lower temperatures. At elevated temperatures, the pad surface asperities become soft and get elastically deformed. As the pad surface asperities do not undergo plasti cally deformation (generation of rough
81 surface) easily at higher temperature it ta kes a longer time to stabilize the COF and completion of the conditioning process. Figure 4.11 Coefficient of friction curv es during pad conditioning at different temperatures This observation can be mainly attributed to typical sensitivity of viscoelsatic pad material towards temperature change. The stabil ization of the coeffici ent of friction is a measure of the end of the conditioning pr ocess . Thus, longer conditioning was required for full pad conditioning at higher temperatures. 0200400600 Time sec 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 COF 38 o C 10 o C 20 o C 24 o C
82 0100200300400500600700 Time sec 0 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 Z mm Loss of pad material during conditioning is another aspect which affects the pad lifetime and also polishing performance. As th e pad loss increases along with a variation in polishing performance pad lifetime decreas es, and so, pads need to be replaced more often. This increases the cost of consumables and the machine down time, which in turn negatively affects the throughput of CMP proces s and overall process operational costs. Figure 4.12 Carriage positi on during conditioning test s plotted versus time Hence, it is crucial to monitor the pad loss during conditioning process. The realtime change in pad thickness and the pad cut rate during conditioning at different temperatures are presented in figures 4.12 and 4.13. The pad loss was high at lowest temperature and decreased thereafter with in crease in water temperature. At elevated 10oC conditioning 20oC conditioning 24oC conditioning 38oC conditioning Carriage position during pad conditioning
83 temperature the pad cut rate was lower due to lower actual conditioning pressure resulting from higher surface contact area  Also, the pad surface asperities deform elastically when they are soft at elevated te mperatures. This preven ts active abrasion and surface roughening process. This effect of soft er pad surface asperiti es is indisputably evident from figure 4.12. The loss in pad thickness at 38oC was almost negligible compared to pad loss for conditioning process at 10oC. These observations indicate an aggressive conditioning process at lower te mperatures, as compared to the elevated temperatures. 0 0.2 0.4 0.6 0.8 1 1.2 010203040 pad conditioning temperature (oC)pad wear (microns/min) Figure 4.13 Pad wear rate versus temperature during conditioning. To study the effect of the change in pad conditioning on CMP performance, patterned copper samples were subsequently polished. Th e process conditions for the
84 CMP experiments are tabulated in Table 4.5. Post CMP samples were accordingly analyzed for their dishing characteristics. Table 4.5. Process parameter set for patterned CMP experiments. Description Value Wafer coupons 2Â” 10,000 copper over patterned low-K dielectric Slurry I-cue 5001 copper slurry Polishing Pressure 4 psi Polishing Velocity wafer carrier Â– 95 RPM; polishing pad Â– 100 RPM The removal rate and coefficient of friction from these tests are plotted versus pad conditioning temperature as shown in figure 4. 14. The coefficient of friction and removal rate values were higher when the pad was conditioned at temperatures below the room temperature. The removal rate and coefficient of friction values were the lowest at the room temperature and they rose as the te mperature was elevated above the ambient temperature. The higher removal rate and coe fficient of friction at lower temperature is attributed to the hardening of polishing pad surface asperities due to lower temperature. The pad surface asperities become hard a nd also they are less flexible at low temperatures due to the dependence of phys ical properties of pol ymer material on temperature. This will result in enhanced ab rasion of thin film surface due to abrasive particles.
85 0 50 100 150 200 250 300 350 400 450 500 01020304050 Pad conditioning temperatureRemoval Rate (nm/mi n 0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29 0.31 0.33 0.35Coefficient of frictio Removal Rate cu polish COF Figure 4.14 Removal rate and coefficient of friction measured during copper polishing after pad conditioning at different temperatures Thus, pad-abrasive-wafer in teraction at lower temperature is more effective, which dominates over the chemical reaction ra te, which is expected to be slow at low temperature [45, 84]. On the ot her hand, at higher temperature the reactionkinetics of the slurry chemicals is high in accordance with Arrhenius relation  and hence a high polishing rate was observed. Even though the mechanism at higher temperatures is predominantly chemical kinetics dependent, there could also be a minor effect of increased area of contact due to softeni ng of the pad surface asperities at higher
86 temperatures . This increase in contact ar ea results in increased shear force adding to increase in removal rate. The dishing values on these samples were measured and found to be increasing with increase in pad temperature as shown in figure 4.15. It can also be noticed that the deviation from mean value decreased with in crease in pad temperature. The standard deviation in figure 4.15 signifies the variation of dishing dept hs at different locations on the sample. 0 50 100 150 200 250 300 01020304050Conditioning temperature oCDishing depth (nm) Figure 4.15 Dishing depth measured at 50 m features for samples polished after pad conditioning at differe nt temperatures
87 This indicates that the level of non-unifo rmity in the polishing process decreased considerably for the wafers polished after c onditioning the pad at elevated temperatures. The fact that the absolute values of dishi ng increased with increase in pad temperature even with improved polishing uniformity, suppor ts the earlier result that greater amount of dishing occurs when the increase in pad temperature is high. 4.4 Summary Dependence of coefficient of friction on sl urry temperature and slurry chemistry during copper CMP has been studied. From th e results it was found that coefficient of friction during copper CMP decr eases with increase in load and platen speed counter intuitive to the popular theory. This trend also indicates that the coeffi cient of friction and removal rate are inversely related. The effect of slurry temperature on coefficient of friction is observed only under certain speci fic process conditions. Kinematic parameters during CMP dictate the effect of temperature on surface tribology. Also from the results it was found that the chemical nature of the exposed surface layer affects the frictional properties during CMP. Friction at the interface increases w ith increase in peroxide content in the slurry. This result suggests that the increase in surface oxidation results in higher coefficient of friction. This could be one of the underlying reasons for an increase in coefficient of friction seen with an increas e in slurry temperature. The slurry attack on the polishing pad surface could also be a reason for the change in tribology during CMP. The surface tribology is found to be influenced by the surface oxidation but not by the pH of the slurry medium. In summary, the co efficient of friction during coppe r CMP is
88 affected by slurry chemistry along with the mechanical component such as polishing pad and abrasive particle concentration. During ex-situ pad conditioning, friction reached steady-state faster at lower temperatures comparing to the elevated te mperatures, thus, full-conditioning at higher temperatures was longer. Highe r removal rates and coeffici ent of friction between pad and wafer surface were noted at both very low and very high temperatures of conditioning. Post-CMP dishing increased wi th increase in the pad temperature. Pad temperatures during both condi tioning and polishing play a ma jor role in the generation of wafer defects like dishing and erosion during copper CMP process.
89 CHAPTER 5: THERMAL MODEL-STEA DY STATE HEAT CONDUCTION 5.1 Introduction Chemical Mechanical Plan arization (CMP) process ne eds to be optimized not only in regard to removal rate pad conditioning a nd coefficient of fr iction but also in several other aspects such as defects during CMP as seen in the pad conditioning results of chapter 4. Process induced defects mainly related to pattern on the wafer surface such as dishing, erosion and metal loss need to be reduced in order to get good yields, which result in lower operational costs. The above mentioned defects are caused largely due to non-uniform pressure distributio n and inconsistent material removal rates. Improving wafer-scale and die scale uniformity in polishi ng would reduce several defects concerned with wafer pattern dens ities and feature size. The thermal aspect of CMP, even though a significant factor affecting the process output, has not been researched as extensively as parameters like pressure, velocity, slurry flow rate, and other chemical aspects. The effect of temperature or heat dissi pation at the interface could af fect the polishing pad surface and the contact area during CMP, which th ereby could induce polishing non-uniformities as observed in the pad conditioning studies presented in the previous chapter. Thus, studying the heat transfer mechanism at th e interface and the su bsequent rise of temperature on wafer surface is hypothesized to provide valuable insight into improving with-in wafer non-uniformity to a great extent. Research work on temperature rise on the surface of polishing pad during interlayer dielectric and metal polishing, re moval rate dependence on temperature and its
90 modeling, effect of slurry flow rate on pad temperature rise etc., has been carried out in the recent past to understand the role of te mperature at interface on CMP performance. However, in all these research works, the re ported temperature rise is either the average temperature on the pad surface or a predicted average temperature on the wafer surface or temperature rise at three isolated locations on the wafer. These works report the overall temperature rise but do not pr ovide the information about th e temperature distribution on the wafer surface. An analysis of the temperature distribution on the wafer surface after the polishing process reaches a steady stat e has not been researched till date. The temperature profile on the wafer surface as a f unction of radius and thickness will provide valuable insight into the extent of temperature rise at different locations on the wafer. Since the material removal rate during copper CMP is so sensitive to temperature, the temperature distribution over the entire wafer will significantly affect the uniformity of material removal over the entire wafer. Understanding the temperature profile will decrease the with-in-wafer non uniformity and thus improves yield by minimizing the number of faulty dies. In this research, we model the conduction heat transfer mechan ism at the interface after a steady state has been achieved by the po lishing process. By solving the model, we present the temperature profile on the wafer surface as a function of wafer radius and thickness. The analytical modeling effort is supported with the finite element analysis using FIDAP package which can handle both conduction and convection heat transfer environments effectively. The coefficient of friction values at di fferent pressures and velocities required to calculat e the heat dissipation at the interface are obtained from copper polishing experiments conducted on a bench top tester.
915.2 Thermal model development In this section, the thermal response of the wafer to a uniform heat flux at one boundary is examined as a non-homogenous st eady state heat c onduction problem. An exact analysis is performed using the method of separation of variab les in a cylindrical coordinate system. The transformation of the heat conduction problem with nonhomogeneous boundary conditions into one with homogeneous boundary conditions is essential for the analytical investigation. Obtaining an expression for the steady stat e temperature distribution T(r, z) along the wafer surface undergoing polish and temperat ure distribution along its thickness is the most critical part of this research. With a substitution ofatmT z r T z r ) ( ) ( Sk q q S 1 1k h H and S 2 2k h H the equation describing the conservation of energy inside the solid can be written as : 0 z z r r z r r 1 r z r2 2 2 2 in 0 r rd, 0 z b (1) The following are the boundary conditions. q H z z r 0 z At1 (2) 0 z z r b z At (3) 0 H r z r, r r At2 d (4) Where Tatm = atmospheric temperature (K); q = heat flux (W/m2); rd = radius of the wafer (m);b = thickness of the wafer (m); h1,2 = convection heat transf er coefficient at the
92 interface and at the wafer edge (W/m2K); r, z =coordinate directions; ks = thermal conductivity of the solid. (W/mK) The above problem was solved analytically with the help of the principle of separation of variables, which involved Be ssel functions. It was necessary to solve Eigenvalues mÂ’s or all positive roots of the following equation: 0 ) r ( J H ) r ( Jd m 0 2 d m 0 m where J0 is the Bessel function of zeroth order (5) The final solution for the temperature distri bution was derived with initial substitution dr r J r b b b z b r r q T z rdr r m m m m m m m m m m d 0 0 7 0 1 2 2 2 2 0 0 2 2, sinh cosh H H J sinh J 2 ) ( T (6) where mÂ’s the eigen values; for m = 0 to 7 are = [162.82, 380.20, 609.03, 844.39, 1083.86, 1325.76, 1813.57, 2058.62] The experiments necessary for the coeffici ent of friction values and also for the validation of the model predictions were car ried out on the bench top CMP tester. The tester holds a 6 inch polishing pad and a 2 inch silicon wa fer. Real time coefficient of friction was measured using a dual force se nsor which measures lateral and normal forces. Features of the bench top tester are pr ovided in detail in previous publication [69, 85]. Before actual polishing of copper wafe rs, the Rodel IC1000 polyurethane perforated pad was conditioned for 20 minutes and a couple of dummy samples were polished to bring the pad surface condition to a steady state and thereaf ter the pad was conditioned for 5 minutes in between each polishing ex periment. Pad conditioning was carried out using DI water at 1 psi and 100 rpm pad rotation keeping the c onditioner stationary.
93 99.99 % pure copper disk of 2 inches in diam eter from Sigma-Aldrich chemical company was then polished for 3 minutes using Icue 5001TM copper slurry from Cabot Microelectronics Corporation. The slurry fl ow rate was kept at 75 ml/min and the polishing parameters were set at 2 Â– 4 psi and 100 Â– 200 RPM pad rotation, which translates to 0.314 m/s to 0.628 m/s linear velocity. The convective heat transfer coefficien t necessary to compute the numerical values for the temperature was obtained from a finite element analysis. Finite element analysis was done using FIDAP package by Mr Jorge Lallave under the guidance of Dr. Rahman, Associate Professor in Mechanic al Engineering department. Four node quadrilateral elements were used in construc ting a finite element model of the polishing interface in presence of a liquid medium. In each element, the velocity, pressure, and temperature fields were appr oximated which led to a set of equations that defined the continuum. Due to a non-linear nature of the g overning transport equations, the NewtonRaphson procedure was used to arrive at th e solution for the velocity and temperature fields. The solution was considered converged when the sum of the residuals in all the degrees of freedom was less than a predefine tolerance value; in this case, 1E-05. Once the convective heat transfer coefficients are obtained, the conduc tion heat transfer problem is modeled in the finite element an alysis and the temper ature distribution is obtained. The laminar slurry flow during th e CMP process is controlled by three major physical parameters: the volumetric flow rate of the slurry (Q = 30 to 100 mL/min), the spinning rate of the wafer disk in conjuncti on with the top insulated plate at a uniform angular velocity ( ) of 10.47 to 31.41 radians/sec or 100 to 300 rpm and machine
94 pressure of 0.5 to 3 psi. An absolute pressure that controls the heat flux is defined as q = frictPV. It ranged from 455 to 10,600 W/m2. The wafer was modeled to have a diameter 2.54 cm and its thickness was kept at a value of 0.75 mm. The wafer material used in the present study was predominantly silicon with a thin layer of cooper. The solid material property was acquired from Bejan  and was assumed to remain uniform and isotropic for the temperature range encount ered in the investigation. 5.3 Temperature contours on wafer surface Figure 1 shows the variation of the interface temperature for different slurry flow rates under a rotational rate of 100 rpm. The surface temperature has a maximum value at the center and decreases towards the edge. As the copper removal rate is greatly affected by the temperature, this could be one of the main reasons for non-uniformity during CMP process. Figure 5.1 Local polishing interface temperat ure for a silicon wafer at different flow rates of alumina as the slurry (b=0.075mm, =100 rpm, q=455W/m2) 294 296 298 300 302 304 306 308 310 312 314 316 00.250.50.7522.214.171.12422252.52.75Dimensionless Radial Distance, r/dnPolishing Interface Temperature, K Q=30mL/min Q=45mL/min Q=65mL/min Q=80mL/min Q=100mL/min
95 The plots in figure 5.1 reveal that interface temperature d ecreases with slurry flow rate. Figure 5.1 confirms to us how an increasing slurry flow rate contributes to a more effective cooling. Figure 5.2 Polishing interface temperature for a silicon wafer at different coefficient of friction (q = 455 to 593W/m2, b = 0.075mm, = 100 rpm) From the solution of the analytical model, surface temperature curves were obtained for different coefficien t of friction values. From figu re 5.2 it can be noted that the coefficient of friction has a significant effect on the temperature increase on the wafer surface. The increase in friction resulted in hi gher temperature on the wafer surface. This is attributed to generation of higher amount of heat at the interface due to higher friction. Effect of wafer rotation on temperature at the polishing interface is illustrated as a function of dimensionless radial distance in figure 5.3. It can be noted from figure 5.3 that the effect of wafer rotational sp eed is statistically insignificant. 300 302 304 306 308 310 312 314 316 318 00.250.50.75126.96.36.199188.8.131.52Dimensionless Radial Distance, r/dnPolishing Interface Temperature, K COF=0.36 COF=0.415 COF=0.47
96 Figure 5.3 Local polishing interface temper ature for a silicon wafer at different ( ) spinning rates and alumina as the slurry (Q=65 mL/min, b= 0.075mm, q =1364W/m2) The temperature across the thickness of the wa fer is also estimated at various radial locations. The cross sectional temperature dist ributions within the wafer are plotted as shown in figure 5. 4. It can be seen from the figure that the temperature distribution inside the wafer decreases li nearly as we move towards the non polishing side of the wafer. As the wafer consisted of predominantly single material, the temperature distribution is completely based on conducti on heat transfer mechanism which changes linearly within the solid. From these temp erature contours on the wafer surface as a function of thickness and radius, it was conclu ded that apart from process parameters that gave different coefficient of fr iction values, slurry flow rate had a major influence on the wafer surface temperature. This can be attribut ed to the ability of the slurry film to transfer heat from the interface using convection heat transfer mechanism. 295 300 305 310 315 320 325 330 335 340 00.250.50.75184.108.40.206220.127.116.11 Dimensionles Radial Distance, r/dnPolishing Interface Temperature,K =300 rpm =200 rpm =100 rpm
97 Figure 5.4 Cross sectional temperature w ithin the wafer during CMP at a rotation speed of =200 rpm (Q = 65 mL/min, b = 0.075 mm, q=1049W/m2) This led the way to study the effect of slurry flow rate on CMP in terms of interfacial temperature rise interface tribology and post CMP surface characteristics. 5.4 Slurry flow rate experiments Slurry acts as a lubricant and coolant at the interface during polishing. From the results obtained by solving the thermal model, it was seen that the increase in slurry flow decreases the temperature rise. The lubricant film separates the sliding surfaces and thus reduces friction between them resulting in le sser amount of heat dissipation and hence low temperature rise. Even though the slurry acts as a lubricant medium during CMP, besides modifying the wafer surface it even dissolves or etches the formed surface compounds in several cases and the abrasives pr esent in the slurry abrade the chemically modified surface. Thus, the effect of slu rry on the tribology is beyond the simple 292 294 296 298 300 302 304 306 308 310 312 314 316 318 00.01250.0250.03750.050.06250.075 Wafer thickness distance, z(cm) Cross sectional temperature within the wafer, K T(0,z) T(0.005m,z) T(0.006m,z) T(0.0075m,z) T(0.010m,z) T(0.012m,z) T(0.0127m,z) Tatm
98 mechanism of lubrication o ffered by a fluid film. Lu et al  investigated the effect of normal force and pad velocity on the slurry film thickness at the interface and correlated it to the friction coefficient data. Their data i ndicated that the thickness of the slurry film at the interface decreases with increase in the applied normal force and decrease in pad velocity. The slurry film thickness data was correlated to the measured coefficient of friction data collected during their experiment s. According to their findings, coefficient of friction increased with an increase in force and decrease in pad speed. An inverse relation was thus observed between the slur ry film thickness and the coefficient of friction, emphasizing the role of slurry as an effective l ubricating medium. As per the study by Runnels et al , it was concluded that the pressure applie d during CMP is partially supported by the slurry film at the interface. Thus we understa nd that slurry film thickness and slurry flow patte rn significantly impact the fri ctional characteristics at the interface, which needs to be studied in much more detail to fully understand the mechanism during copper CMP. Along with the effect of slu rry flow on friction characte ristics, dependence of post CMP surface characteristics on slurry flow is an aspect of interest to this study. Due to the pattern density variations across the wafer, there is a difference in individual removal rates and step height reductions of patterns mainly depending on the density and width of the pattern lines. Due to this difference in re moval rate, global plan arization faces issues such as non-uniformity across the wafer of wi thin a single die subsequently leading to defects such as dishing and er osion of the interconnect materials. Dishing is the loss of the copper from the copper lines resulting in a deviation from the desired flatness of the metallization layer [7, 8]. Erosion is the loss of dielectric material due to its removal
99 during over polishing step (practiced in orde r to remove even the final trace of copper between the metal lines). There are many fact ors that influence the generation of these anomalies on the surface. Some of such factors are width of the lines, pattern density, down force, and physical properties of the polishing pads. Even though models and investigations on dishing have been done in the past [27, 5052] to investigate the effect of down force, slurry chemistr y and pattern dependencies, very little effort has been put into studying in depth the sources of generati on of dishing and eros ion. From the above mentioned past investigations, down-force and pattern line width have a substantial influence on dishing depth. This indicates a pr edominant role of the mechanical aspect of polishing on the generation of dishing. Since, the slurry film at the interface is found to significantly influence the mechanical intensity of abrasion during CMP , slurry flow rate is hypothesized to have a direct impact on the dishin g and erosion characteristics. Also, the slurry being delivered at the inte rface of the pad and wafer contact, takes away major part of the heat from the interface thr ough convective heat tran sfer [41, 67]. Thus, slurry flow affects heat transf er at the interface, which in turn influences the physical properties of the polishing pad. As discussed earlier, the polishing pad softens due to the temperature increase, and so the area of c ontact at the interface increases, which would again impact the post CMP surface planarity. U nderstanding the effects of slurry flow rate and related pad surface temperature dur ing copper CMP process on the generation of dishing and erosion thus prove s highly beneficial to optimize the CMP process and to minimize defects.
1005.4.1 Materials and techniques The polishing experiments were c onducted on an upgrade bench-top CMP machine model CP-4, manufactured by CETR Inc. (refer figure 5.5). This polisher provides a fully controlled CMP process, wh ich imitates closely any large-wafer fab production processes. The polisher can accommoda te 2Â” to 4Â” wafers, the platen can hold up to a 9Â” pad, which imitates the CM P process more closely and with greater control than the prev ious version explained in Chapter 3. Figure 5.5 Bench top CMP tester mod. CP-4 Post-CMP surface characterization was carried out using both KLA-Tencor surface profiler and Pacific Nanotechnology (PN I) atomic force microscope integrated
101 with the Universal Nano+Micro Tester ( UNMT-1), manufactured by CETR Inc. The instrument has a Â“move-n-scanÂ” procedure, which facilitates imaging at several locations on the wafer in a single operation. The UNMT-1 with AFM head is used for large-area automated multi-scanning and facilitates imaging of multiple locations on wafers up to 8Â” in diameter. Its large sample stage can rotate with a sub-micron angular positioning resolution, while the AFM head on the latera l slider has a long translational motion to provide precision positioning on various wafer ra dii. The co-ordinates of the features on the wafers were pre-determined to be used as inputs to the Â“move-n -scanÂ” procedure. The AFM was operated in the contact mode imagi ng, scanning in the direction perpendicular to the direction of cantilever holding the tip. Scan size was set at 80 m, frequency of the cantilever was set at 1 Hz. The post proces sing on the images is performed in the Nanorule software. Upon processing, a line an alysis was performed and an average of 10 dishing depth measurements on every im age was taken to obtain statistically meaningful data. The measurements of me tal loss and erosion were performed on the KLA-Tencor surface profiler, as imaging of the total pattern width of 1500 m is not possible on AFM due to scan size limits. Similar to the AFM measurements, 10 measurements at each feature were taken on the profiler. 5.4.2 Experimental procedures and samples In the present research, the effects of slu rry flow rate were studied in the first series of experiments, the effects of pad temperature and conditioning on the generation of wafer defects during polishing were studied in the second series of experiments. The consumables and the polishing parameters employed for the experimentation are presented in Table 5.1.
102 Table 5.1 Consumables and process parame ters in slurry flow rate experiments Description Value Wafer coupons 2Â” 10,000 copper over patterned low-K dielectric Polishing Pad IC 1000 perfor ated/ Suba IV sub pad Slurry I-cue 5001 copper slurry Oxidizer 30% hydrogen peroxide Slurry Flow rates 20, 30, 45, 55 and 75 ml/min Polishing Pressure 4 psi Velocity Wafer carrier Â– 95 RPM; Polishing pad Â– 100 RPM In-situ conditioning pressure 4 psi Water Flow Rate during conditioning 200 ml/min Soft conditioner ( 4Â” diameter disk) for in-situ condition Disk conditioner by 3MTM Â– 400 grit size. The chosen slurry flow ra tes scale up to the range of 100 ml/min Â– 375 ml/min on an 8Â” wafer polishing system. The polishing experiments constituted of preliminary tests for removal rate determination and main tests for dishing and erosion characteristics. The preliminary tests for removal rate were conduc ted with varying slurry flow rate. The endpoint of copper planarization was determined from the coefficient of friction changes during polishing, which shows a characteristic transition at the time of copper removal as the underlying barrier layer is exposed (see fi gure 5.6). Thus, removal times and removal
103 rates were estimated. The main experiments for dishing and erosion characteristics were then carried out at varying slurry flow rate with a 20% over-polish time to fully expose the underlying barrier layer. 020406080100120140160 Time sec 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 COF Figure 5.6 Coefficient of friction realtime graphs for three slurry flow rates Patterned 2Â” wafers with 10 k electroplated copper layers and an MIT 854 pattern were used for our experiments . Isol ated lines of 50 m wide at two locations from a single die were chosen for dishi ng depth measurements. Two wide metal line patterns, 50 m line width with 98% density and 100 m line width with 99% density, were chosen from each die for metal loss da ta. Two thin metal line patterns, 10 m line with 50% density and 1.5 m line with 67 % density were chosen from each die for erosion data. Thus, 8 locations per wafer were available for dishing, erosion and metal loss measurements. 30 ml/min 75 ml/min 45 ml/min
1045.4.3 Results and discussion for flow rate experiments Coefficient of friction at the interface is a function of various factors such as surface characteristics, nature of interacting materials, kinematic aspects of polishing etc. It also determines the regime of lubrica tion at the interface  The coefficient of friction also gives a measure of polishing inte nsity at the interface wh ich would result in heat dissipation (generation of thermal ener gy from mechanical interaction). Thus the measure of the coefficient of friction at th e interface during CMP gives vital information about the polishing and remova l mechanism. The coefficient of friction between the pad and wafer surface during polishing and removal ra te data from the first series of slurry flow rate experiments are presented in figure 5.7. 0.45 0.47 0.49 0.51 0.53 0.55 020406080Slurry flow rate (ml/min)Coefficient of friction500 550 600 650 700 750 800 850Removal Rate (nm/min) COF Removal rate Figure 5.7 Coefficient of friction and coppe r removal rate versus slurry flow rate The numerical values in figure 5.7 are aver ages of data collected from four samples polished at each slurry flow rate. Y-axis erro r bars represent the sta ndard deviation of the data about the average values.
105 100 150 200 250 300 350 400 450 01020304050607080 Slurry flow rate (ml/min)Dishing depth (nm) 50 micron location 2 50 micron location 1 20 30 40 50 60 70 80 90 100 01020304050607080 slurry flow rate (ml/min)Erosion (nm) cu10-50% density cu1.5-67% density 150 170 190 210 230 250 270 290 310 330 01020304050607080 Slurry Flow Rate (ml/min)Metal Loss (nm) cu50:98% density cu100:99% density Figure 5.8 Effects of slurry flow rate on (a) dishing, (b) erosion and (c) metal loss a b c
106 From the figure 5.7 it can be seen that coefficient of friction decreased and removal rate increased with an increase in slurry flow rate. As the higher flow rate decreases the temperature at the interface, the observed decrease in coefficient of friction is in agreement with the results from other i nvestigations [40, 84]. The observed trend of the removal rate can be attributed to the inadequate chemical component during polishing at lower slurry flow rates. During CMP process, slurry should act as both a surface oxidant and dissolver of the fragments of copper detached from the wafer surface. If the amount of slurry available on the pad is not sufficient to carry out these chemical activities, the removal rates decrease. The dishing, metal loss and erosion data, plotted versus slurry flow rate, are presented in figur e 5.8. Polishing was repeated 4 times at each condition to confirm data reproduc ibility. From figure 5.8 it can be seen that all the three levels of dishing, erosion and metal loss decr eased with an increase in slurry flow rate. High amounts of dishing, erosion and metal loss at lower slurry flow rates may be due to a combination of two reasons: a) rela tively high temperatures at the interface at low slurry flow rates, causing local softening of the pad. Resulting softer asperities of the pad reach deeper into the trenches compared to the stiffer pad asperities, resulting in increase of dishing, erosion and metal loss; b) the second reason might be due to increased chemical activity be tween the copper and the slurry at elevated temperatures. Figure 5.9 shows AFM images of dishing profil es of 50 m features at different slurry flow rates.
107 Figure 5.9 AFM images of dishing profiles of a 50 m wafer feature at a) 20 ml/min b) 30 ml/min c) 45 ml/min d) 55 ml/min and e) 75 ml/min slurry flow rates. a b c d e
108 The rise in the pad surfa ce temperature during polishing experiments at different slurry flow rates is shown in figure 5.10. Fr om the figure 5.10 it coul d be seen that the amount of rise in pad surface temperature decr eased with increase in slurry flow rate. Figure 5.10 Measured temperature rise on the pad surface during copper polish at different slurry flow rates 5.5 Summary The steady state conduction heat transfer model is developed and is solved analytically to obtain the surf ace temperature on the wafer as a function of its radius and thickness. The polishing interface temperatur e and local heat tran sfer coefficient are significantly affected by slurry flow rate a nd tribology at the interf ace. Increase in the slurry flow rate and decrease in friction coefficient results in lower wafer surface temperature. The surface temperature is highe st at the center and decreases towards the edge. This steady state surface temperature pr ofile could be one of the reasons for nonuniformity within wafer during CMP process. The temperature across the cross section of 050100 Time sec 0 1 2 3 4 5 6 T1 C 10 ml/min 55 ml/min 75 ml/min 45 ml/min 30 ml/min Tem p erature rise on p ad
109 the wafer decreases linearly reaching room te mperature on the back side of the wafer. The effects of slurry flow rate, pad temperature and conditioning temperature on the copper CMP performance have been studied. During copper CMP process, higher slurry flow rates resulted in decreased levels of fr iction, dishing, erosion and metal loss, while increased copper removal rate.
110 CHAPTER 6: NOVEL SLURRY DEVELO PMENT TO REDUCE CMP DEFECTS 6.1 Introduction Besides global planarization and high po lish rate, the CMP process should also achieve high material selectiv ity (high polishing rate of on e material compared to the other), high-quality surface finish, which is de void of scratches, patte rn related defects, pits, delamination and particle contamina tion [99-104]. Oxide CMP is conducted during shallow trench isolation (STI) in logic devi ce fabrication and also in many other novel applications. Achieving a supe rior surface quality includes fe wer scratches, with minimal oxide dishing and nitride erosion, particular ly in case of STI CMP. CMP defects can be due to contamination issues from slurry chem icals, particle contamination (residue) from abrasive, scratches during polishing due to agglomerated abrasive particles, pattern related defects like dishing and erosion, delamination an d dielectric crushing due to mechanical damage of dielectrics . Su ch defects during CMP hamper the device yield and reducing the defects is thus highly important. Thes e defects result in nullifying the advantages of using CMP as a global planarization technique. The quality of the post CMP wafer su rface is significantly dependent on the characteristics of the abrasive particles pr esent in the slurry. The polishing process involves active abrasion of th e wafer surfaces using abrasive particles present in the slurry. Such an abrasion results in generating surface scratches on the wafer being polished. The generation of surface scratches de pends on a wide variety of factors such as
111 the process conditions, characteristics of the abra sive particles, their content in the slurry, hardness of the pad, chemistry of the slurry etc. Of particular interest in the present research are the characteristics of the abrasi ve particles. Abrasive particles at times agglomerate in the slurry and the effective si ze of the particles can be much higher than the specification of the slurry Such agglomerated particles cause deep scratches in the surface and result defects that cannot be removed by any other post processing techniques. Commonly used cera mic abrasive particles are much harder than the low dielectric constant materials and copper. These particles can eas ily scratch the surface and if agglomerated can result in permanent sc ratch defects. Thus, the inherent nature of the particle plays a significant role. These surface scratches in turn result in formation of puddles in further layers of metallization causi ng an electrical shor t circuit . Also, the abrasive particles that re sult in low friction at the in terface are beneficial to the process as lower friction helps reduce surf ace damage during CMP . Another aspect of polishing related to slurry abrasives that n eeds to be countered in order to improve the yield and effectiveness of polish is particle residue on the wafer surface after CMP . Researchers in the recent past have studied mixed or modified abrasive particles in order to reduce defects during CMP (107-11 1). These studies mostly use abrasives of different inorganic oxides and of different sizes or use micelles etc. Minimal success has been achieved in reducing both surface scratches and particle residue at the same time as the inherent material characteri stics of the abrasive particle that meets the wafer surface is still hard and has the same surface properties.
112 The focus of this research is to reduce surface defects during polishing by developing slurries using nove l inorganic-organic composite particles. These particles consist of polymer, surface of which is mos tly modified using silica functional groups and ceria nanoparticles. Presence of ceria nanoparticles is pr oven to be highly beneficial for oxide CMP, both in terms of removal rate and selectivity . The developed composite particles are inherently soft due to the presence of polymer. These particles exhibit controllable surface hardness and ch emical nature and hence are hypothesized to prevent aggressive scratching, leave partic le residue, or apply high mechanical stress during polishing. The incorpor ation of functional groups ont o polymer latex surfaces to form new hybrid materials represents an em erging discipline for the synthesis of novel materials with diverse architectures. Pa rticle synthesis and characterization was conducted by Mr. Cecil Coutin ho under the guidance of Dr. Vinay Gupta in the Department of Chemical Engineering. 6.2 Silica hybrid particles 6.2.1 Hybrid particle synthesis Unless otherwise noted, all chemicals were purchased from Sigma-Aldrich (WI) and used without further purif ication. The monomer NIPAM (TCI), was recrystallized from hexane before use. With the goal of developing novel slurry for CMP applications, polymer-siloxane (hybrid) microgels were formed by the surfactant free precipitation polymerization of NIPAM (5g) in aqueous media (800 ml) using N,NÂ’methylenebisacrylamide (0.2g) as the cross-linker. Following purging with N2 for 1h, the reaction mixture was heated in an oil bath to 75C and the ionic initiator potassium
113 persulfate (0.1g) was added to instigate polymerization. After an initial polymerization of 2 hours, 3-(trimethoxysilyl)pr opyl methacrylate (1g) was a dded to the reaction mixture and the polymerization continued for a furt her 90 minutes. The microgels formed were collected and purified by re peated centrifugation (7800g, 30 minutes) and re-dispersed with deionized water. 6.2.2 Particle characterization Microgel sizes and polydispersities were de termined via dynamic light scattering (DLS) using a Zetasizer Nano-S (Malvern, PA). Samples were sonicated prior to analysis. A 1ml of the microgel solution wa s placed into a cuvette and allowed to thermally equilibrate to a certain temperature for 10 mins before each measurement. Data fitting was done using a multi-modal algor ithm supplied by Malvern. The collected correlelograms were fitted to diffusion co -efficients and converted to a hydrodynamic diameter using the Einstein-S tokes equation. The polymer-CeO2 composites were examined using TEM to visually determine the extent of CeO2 loading and dispersion within the polymer matrix. A drop of the sample solution was placed on a Formvarcoated Cu TEM grid that was examined using a FEI Morgagni 268D. Bulk FTIR spectrum of the microgels was measured us ing a Nicolet Magna-IR 860 spectrometer by pelletizing a small amount of dried gel with KBr. 6.2.3 Post CMP surface characterization Qualitatively, surface quality of the oxi de surface post CMP was examined by optical microscopy using a Leitz Ergolux Optic al Microscope. Quantitatively, the surface
114 roughness was measured using a Digital In struments Dimension 3100 Atomic Force Microscope. The bench top CMP tester provid ed real-time measurements of the friction coefficient during polishing and the average value after the proc ess has reached the steady state has been reported. Removal rates of the silica film were measured using a home-built ellipsometer. 633 nm wavelengt h light from a helium-neon laser was polarized by a Glan-Thompson polarizer both manufactured by Melles Griot. The polarization of the light was subsequently m odulated using a liquid-cr ystal (LC) variable phase retarder (by Meadowlark Optics) and directed onto the silicon dioxide film of the wafer surface at an incident angle of 64. Th e laser spot measured approximately 1mm 2mm at the surface. Reflected light was analyzed using another Glan-Thompson polarizer, and the inte nsity was measured using a Si photodiode manufactured by Thor Labs. Control of the variable LC retarder a nd the data acquisition from the detector was performed using a HP-VEE (version 4.0) prog ram. Thickness measurements were made at a minimum of six different spots on each s ubstrate, and the average value is reported. 6.2.4 Experimental conditions for silica hybrid particle slurry testing Upon synthesis and re-dispersion of the hybr id particles in water, the pH of the solution was adjusted to 12 using KOH soluti on. Conventional silica particles of 50 nm and 150 nm were used to make silica slurries for comparison with the hybrid particles. The details of the slurry samples made using these particles have b een tabulated in Table 6.1. All the slurries were formulated to have equal amount of weight percentage of abrasive particles.
115 Table 6.1 Details of the slurry sample s made out of hybrid and silica particles Slurry Name Particle Type Particle size (nm) Wt % Slurry 1 Hybrid polymer-oxide particle500 (Less surface Oxide) 1.5 Slurry 2 Hybrid polymer-oxide particle300 (More surface Oxide) 1.5 Slurry 3 Pure silica NEI corp. 150 1.5 Slurry 4 Pure silica -Fuso chemical co.50 1.5 The slurries were then employed for perf orming CMP of 1Â” thermal oxide wafers on the bench top CMP tester. The testing of the slurry samples was carried out at the process conditions as summarized in Table 6.2 Table 6.2 Experimental c onditions for slurry testing Parameter Value Pressure 4 Psi Pad RPM 200 RPM Slurry flow rate 75 ml/min Pad IC 1000 K grove polishing pad Slider stroke and velocity 7 mm and 3mm/sec respectively 6.2.5 Results and discussion Â– silica hybrid particles Hybrid particles synthesized using precip itation polymerization were characterized using TEM and the average size of the part icles was noted to be approximately 500 nm. The TEM image of the hybrid particles is pr esented in the figure 6.1 along with the 50
116 nm silica particles. The patchy surface of the pa rticle can be clearly seen from the image, which indicates the presence of sili ca functional groups on the surface Figure 6.1 TEM images of the Hybr id particles and silica particles The coefficient of friction data measured in-situ by the polisher is presented in Figure 6.2. The removal rate measurements as presented in Figur e 6.3 were obtained by using a Rudolph AutoEL III ellipsometer. From th e removal rate data, it can be noticed that the hybrid particle s performed similar to the silica particles. Figure 6.2 COF data measured in-situ dur ing CMP of thermal oxide wafers using various kinds of abrasive particles. 0.2 0.22 0.24 0.26 0.28 0.3 0.32 300 nm hybrid500 nm hybrid50 nm silica150 nm silicaCoefficient of friction 0.5 m 0.1 m
117 Figure 6.3 Removal rate measurements dur ing CMP of thermal oxide wafers using various kinds of abrasive particles. Friction data suggested that the hybrid particles demonstrat ed better frictional characteristics as compared to the 50 nm and 150 nm silica particles. This indicates that the hybrid particles perform much better than the traditional silica particles resulting in higher removal rate and having less friction at the interface. The 300 nm hybrid particles resulted in lower coefficient of friction and higher removal rate as compared to the 500 nm hybrid particles. This resu lt could be due to the increas ed hardness of the 300 nm particle as compared to the 500nm particle. The thermal oxide wafer surface was characterized before and after CMP usi ng Fourier Transform Infrared (FTIR) Spectroscopy to ensure any deposition of the polymer material onto the wafer surface during polishing. From the FTIR spectrum (see figure 6.4), it can be seen that polishing 0 2 4 6 8 10 12 14 16 18 300 nm hybrid500 nm hybrid50 nm silica150 nm silicaRemoval rate (nm/min)
118 with the hybrid or silica partic les did not result in any change in the surface properties of the thermal oxide wafer. Figure 6.4 FTIR spectroscopy on thermal oxide wafer before and after CMP. Surface roughness imaging using Atomic Force Microscopy (AFM) was conducted to probe the surface quality and roughne ss. Also an estimate of particle residue was obtained from the AFM images (10 X 10 m images at 200 nm data scale) as shown in Fig 6.5 (a-d). From the AFM images and th e numerical data, it could be seen that the hybrid particles performed much better than the pur e silica particles both in terms of achieving lower surface roughness and more im portantly lowering the particle residue, which helps eliminate rigorous post CMP clean steps . The numerical values of the surface roughness are presented in the table 6.3.
119 Figure 6.5 AFM images of the thermal oxi de wafer surfaces polished with (a) 300 nm hybrid (b) 500 nm hybrid (c) 50 nm silica and (d) 150 nm silica Table 6.3 Surface roughness of wafer afte r CMP using various abrasive particles Abrasive particle Abrasive particle size Surface roughness (nm) rms roughness 500 0.86 Hybrid polymer particle 300 1.26 150 2.86 Pure silica particle 50 5.32
120 From the above results and discussion, it can be noted that the hybr id particles exhibit superior performance as compared to the conven tional silica particles in regard to surface finish after CMP. Moreover, the amount of particle residue on the polished wafers is minimal in the case of hybrid particles when compared to the silica particles. However, the oxide removal rates during CMP upon using th e slurries made of hybrid particles are negligible. Even though the obs erved negligible removal rate can be attributed to the absence of any additives and dispersants th at are used in commercial slurries, the inclusion of ceria particles on the surface of the hybrid particle along with silica functional groups is hypothesized to improve the oxide removal rate during CMP. This hypothesis led us into the modification of th e surface chemical nature of the hybrid particles making them into Â‘Interpenetrating hybrid ceria compositeÂ’ particles. 6.3 Ceria composite particles In previous work, we have reported the performance of hybrid microgels consisting of soft polymeric networks cont aining hard inorganic siloxane segments. Although the slurries made from these hybr ids produced a superior surface finish compared to traditional silica slurries, the low removal rates of silica from the wafer surface hinders its potential for commercial CMP applications. As an extension to the above study, in order to improve oxide removal rates during CMP, we chose to incorporate ceria on the surface of the microge l. Traditionally ceria particles are well known for achieving significant removal rate during oxide CMP and moreover, they enhance the selectivity of the slurry thereby avoidi ng dishing during STI CMP [114-116]. However, these ceria particles produce bot h major and minor scratches on the wafer
121 surface during polishing. In our approach ceria nanoparticles on the surface of a polymeric microgel (hybrid composites) will conceivably prevent aggressive abrasion of the ceria on the wafer surface resulting in smoother surfaces with reduced surface damage. These hybrid composite particles are hypothesized to provide a cushioning effect to the wafer being polished due to the soft nature of the po lymer and at the same time achieve appreciable oxide layer removal rate due to the chemical nature of the surface silica and ceria pa rticles. Since the polishing is not suspected to be aggressive, the coefficient of friction while pol ishing with the slurries cont aining hybrid particles is also expected to be lower than dur ing polishing with slurries c ontaining conventi onal abrasive particles. 6.3.1 Composite particle synthesis Hybrid microgels were formed similar to the procedure descri bed in silica hybrid particle synthesis but in addition poly(acrylic acid) sodium salt (~10g, MW ~15000 g/mol) was added during initia l polymerization to achieve in terpenetrating chains of poly (acrylic acid) within the hybrid microgel, thus forming IP-hybrid microgels [117-119]. These IP-hybrid microgels enhance the incorporation of ceria nanoparticles. Nanoparticles of CeO2 suspended in deionized water, were mixed with the IP-hybrid microgel solution in a desired loading ratio (50wt %) at a pH 5. The resulting composite settled to the bottom and the supernatant was removed. The composite slurry was homogenized by washing three times with deionized water. All pH adjustments were done using 0.1M NaOH and 0.1M HCl.
1226.3.2 Experimental conditions for co mposite particle slurry testing The planarization of 1.5Â” square PEC VD oxide wafers was carried out on the CMP bench top polisher using slurries consisti ng of (a) 0.5wt% composite particles, (b) 0.5wt% of ceria (same weight fraction of th e composite) and (c) 0.25wt% of ceria (same number fraction of ceria as in composites). A ll slurries were disper sed in deionized water at a pH of 5 to maintain a slight positive charge on the CeO2 (Iso-Electric Point (IEP) ~ pH 6.0) that helps disperse the ceria more evenly and aids in abrading the negatively charged silica surface (IEP ~ pH 2.3). All the slurries were well agitated during experimentation to prevent sedimentation of th e abrasive particles. The planarization was conducted for 3min at room temperature a nd all CMP experiments were repeated to ensure reproducibility. The process conditions for the polishing experiments were the same as tabulated in the Table. 6.1 except that 7psi down pressure was used instead of 4psi. 6.3.3 Results and discussion Â– composite particles Upon controlling the mixing ratios of the microgel and ceria solutions precisely, the mass fraction of ceria within the composite can be easily tailored. The composite particles prepared contain approximately 50wt% ceria and is shown in the TEM image in figure 6.6. The dark spots indicate ceria (~ 20nm) that is well disp ersed and unaggregated within the polymer microgel. The synthesis and development of the slurry with these hybrid composites is under the proces s of patenting (Ref # 06B115PR).
123 500 nm Figure 6.6. TEM image of the composite particle dark spots indicate CeO2 As mentioned earlier, PNIPAM has become one of the best studied responsive polymers. A hydrophilic-hydrophobic balance that exists between the amide and isopropyl side chains  causes the polymer microgel to phase separate out of solution above the transition temperature (~32C). The co-polymerization of NIPAM with MPS and the presence of interpenetrating chains of PAAc do not significantly alter the temperature response behavior of the IP-hybrid microgel. 850 800 750 700 650 600 550Hydrodynamic Diameter (nm) 40 38 36 34 32 30 28 26 Wavenumber (cm-1) Figure 6.7 DLS of microgels where the pa rticle phase separates at nearly 32C
124 The phase separation of the IP-hybrid mi crogel is described by the DLS data in figure 6.7 where the transition results in a 70% decrease in the volume of the microgel. Hence, by controlling the temperature, the hardness of the particle can be easily manipulated to create harder or soft er abrasive particles when desired. To examine the removal rates of the s ilica from the wafer surface and test for organic residue on the polished surface (data no t shown), we used infrared spectroscopy. FTIR characterization of the wafer surface be fore and after CMP confirmed that there was no polymer deposition onto the wafer surface during CMP. The intensity of the absorption peak of Si-O-Si at 1075cm-1 was evaluated and this showed substantial removal of the oxide layer by the ceria and composite slurry as seen in figure 6.8. Qualitatively, it can be seen that the 0.25wt% ceria slurry and 0.5wt% composite slurry achieve nearly identical removal while the slurry consisting of 0.5wt% ceria achieves about double that removal. 1.00 0.95 0.90 0.85 0.80 0.75 0.70 Normalized Absorbance 1120 1100 1080 1060 1040 Wavenumber (cm-1) Blank CeO2_0.25wt PSC CeO2_0.5wt Figure 6.8 FTIR characterization of silica removal from the wafer surface.
125 Thereby, it can be concluded that the ceria is the primary factor responsible for removal of silica, while the polymer microgel is the entity that ensu res a planar surface. Quantitative thickness measurements of the wafer were measured very accurately using ellipsometry. The coefficient of friction during polishi ng was obtained from the ratio of lateral and normal forces measured in-situ using a dual force sensor in stalled to the upper carriage of the machine carrying the wafer car rier. The average coefficient of friction after the process has reached th e steady state has been noted. The numerical values of the average coefficient of friction along with the standard deviat ion are tabulated in the table 6.4. The numerical values for remova l rate were obtained from Ellipsometry measurements. Table 6.4. Numerical values for coefficien t of friction, surface roughness and removal rate COF Surface Roughness (nm) Removal Rate (nm/min) Composite 0.16 0.01 1.06 0.44 104 4.24 0.5wt% CeO2 0.22 0.01 2.31 0.84 241 28.28 0.25wt% CeO2 0.11 0.01 4.4 2.2 125 0.71 From the average values it can be seen th at the slurry containing 0.5 wt % ceria composite particles resulted in lower coefficient of friction as compared to the slurry
126 containing 0.5 wt % conventional ceria particles. This shows that the composite particles aid in reducing the friction at the polishing in terface. This could be attributed to their relative softness and the resulting cushioning eff ect. However, it was noted that 0.25 wt % ceria resulted in lower coefficient of fric tion than the 0.5 wt % conventional ceria and also the 0.5 wt % ceria composite particles. Th is observation can be attributed to the fact that slurry with 0.25 wt % ceria will have a fewer number of particles of same size as compared to 0.5 wt % ceria, thus resulting in milder abrasion and hence lower coefficient of friction. 0.25 wt % ceria will have much smaller volume fraction in comparison with the composite particles, which could be the pl ausible reason for the lower coefficient of friction observed. 6.3.4 Post CMP surface characterization Even though from the removal rate and coefficient of friction measurements it appears as if 0.25 wt % convent ional ceria would be benefici al due to their low friction characteristics, it is crucial to study th e post CMP surface characteristics to draw any conclusions regarding the perf ormance of ceria composite pa rticles. Optical microscopy images of the post CMP oxide surface at 5X magnification are shown in figure 6. As revealed from figure 6.9, slur ries with conventional ceria pa rticles resulted in severe scratches on the wafer surface. Conversely, sl urries consisting of the composites resulted in relatively much less surface defects.
127 10 um 10 um 10 um Figure 6.9 Optical microscopy (5X magnification) images of wafers polished with (a) ceria composite particles, (b) 0.5wt% CeO2, (c) 0.25wt% CeO2 nanoparticles This reduction in surface scratches can be plausibly attributed to polymer microgels maintaining an even dispersion of the ceria nanoparticles and preventing aggregate formation during the duress planarizat ion. Further, the cush ioning effect of the polymer particles helps in re ducing the aggressiveness of po lishing thereby resulting in reduced surface damage. (b) (c) (a)
128 Figure 6.10 AFM images of wafers polished with (a) 0.25wt% CeO2 (b) 0.5wt% CeO2 nanoparticles and (c) composite particles Additionally, AFM images in figure 6.10 show particle contamination on wafer surfaces polished with only ceria nanoparticles. The wafer polished with the composite is devoid of pitting and minor scratches. Th e above results clearly indicate that the composite particles with controlled softness/ hardness can be potentia lly beneficial and
129 can be successfully implemen ted for polishing in the final stage of CMP process where only moderate amounts of material needs to be removed but superi or surface quality is required. 6.4 Summary Novel slurries based on hybrid particle s consisting of polymer with inorganic component on the surface are successfully developed. The hybrid silica microgels resulted in superior surface finish as compar ed to the conventional silica particles. The hybrid particles resulted in lower coefficien t of friction and lower particle residue as compared to the conventional silica particles. However, the oxide removal rates during CMP upon using the slurries made of hybrid particles are negligib le. Improved surface finish, lower COF make these particles potential candidates for next generation stringent polishing requirements. In order to improve the removal rate during oxide CMP, ceria particles were embedded into the hybrid microgels and also were incorporated onto the surface of the hybrid particles to form ceria hybrid co mposites. These hybrid composite particles resulted in moderate removal rates but at the same time did not compromise the surface quality. The oxide surface pol ished with the hybrid composites was superior when compared to the surface polished with conve ntional ceria particle s. Such performance makes these particles potential candidates for CM P slurries to be used for next generation CMP processes.
130 CHAPTER 7: SUMMARY AND FUTURE WORK A detailed investigation of effect of temperature on Cu CMP process has been conducted in this dissertation. The e ffect of temperature not only on CMP process but also on the electrochemical, surface chemical analysis, coefficient of friction and pad conditioning process has been explored. Beside s temperature, the eff ect of slurry flow rate on CMP process has also been studied. Finally, novel slurries are developed to reduce post CMP surface damage and friction during CMP process. All the above mentioned studies are summarized below. 7.1 Effect of temperature on CMP process The effect of temperature on CMP process and interface tribology has been studied. This research demonstrated th at the increase in removal rate during CMP due to an increase in temperature is not only due to increase in oxidation rate but also an increase in dissolution rate of the dissolv able surface copper oxides and hydroxides in to the slurry. It was noted that the increase in temperature did affect the surface quality as the scratch depth increased with increase in slurry temperature above room temperature. The temperature study on the patterned coppe r samples revealed th at the non-uniformity in polish decreases from below room temperat ure to room temperature and then increases with increase in temperature. This along w ith surface quality shows that the polish was predominantly mechanical at lower temperat ures. The surface mechanical properties did
131 not change due to polishing at different temperatures which suggests that the increase in coefficient of friction is mainly due to the change in contact area and pad surface asperity deformations. The study of coefficient of fric tion plotted as stribeck curves indicated no change in the polishing regime with a change in temperature. Further, the effect of temperature on coefficient of friction was st udied at different process conditions, which revealed the existence of three distinct regime s of the effect of temperature, determined by the process parameters pressure and veloc ity. Thus, it was concluded that the change in coefficient of friction is mainly due to the change in pad surface asperity deformation and the change in contact area. The effect of temperature was furt her studied on pad conditioning process which indicated an aggressive conditioning pr ocess at low temperatures but a rather ineffective conditioning process at elevated temperatures. However, the pad loss was found to be the most during conditioning at low temperatures and minimal pad loss was noted during conditioning at elevated temperat ures. Thus, it was shown that temperature has a pronounced effect of pad conditioning process and an optimized setting of temperature is needed for each specific case depending on the process requirements. Also, the dishing on the post CMP surface after subsequent polishing at different pad conditioning temperatures was measured. It was found that the dishing increased with increase in pad surface temperature but th e with-in wafer uniformity increased at the same time. This shows a significant differenc e in the area of c ontact at various pad surface temperatures.
1327.2 Effect of slurry flow rate on CMP process Thermal model to estimate wafer temperature as a function of thickness and radius of the wafer has been developed and the temperature on the wafer surface as a function of radius and thickness of the wafer ha s been estimated. From the finite element analysis of the model, slurry flow was s hown to reduce the temperature rise on the surface, which in turn would reduce patte rn effects on post CMP wafer surface. A study of the effect of slurry fl ow rate on tribology during Cu CMP has been conducted. It was seen that the coeffi cient of friction decr eases with increasing slurry flow rate and at the same time, the re moval rate increases w ith increase in slurry flow rate. Post CMP surface characteristics i ndicated a decrease in dishing, erosion and metal loss with increasing slurry flow rate. Also, a smaller rise in pad surface temperature was noted at higher slurry flow rates. Th e above mentioned two results confirm the finding from pad conditioning studies that dishing increases with an increase in pad surface temperatures. These results suggest that maintaining the pad surface asperities at a constant temperature, dishi ng and erosion can be reduced to a considerable extent. 7.3 Novel slurry development for reduced surface defects Also, low defect slurries using novel polymer based hybrid particles have been developed. The proposed slurries result in superior surface finish as compared to the surface finish obtained using the slurries co ntaining conventional ab rasive particles. Slurries containing polymer-silica hybrid part icles even though gave a superior surface finish, they did not result in a substantial removal rate. Ceria particles were then
133 embedded onto the polymer silica hybrid particles. These particles resulted in generating a superior surface quality as compared to the conventional ceria particles and at the same time resulted in lower coefficient of friction. The removal rate obtained using the slurry consisting of polymer-silica-ceria particles ev en though was not as much when compared to the conventional ceria slurries, the coeffici ent of friction, surface damage and scratches following CMP were found to be much less. Thus, the slurries containing polymer particles prove to be potential slurries for th e generating a superior surface after polishing sans any defects. Thus, these slurries provide a valuable solution fo r the next generation stringent CMP output requirements. 7.4 Future Work There are several aspects of this res earch that could be extended to gain valuable in-sight into the pr ocess mechanisms and several methods of detecting defects in-situ can be improved. Slurry viscosity and its importance to CMP could be studied drawing the analogy of the working of a jour nal bearing to the CM P polishing interface. 7.4.1 Thermal model Thermal model of the wafer along with carrier assembly needs to be developed to simulate the industry set up an d to estimate the temperatures on the wafer which are more representative of the actual process temperature profiles. In the same context, steady state heat transfer therma l model of the polishing pad can also be developed, which can improvise the desi gn and manufacturing of the polishing pad. Analytical model improvised by more realisti c geometrical model along with pad model
134 supported by thermal imaging data on the wafer carrier and polishing pad could be extremely helpful. 7.4.2 Electrochemistry and impedance spectroscopy Electrochemistry (potentiodynamic polarization) and Electrochemical Impedance Spectroscopy (EIS) studies can be conducted at different temperatures and for varying chemistries. The electrochemical study of different slurry chemistries at different temperatures will provide information about the dependence of oxidation and dissolution rates of various chemistries on slurry te mperature. Such a study could be highly complimentary to the present research work. 7.4.3 Low defect slurry development The novel low defect slurry develope d using hybrid composite abrasives could be improved for its pa rticle dispersion characte ristics by adding appropriate additives and thereby improving the shelf life and possibly the removal rates of oxide CMP further. Thus developed slurries could be tested on STI (patterned) oxide wafers for their selectivity with the unde rlying nitride layer on STI wafers. A study on dishing and erosion during STI CMP could be conducted and the performance of hybrid abrasives be evaluated in comparison with conventional ce ria particles. The applicability of these abrasive particles for copper CMP could also be evaluated as they provide relatively lower coefficient of friction as compared to the conventional particle s, which could prove highly beneficial for next generation low pressure sensitive CMP of Copper/Low-K metallization stacks.
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143 ABOUT THE AUTHOR Subrahmanya Mudhivarthi was born in 1980 in the state of Andhrapradesh located in Southern part of India. The author received his BachelorÂ’s degree in Mechanical Engineering in 2001 from Sri Kris hnadevaraya University, India. He then pursued his MasterÂ’s studies in the field of Tribology under the guidance of Dr. Daniel Hess and received his Master of Science in Mechanical Engi neering degree at University of South Florida, Tampa in the December of 2003. He then joined Dr. Ashok KumarÂ’s group in 2004 to pursue docto ral studies in the area of Chemical Mechanical Planarization. During his doctoral studies the au thor has worked as a Graduate Intern at Center for tribology Inc. and Intel Corporation. The author is a member of the Tau Beta Pi honor society for engineers and American Society of Mechanical Engineers.