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Manavalan, Sriraj G.
Structural and electrical properties of barium strontium titanate thin films for tunable microwave applications
h [electronic resource] /
by Sriraj G. Manavalan.
[Tampa, Fla.] :
b University of South Florida,
Thesis (M.S.E.E.)--University of South Florida, 2005.
Includes bibliographical references.
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ABSTRACT: The dependence of dielectric permittivity on the applied electric field, high dielectric constant and low cost makes barium strontium titanate (BST) a promising ferroelectric material for applications in tunable microwave devices. High tunability and low dielectric loss is desired for tunable microwave devices. The primary objective of this research was to optimize the tunability and dielectric loss of BST thin films at microwave frequencies with different deposition techniques. Ba0.5Sr0.5TiO3 thin films were grown on Pt/TiO2/SiO2/Si, by pulsed laser deposition (PLD) and sputtering. Parallel plate capacitor structures were designed using ADS and fabricated. The microstructural and phase analysis of the BST films were performed using X-ray diffraction (XRD) method. The diffraction patterns are attributed to cubic (perovskite) crystal system. The analysis of surface morphology was done using atomic force microscopy.
Adviser: Dr. Ashok Kumar.
Co-adviser: Dr. Shekhar Bhansali
x Electrical Engineering
t USF Electronic Theses and Dissertations.
St ructural and Electrical Properties of Barium Strontium Titanate Thin Films for Tunable Microwave Applications by Sriraj G. Manavalan A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering Department of Electrical Engineering College of Engineering University of South Florida Co Major Professor: Ashok Kumar Ph.D. Co Major Professor: Shekhar Bhansali Ph.D. T homas Weller Ph.D. Date of Approval: March 23 200 5 Keywords: bst, rf, tunability, pld, sputtering Copyright 2005 Sriraj G. Manavalan
ACKNOWLEDGEMENTS I am grateful to everyone who helped me throughout my research work to make this work successful. First of all I thank God and my Family for their love and support. I express my deep gratitude and thankfulness to co major Professor, Dr. Ashok Kumar for providing me with this opportunity to conduct the thesis and also for his encouragement throughout my research work. I would also like to express my sincere thanks to co major Professor, Dr. Shekhar Bhansali for his valuable time and suppor t. I am very thankful to thesis committee member, Dr. Thomas Weller for his valuable suggestions, guidance and for allowing me to use his labs and facilities. Special thanks go to my friend Saravana P. Natarajan, who has helped me a lot throughout my rese arch work. His help during the fabrication process and RF measurements is highly appreciated. I am also grateful to my friends in the group, Harish Jeedigunta for his time and effort in sputtering and his support during research work, Lavanya Sriram and R aghu Mudhivarthi for their support and motivation. I take this opportunity to thank all my colleagues and friends for their encouragement and moral support during the research period.
i T ABLE O F C ONTENTS LIST OF TABLES i v LIST OF FIGURES v ABSTRACT vi i i CHAPTER ONE: INTRODUCTION 1 1 .1 Overview 1 1.2 Thesis Outline 3 CHAPTER TWO: BACKGROUND 4 2.1 Ferroelectrics 4 2. 2 Barium Strontium Titanate (BST) 8 2. 3 Competing Technologies 10 2. 4 Factors that Influence BST Thin Film Properties 1 1 2. 5 Summary 1 6 CHAPTER THREE: FABRICATION AND DEVICE STRUCTURE 17 3.1 Deposition Techniques 17 3 .1.1 Pulsed Laser Deposition 17 3.1.2 Sputtering 22 3.1.3 Sol gel 25 3.2 Capacitor Configurations 2 6
ii 3. 2 .1 Parallel Plate Cap acitor s 27 3. 2. 2 Interdigital Capacitor s 2 7 3. 3 Fabrication of BST Capacitor 2 8 3. 4 Summary 31 CHAPTER FOU R : STRUC T URAL CHARACTERIZATION 3 3 4.1 Experimental Matrix of Pulsed Laser Deposition 3 3 4.1.1 XRD Analysis of PLD BST Films on Pt/T iO 2 /SiO 2 /Si 33 4.1.2 AFM Analysis of PLD BST Films on Pt/TiO 2 /SiO 2 /Si 3 6 4.2 Experimental Matrix of RF Sputtering 38 4.2.1 XRD Analysis of Sputtered BST Films on Pt/TiO 2 /SiO 2 /Si 3 9 4 2 2 AFM Analysis of Sputtered BST Films on Pt/TiO 2 /SiO 2 /Si 4 2 4. 2.3 XRD Analysis of Sputtered BST Films on MgO, Al 2 O 3 and LaAl 2 O 3 43 4. 2. 4 AFM Analysis of Sputtered BST Films on MgO and Al 2 O 3 44 4.3 XRD Analysis of S ol gel D epos ited BST T hin F ilms 45 4 4 Summary 4 6 CHAPTER F IVE : ELECTRICAL CHARACTERIZATION 4 8 5 .1 Measurement Set up 4 9 5.2 Results and Discussion 5 1 5 2 .1 Pulsed L aser Deposited BST T hin F ilms 5 1 5 2 2 RF Sputtered BST T hin F ilms 5 5 5 3 Summary 59 CHAPTER SIX: INTERDIGITAL CAPACITOR 60 6 .1 Design of I nterdigital Capacitor 60
iii 6.1.1 Capacitor Simulation 60 6.2 Capacitor Measurements 6 3 6 3 Summary 6 6 CHAPTER S EVEN : C ONCLUSIONS AND FUTURE WORK 6 8 7 .1 Co nclusions 6 8 7 .1.1 Pulsed Laser Deposited BST T hin F ilms 6 8 7 .1.2 RF Sputtered ST Th in F ilms 6 9 7 .1.3 Sol gel Deposited BST T hin F ilms 71 7 .2 Future Work 71 REFERENCES 7 3
iv LIST OF TABLES Table 2.1 Types of Perovskite Oxides 5 Table 2. 2 Comparison of Competing Technologies with BST 1 1 Table 2.3 Summarized Literature Review of BST Capacitors 15 Table 3 .1 Differen t D eposition Techniques and T heir A dvantage s and D isadvantages 2 6 Table 4 1 Intensity of A ll the Diffraction Peaks at 450C, 550C and 650C 3 4 Table 4 2 Variation of Average Gr ain Size and Surface Roughness with Different Parameters 37 Table 4 3 Intensity of A ll the Diffraction Peaks at 550C, 600C and 650C 39 Table 4 4 Intensity of A ll the Diffraction Peaks at Different Ar/O 2 Ratios 4 1 Table 4.5 Intensity of All the Diffraction Peaks of BST on Different Substrates 44 Table 5.1 Electrical Characteristics of Pulsed Laser Deposited BST Thin Films 5 4 Table 5.2 Optimum Conditions for PLD BST Thin Films 55 Table 5. 3 Optimum Conditions for Sputtered BST Thin Films 58 Table 7.1 Summary of BST Capacitors Fabricated 71
v LIST OF FIGURES Figure 2 1 Perovskite Structure (ABO 3 ) 5 Figure 2.2 Transition Characteristic of Ferroelectrics 6 Figure 2.3 Perovskite BTO U nit cell a) Paraelectric Phase b) Ferroelectric P hase 8 Figure 2. 4 Temperature Dependence of Dielectric Constant in Ceramic and Thin Films 12 Figure 3.1 Schematic Representation of Laser Solid Interactions 1 9 Figure 3.2 PLD System at USF 2 1 Figure 3 3 Schematic of the PLD System at USF 2 2 Figure 3 4 Sputtering S ystem at USF 2 5 Figure 3 5 Schematic of Interdigital and Parallel Plate Capacitors 2 8 Figure 3.6 Schematic of the CPW Parallel Plate BST Capacitor 2 9 Figure 3.7 Mask Scheme for BST Parallel Plate Capacitor 30 Figure 3.8 Process Flow for Fabrication of BST (a) Parallel Plate and (b) Interdigital Capacitors 31 Figure 4 1 X ray Diffraction Patterns Showing the Effect of Deposition Temperature on the Crystallinity of BST Thin Film s 3 5 Figure 4 2 X ray Diffraction Patterns Show ing the Effect of Oxygen Pressure on the Cr ystallinity of BST Thin Film 3 5 Figure 4.3 AFM Micrographs of BST Thin Films 3 7
vi Figure 4.4 X ray Diffraction Patterns Showing the Effect of Deposition Temperature on the Crystall inity of Sputtered BST Thin Films 40 Figure 4.5 X ray Diffraction Patterns Showing the Effect of Ar/O 2 Ratios on the Crystallinity of Sputtered BST Thin Film s 4 1 Figure 4.6 Surface View of BST Films Sputtered at Ar/O 2 R atios (a) 90/10 (b) 70/30 4 2 Figure 4.7 X ray Diffraction Patterns of Sputtered BST Thin Film on MgO, Al 2 O 3 and LaAl 2 O 3 43 Figure 4.8 Surface View of BST Films Sputtered on MgO and Al 2 O 3 45 Figure 4.9 Flow Diagram to Prepare BST Thin Films Using Sol gel 4 5 Figure 4.10 X ray Diffraction Patterns of Sol gel Deposited BST Thin Film on MgO 46 Figure 5.1 Measurement Setup Used for BST Capacitors 50 Figure 5.2 Magnitude of Z for the Load 50 Figure 5.3 Variation of Permittivity with Applied Bias for Differen t Thicknesses 52 Figure 5.4 Variation of Quality Factor with Applied Bias for Different Thickness es 52 Figure 5.5 Variation of Capacitance with Frequency for Different Bias Voltages 53 Figure 5.6 Variation of Q Factor with Frequency 5 3 Figure 5.7 Tunability of Sputtered BST Thin Films at Different Ar/O 2 Ratios 5 6 Figure 5.8 Variation of Q Factor with Applied Bias for Ar/O 2 Ratios 5 6 Figure 5.9 Variation of Capacitance with Frequency for Different Bias Voltages 57 Figure 5. 10 Variation of Q Factor with Frequency 5 8 Figure 6.1 Layout of an Interdigit al Capacitor with CPW T opology 61 Figure 6. 2 EM Simulation Plots of S 11 dB as a Function of Frequency 62 Figure 6.3 EM Simulation Plots of S21 (dB) as a Function of Frequency 62 Figure 6. 4 EM Simulation Plots of S21 P hase (deg) as a Function of Frequency 63
vii Figure 6. 5 Mask Scheme for BST Interdigital Capacitor 64 Figure 6. 6 Measured R esponse of S 11 (d B ) as a Function of Frequency 65 Figure 6. 7 Measured R esponse of S 21 (d B ) as a Function of Frequency 65 Figure 6. 8 Measured Response of S21 P hase (deg) as a Function of Frequency 66
viii STRUCTURAL AND ELECT RICAL PROPERTIES OF BARIUM STRONTIUM TI TANATE THIN FILMS FO R TUNABLE MICROWAVE APPLICATIONS Sriraj G. Manavalan ABSTRACT The dependence of dielectric permittivity on the applied electric field, high dielectric constant and low cost makes barium strontium titanate (BST) a promising ferroelectri c material for applications in tunable microwave devices. High tunability and low dielectric loss is desired for tunable microwave devices. The primary objective of this research wa s to optimize the tunability and dielectric loss of BST thin films at micro wave frequencies with different deposition techniques. Ba 0.5 Sr 0.5 TiO 3 thin films were grown on Pt/TiO 2 /SiO 2 /Si, by pulsed laser deposition (PLD) and sputtering P arallel plate capacitor structures were designed using ADS and fabricated. The microstructura l and phase analysis of the BST films were performed using X ray diffraction (XRD) method. The diffraction patterns are attributed to cubic (perovskite) crystal system. The analysis of surface morphology was done using atomic force microscopy. Electrical p roperties of parallel plate capacitors were measured using LCR meter and t unability of 2 .4:1 and loss tangent of 0.05 was achieved at low frequencies for laser deposited BST thin films. T unability of 2 .8:1 and loss tangent of 0.03 was achieved at low frequ encies for sputtered BST thin films. The correlation of optimized structural and dielectric properties of thin films deposited by pulsed laser deposition and sputtering
ix technique was analyzed and c om pared. The structural characterization of sputtered BST thin film on MgO, Alumina and LaAl 2 O 3 was achieved for the fabrication of interdigital capacitors. Interdigital capacitor has been design ed using ADS momentum
1 CHAPTER ONE INTRODUCTION 1.1 Overview W ith the rapid growth of communication systems in recent years, there has been significant demand to minimize the device size and implement affordable tunable RF and microwave compo nents. Small size, low cost, high fi eld dependent permittivity and low loss are the desirable characteristics for the current and next generation tunable microwave applications. These requirements impose significant challenges on existing technologies and create the necessity for new materia ls and technologies. The current competing technologies for tunable circuits are semiconductor varactor diodes, MEMS and ferroelectric devices. Ferroelectric technologies have received extensive attention because of their ability to achieve desirable chara cteristics for tunable microwave applications due to the dependence of dielectric permittivity on the applied electric field [1 5] As the dielectric constant of the ferroelectric thin films change with the electric field, the phase velocity of the films c hange which allows tuning of the films. For optimum performance at microwave frequencies, the critical parameters needed are high tunability and low dielectric loss. Some materials which have shown a variable permittivity with electric field are SrTiO 3 ( Ba, Sr)TiO 3 (Pb, Sr)TiO 3 (Pb, Ca)TiO 3 Ba (Ti, Sn)O 3 Ba (Ti, Zr)O 3 and KTaO 3 In particular, Barium S trontium T itanate (BST) has been found as one of the most suitable ferroelectric materials because of the higher
2 dielectric constant, high tunability and low dielectric loss at room temperature [6 10]. Its high capacitance density allows the construction of high value capacitors in a very small area. T unable capacitor is one of the critical components in tunable RF and microwave devices and applied in commercial and military systems such as tunable band select filters for wireless communications, phase shifters for electronic scanning antennas, tunable radiating structures for frequency hopping, and tunable transformers to reduce RF impedance mismatch The p arameters that have been identified to affect the dielectric properties of BST film s are processing technique film composition crystalline structure, microstructure, surface morphology, film thickness and electrode materials. In this work, B a 0.5 S r 0 .5 T iO 3 thin films have been deposited by three different deposition techniques p ulsed laser de position, RF sputtering and sol gel The goal is to characterize the structural and e lectrical properties of BST thin film varactors The microstructural analysi s of BST films is performed using X ray diffraction (XRD) method. The analysis of surface morphology has been done using atomic force microscop y and scanning electron microscop y I nterdigital and parallel plate varactors a re designed in a coplanar wave gui de structure topology. For parallel plate capacitors, BST is deposited on Pt/TiO 2 /SiO 2 /Si substrate s. For interdigital capacitors, BST is deposited on MgO, LaAl 2 O 3 and Al 2 O 3 substrate s and gold is used as top electrode for both capacitor structures. LCR me ter was used to measure the d ielectric constant tunability and dielectric loss of BST thin film s. Electrical and structural properties of
3 B ST thin films deposited by different deposition techniques has been analyzed and compared. 1.2 Thesis Outline Signif icance of BST in microwave applications along with brief description of competing technologies has been presented in chapter 2. Factors affecting the electrical properties of BST thin films and literature review of the current BST capacitors have also been discussed Chapter 3 discusses the detailed description of three deposition techniques that have been used in this work namely pulsed laser deposition, sputtering and sol gel. Introduction to parallel plate and interdigital capacitors has been made and f abrication process for both structures is presented. The microstructural analysis and surface analysis of BST films deposited by different deposition techniques is presented in chapter 4. The e ffect of substrate temperature o n structural properties is also discussed. T he dielec tr ic properties of measured devices are discussed in Chapt er 5. The measured frequency response data is presented in this chapter. The dielectric constant and Q factor of the capacitor with respect to different bias voltages extracte d at a particular frequency is presented. The optimized deposition condi ti ons for pulsed laser deposition and RF sputtering is presented
4 CHAPTER TWO BACKGROUND 2.1 Ferroelectrics Ferroelectric materials are dielectric materials with unique properties l ike field dependent dielectric permittivity. When an electric field is applied to a dielectric material, some of the charges are discharged at the electrodes while the remaining charges are bound and oscillate. This phenomenon is termed polarization and is expressed as the sum of the electric dipoles per unit volume. In some materials, even without the application of an electric field, polarization exists and such materials are said to be spontaneously polarized. Ferroelectrics are pyroelectric crystals who se spontaneous polarization can be reversed by the application of an electric field not exceeding the breakdown limit of the crystal. These crystals have a unique direction/axis as well as dipole in their unit cell. Ferroelectric crystals are most commonly found to have the perovskite (ABO 3 ) structure as shown in Figure 2.1. In Figure 2.1, A represents the large cations located at the corners of the unit cell, B represents the smaller cations located at the body center and O is the oxygen atoms positioned a t the face centers. The perovskite structure includes A 1+ B 5+ O 3 A 2+ B 4+ O 3 A 3+ B 3+ O 3 types and oxygen and cation deficient phases . Examples of each type are presented in Table 2.1. Crystals with ideal cubic perovskite structure as well as crystals with structures derived from the ideal by small lattice distortions or omission of few atoms are considered as perovskites.
5 Figure 2.1 Perovskite Structure (ABO 3 ) Table 2.1 Types of Perovskite Oxides  Group Oxide Type Examples I A 1+ B 5+ O 3 NaNbO 3, KTaO 3, NaTaO 3 II A 2+ B 4+ O 3 BaTiO 3, PbTiO 3, SrTiO 3, III A 3+ B 3+ O 3 YCrO 3, LaAlO 3, YAlO 3 Ferroelectric materials are very promising for a variety of applications such as high permittivity capacitors, ferroelectric memories, tunable microwave devices, pyroelectric sensors, piezoelectric transducers, electro optic devices and PTC thermistors. This wide range of applications is mainly attributed to the phase transitions in ferroelectrics. Ferroelectric materials tend to become paraelectric beyond a transition tempera ture called curie temperature T c At the Curie temperature, the ferroelectric
6 materials undergo a structural change from ferroelectric to paraelectric attaining highest dielectric constant. The transition characteristic of ferroelectrics is shown in Figure 2.2. Figure 2.2 Transition Characteristic of Ferroelectrics [ 12] As shown in Figure 2.2, in the ferroelectric phase, the dielectric constant of the increases as the temperature increases. While in the paraelectric phase, t he dielectric constant decreases with increase in temperature obeying the Curie Weiss law . The Curie Weiss law is given as follows: e r = C ( 2. 1) T T c where e r is the relative dielectric constant, T c is the curie temperature, T is temperature and C is a constant. e r attains its maximum value at T c and for T > T c e r decreases s harp ly. In the ferroelectric phase, a s trong hysteresis behavior is observed and it makes
7 the ferroelectric material suitable for non volatile memory applications. Above the Curie temperature, in the paraelectric phase the material no longer has spontaneous p olarization. However the dielectric constant still remains high and hence the ferroelectric material is highly suitable for DRAM and tunable microwave applications. The property of ferroelectricity is often associated with a crystallographic phase change from a centro symmetric non polar lattice to non centro symmetric polar lattice. Below the curie temperature, the position of Ti ion and the octahedral structure changes from cubic to tetragonal symmetry, with the Ti ion in an off center position correspo nding to a permanent dipole. These dipoles are ordered, giving a domain structure with a net spontaneous polarization within the domain. Above Curie temperature, the thermal energy is sufficient to allow the Ti atoms to move randomly from one position to a nother and so there is no fixed asymmetry The open octahedral site allows the Ti atom to develop a large dipole moment in an applied electric field, but there is no spontaneous alignment of the dipoles. In this symmetric configuration material is paraelec tric. The crystallographic phase change associated with BT O unit cell is depicted in Figure 2.3.
8 Figure 2.3 Perovskite BTO U nit C ell a) P araelectric P hase b) F erroelectric P hase 2.2 B arium Strontium Titanate (B ST ) BST (Ba x Sr 1 x TiO 3 ) is derived from t he prototype BaTiO 3 (BTO) perovskite. B TO was shown as the first perovskite compound exhibiting ferroelectric properties. The BTO structure is simple cubic and consists of large barium ion s at each corner of the unit cell, surrounded by twelve nearest oxyg en ions The titanium ion is at the center and has six oxygen ions in octahedral arrangement As in the case of normal ferroelectrics, BST undergoes phase transition at Curie temperature. However, Curie temperature depends on the Ba:Sr ratio. The isovale nt additive, strontium (Sr + 2 ), has a high solid solubility and is the same valency as the replaced barium ion. The a ddition of strontium shifts the C urie temperature for example, Ba 0.5 Sr 0.5 TiO 3 has a curie temperature of about 50C and Ba 0.75 Sr 0.25 TiO 3 h as a curie temperature of about +40C. Interestingly, (Ba, Sr) TiO 3 solid solutions have higher dielectric constant at Curie temperature than pure BTO. BST is purely ferroelectric and has spontaneous polarization below T c The tunability of BST is also ver y high in the
9 ferroelectric phase especially near T c However the dielectric losses are also very high in this region and hence this phase of BST finds applications in non volatile memories. Above T c BST becomes paraelectric and the hysteresis effect is n ot predominant. This region serves well for tunable microwave device applications due to the associated high dielectric constant and low losses. BST has been extensively studied for tunable microwave applications due to its two main attractive features. T he BST characteristics like the composition dependent Curie temperature and the electric field dependent dielectric permittivity have found applications in tunable filters, phase shifters and tunable antennas and are discussed in the following sections. Ap plication of BST in tunable microwave devices is manifold and requires precise film optimization and capacitor design. Tunable filters are widely used in receiver front end and have potential applications in most military and satellite communication system s. Presently, mechanical tuners or semiconductor based varactors are most commonly used in tunable filters. The disadvantages of mechanical tuning are low tuning speed and large size. On the other hand, semiconductor varactors are much faster but they have low power handling capabilities. An attractive alternative would be BST varactors. BST varactors have the potential to overcome these difficulties and can be used in low pass and band pass tunable filters. Phase shifters are important elements in electro nically scanned phased array antennas that are used in fighter aircraft radar and certain commercial systems such as cellular telephone base stations. Phase shifters in an electronically scanned array antenna
10 allow the antenna beam to be steered in the des ired direction without physically re positioning the antenna. Their ability to change the phase of a signal also aids in filtering the unwanted harmonics. High speed semiconductor based phase shifters are the most commonly used phase shifters. However the high losses at microwave frequencies and low power handling capabilities of these devices have driven interest in this material. Compared to semiconductor varactors, BST technology offers lower loss and better power handling capacity. Furthermore it is ine xpensive and does not have reliability issues, which will allow ferroelectric phase shifters to be used more widely. A widely used tunable device for which BST has been investigated is the microstrip antenna. Tunable microstrip antennas have been achieved by introducing multiple varactor loading at the radiating edges. 2.3 Competing Technologies There are several technologies available for tunable microwave applications as previously discussed. The other technologies besides BST are ferrite, mechanical tuni ng, semiconductor varactor diodes and micro electro mechanical systems (MEMS). Mechanical devices are large and have very high loss. On the other hand, ferrites can handle large power levels and have better tuning speed, but they need tunable magnetic fiel ds to operate. Semiconductor technology is dominant in current generation devices. Semiconductor diodes have good tunability, fast tuning speed, small size and are compatible with monolithic microwave integrated circuits. But on the other hand, they have j unction noise, poor power handling capability and need reverse bias to keep them capacitive. MEMS based devices are promising as they have very low loss, very high
11 tunability and are linear devices [13 15] but they are relatively slow, possess reliability issues and have high packaging cost BST varactor is advantageous over semiconductor varactor s and MEMS devices. Different deposition technologies are available for BST thin films and they offer low cost, higher break down voltages, higher power handling capability and lower packaging cost. The current competing technologies for tunable circuits are compared in Table 2.2 Table 2.2 Comparison of Competing Technologies with BST Properties Semiconductor MEMS BST Tunability (High Q) Good (2 3:1) Low (<1.5 :1) Good 2 3:1 RF loss Moderate (Q<60 typ) Very Good (Q<200) Moderate (Q<100 typ) Control Voltage <10 V <60 V <5 30 V Tuning speed Fast 1 5 nsec Slow >5 sec Fast <30 nsec Power Handling Capability Poor Excellent Trades with Control Voltage 2.4 Fa ctors that Influence BST Thin Film Properties BST thin films are preferred over their bulk counterparts for tunable microwave applications. At high frequencies, they offer the advantage of applying large electric fields at relatively low bias voltages. Whe n devices are fabricated using BST thin films, reduc tion in size and weight of the device improve s the compatibility with planar microwave circuits. It has also been observed that the dielectric constant does not vary as sharply in thin films as in bulk [1 6] and is shown in Figure 2. 4
12 Figure 2. 4 Temperature Dependence of Dielectric Constant in Ceramic and Thin Films  The goal of this research work is to optimize the deposition parameters to achieve high tunability and low dielectric loss. Several fa ctors have been identified to affect the dielectric properties of BST thin film capacitors and they are discussed below. Processing Methods A variety of deposition techniques, like rf magnetron sputtering, metal organic chemical vapor deposition ( MOCVD ) p ulsed laser deposition (PLD) and sol gel have been used to synthesize BST thin films [17 20] It is necessary to obtain the lowest possible process temperature to comply with other CMOS technologies besides minimiz ing post deposition thermal treatments und er low oxygen partial pressure to maintain the resistance of the films. While maintaining precise control of the
13 stoichiometry, it i s also essential to achieve good step coverage and large area deposition. These can be achieved by both MOCVD and sputtering has been promising for mass production. Since each deposition technique has its own advantages and disadvantages, this work presents the comparison between different deposition techniques and their influence in properties of BST thin films. Film Compositi on The dielectric constant can be varied by changing the Ba/Sr ratio [ 21]. I t is shown that dielectric constant increases with decrease in Sr c ontent. It has been conferred that highest dielectric constant is obtained at room temperature when (Ba+Sr)/Ti ra tio is 1:1  The compositionally graded BST thin film has large dielectric constant compared to conventional BST thin film[23 26]. Ba 0.5 Sr 0.5 TiO 3 is the most commonly investigated material, since it has the highest dielectric constant at room temperatu re. It has been reported that by varying the oxygen pressure during sputtering, Ti incorporation in the films were varied and films with (Ba+Sr)/Ti of 0.93 achieved the maximum tunability . Its also observed that the films grown at higher oxygen par tial pressures in RF sputtering has excess Ti incorporation which results in lower tunabilities but higher quality factors. Crystalline Structure Crystalline BST films are usually obtained at relatively high substrate temperatures  BST film deposited at high temperatures, low dielectric constant interlayer is formed and specific grain structures are formed that cause decrease in dielectric constant and high leakage current  The study shows that a metastable phase nucleated at around 500 600 C at the interface plays a major role in the crystallization process. Films deposited on Pt surface require sufficiently lower
14 temperatures to produce crystalline film. Its shown that textured film results in higher tunabilities . An alternative approach is to grow amorphous BST films at low temperature s and then crystallize them in a post annealing process. Dielectric constant of polycrystalline film is found to be lower than the epitaxial film s. Microstructure The grain size profoundly affects the dielec tric properties of the BST thin films The grain size decreases and grain boundary increases, as thickness of the film reduces. Hence thinner film has higher leakage current density and lower dielectric constant compared to thicker films . It has been reported that increased dielectric constant and increased temperature variation is observed with increased grain size [ 31 ]. Surface Morphology As the O 2 /(O 2 +Ar) mixing ratio (OMR) is increased during rf sputterin g, the surface roughness of BST thin film s increases and this largely affects the l eakage current characteristics of BST thin films  When OMR is increased, the diffusion energy of the sputtered atoms is probably reduced, as collisions between sputtered atoms and oxygen atoms increase and mea n free path is shorter. Surface roughness of BST thin film is also dependen t on bottom electrode materials. Film Thickness BST thin film s behave differently than bulk BST The dielectric constant of BST thin film is much smaller than bulk BST It is shown that dielectric constant decreases with decrease in thickness of BST thin film [33, 34] The thickness dependence of the dielectric constant varies with the substrate temperature and the grain size effect. The thickness dependence of permittivity is explai ned in Schottky barrier model. In this model, the variation in apparent capacitance with bias is explained via a voltage
15 dependent interfacial layer and capacitance in series with the capacitance of the bulk of the film, whose permittivity is taken to be b ias dependent. Electrode Materials E lectrode materials must meet certain requirements such as high conductivity, sufficient resistance against oxidation, good adhesion to BST and interfacial smoothness to reduce leakage current and increase capacitance. I n addition, factors such as grain size distribution, crystalline orientation, interface and surface structures significantly influence the performance of electrode materials. Metal electrodes such as Pt, Ir and Ru and conducting oxide electrodes like RuO 2 IrO 2 and SrRuO 3 ha ve been investigated for BST varactor s [35 4 1 ] Among these, platinum is promising due to its good oxidation resistant at higher temperatures and high conductivity. But on the other hand, Platinum has some drawbacks like hillock formatio n at higher temperatures, poor adhesion and difficulty with patterning The literature review is summarized in Table 2.3. Table 2.3 Summarized Literature Review of BST Capacitors Parameters Orlando Auciello[ 4 2 ] Amir Mortazawi[ 4 3 ] Y ork R.A[ 18 ] Pond[ 4 4 ] Y o rk R.A[ 4 5 ] Substrate Platinized Silicon (HR) Platinized Silicon (HR) Platinized Silicon MgO (Interdigital) Platinized Sapphire Deposition Method Sputtering MOCVD Sputtering PLD Sputtering Chamber Pressure 22 50 mTorr 50 mTorr 200 mTorr 45 mTorr Temp erature 650 C 640 C 550 C 750 C 700 C Roughness(RMS) 2 3 nm 5 nm Tunability 72% @ 1 MHZ 3.4 : 1 @ 45 MHZ 4:1 1.7 :1 @ UHF 9.38 : 1 @ 1 MHz Loss tangent 0.013 10 30 Q Factor 40 80 50 Breakdown Voltage 1.5 MV/cm 1.4 MV/cm 1 MV/cm 80 kV /cm 4.7 MV/cm
16 2.5 Summary Tunable microwave applications require high speed, low loss at microwave frequencies and an electric field dependent dielectric permittivity. All these are provided by ferroelectric materials in the paraelectric regime. In part icular BST has been extensively investigated for tunable microwave applications such as tunable filters, phase shifters due to its large field dependent dielectric permittivity and composition dependent Curie temperature. BST varactor is fast with lower lo ss at microwave frequencies, higher power handling capacity and lower cost as compared to other technologies like semiconductor and MEMS. Larger electric fields at low bias voltages, device size and weight reduction, and small variation in dielectric const ant with temperature make BST thin films far more attractive than ceramic BST. However, there are several factors affecting the dielectric properties of the BST thin films and careful control of each is needed to achieve high tunability and low dielectric loss.
17 C HAPTER THREE FABRICATION AND DEVI CE STRUCTURE 3 .1 Deposition Techniques A bett er understanding of BST thin film deposition using different techniques and the effect of substrates on the BST film properties is needed t o exploit its advanta ges over semiconductor and MEMS technologies for tunable microwave devices In this work, BST thin film has been deposited by pulsed laser deposition, sputtering and sol gel. This chapter discusses the working principle of each of these deposition techniqu es along with a brief description of the systems at University of South Florida (USF). 3.1.1 Pulsed Laser Deposition Pulsed laser deposition (PLD) is an important technique for thin film deposition. The laser ablates the target and thin film is formed b y condensing the ablated material, with or without intermediate reactions with the ambient medium, on the substrate surface. The fundamental principle of PLD technique is the interaction between the laser and solid surface. This interaction involves (1) ab sorption of photon energy by the target and heat conduction, (2) surface melting of oxide target and (3) evaporation and ionization of oxide target. Figure 3.1 is a schematic representation of the laser target interactions. Shining laser on the solid surfa ce leads to conversion of the
18 electromagnetic energy into electronic excitation and then into thermal, chemical and mechanical energy which cause evaporation, ablation, excitation and plasma formation. When the surface of the target is exposed to the extr emely short laser pulses (20 ns), photons are adsorbed on the surface and the surface temperature of the target material increases rapidly to a few thousands of degrees celsius The bulk of the target material however, remains at room temperature while a m olten layer is formed on the surface. The mass transport involved in this very short vaporization process is considerable and creates a burst of evaporants. These are in turn deposited onto the substrate thereby producing a thin film with the same composit ion as of the target surface. A plume of evaporants is formed during the vaporization process. Energetic species such as atoms, molecules, electrons, ions, clusters, small solid particulates and molten globules constitute the evaporants. This plume widens rapidly from the target surface with high forward directed velocity distribution. There is a tight confinement of the evaporants within the plume and hence when the evaporants condense on the substrate, there is very little contamination. Better quality fi lms are further produced by the secondary interaction between the plume and the laser beam which increases the plasma temperature as well as the energy of the evaporants thereby increasing their surface mobility.
19 Figure 3.1 Schematic Representation of Laser Solid Interactions The laser beam solid interaction is found to be wavelength dependent Using the lasers available in a broad spectrum of wavelengths, pulse energies and pulse width, thin films of almost any material can be deposited by PLD techn ique. T hin films of compounds such as oxides, semiconductors, ferroelectrics, polymers and ceramics have
20 been deposited using PLD Technique. The pulsed laser deposition is a simple technique and has promising advantages The stoichiometry of the target is preserved in the deposited film. The technique supports a dynamic range of deposition pressures as compared to other deposition processes. F ilms can be deposited at high oxygen pressure s and crystallization of the deposited film is possible at relatively l ow substrate temperature s A reactive gas such as oxygen binds the volatile species to the substrate and helps to preserve the film stoichiometry and the high surface mobility involved during deposition produces well oriented and crystalline films. Also ma terials having high melting temperatures can be deposited easily if they strongly absorb the laser light. The PLD technique is fast, has easy operation and is inexpensive as there are only a few control parameters. The main drawback of the system is with p articulate formation. Particles are formed by laser induced subsurface boiling, shock wave recoil and exfoliation. Subsurface boiling is caused by the superheating of the subsurface layer before the surface layer reaches its evaporation point and thus caus es the surface to break into large micron sized globular particles due to on the expansion of the subsurface layer. This phenomenon is common in materials having low melting and boiling points. Shock waves are caused by the rapid expansion of the plume and carry liquid droplets from the target surface to the substrate where they get condensed and form particles. The process of exfoliation is similar to shock wave. Fragile structures or irregularities on the target surface caused by the laser ablation are di slodged and transported to the substrate via the plume. The use of highly dense and smooth target surfaces and proper positioning of the substrate have been found to be effective against particulate formation. Care should also
21 be taken to rotate the target to avoid crater formation and to obtain uniform erosion. T he pulsed laser deposition system at University of South Florida is shown in Figure 3.2. Fig ure 3.2 PLD System at USF The PLD system at the University of South Florid a (USF) consists of a laser generating unit, a six way cross vacuum chamber, vacuum pumps, target and substrate holder, optics and other relevant equipment. Its such as leak valves, mass flow and temperature controllers. The schematic of the PLD system is given in Figure 3.3. The system is capable of depositing multi layer structures composed of up to four different materials at various substrate temperatures up to 650C, in HV ( = 10 6 Torr) or with partial pressure of nitrogen, oxygen and argon. There are four target holders mounted on a disk, which can be rotated to bring the desired target for laser irradiation. Each target holder in turn can be rotated around its center during laser ablation. The laser beam is directed into the chamber and onto the targ et at an angle of 45 with high power UV optics to scan across the target surface. The target material surface forms a rapidly
22 expanding plume, which travels perpendicular to the target and is deposited on the substrate. Figu re 3.3 Schematic of the PLD System at USF 3.1.2 Sputtering Sputtering is one of the most widely used technique s for deposition of BST thin films and has reached a high degre e of maturity It promotes uniformity good adhesion to the substrate, a smoother s urface and low er deposition temperature. The sputtering technique involves material removal by bombarding the surface of the target with accelerated ions. The sputtering system consists of a deposition chamber connected to a high vacuum pumping system. Ins ide the chamber, the target to be sputtered is bonded on to a one circular electrode which acts as cathode and is placed in front of substrate to be coated. Gas is introduced and maintained at a suitable pressure in the chamber which is
23 ionized by a suitab le voltage applied between the two electrodes. The ions get accelerated and bombard the target and cause the disintegrated target material to leave the target and get deposited on the substrate which is placed in the anode position. Inert gases produce a film that has the same composition as the target while mixture of inert and reactive gases produces a film that is a combination of the reactive gas and target species. Various configurations like diode sputtering, triode sputtering and magnetron sputterin g and various modes of operation are available for a sputtering system. Diode sputtering is the oldest and simplest system of all Erosion of the target is uniform in this system, but the deposition rate is low and the substrate gets overheated Triode sys tem provides better deposition rate due to the addition of hot filament which supplies electrons for supporting the discharge, but reactive gas can rapidly destroy the filament. In magnetron sputtering, deposition rate is further increased by introduction of static magnetic field. Among the various configurations available, the most commonly used is magnetron sputtering with circular target. Magnetron sputtering works on the principle of magnetic confinement of charged particles thereby increasing ionizati on within regions close to the target. In such a situation, the electrons are trapped within the magnetic field flux lines and ions are accelerated towards the target. The application of high voltage initiates a plasma between the cathode and the anode at pressures in the mTorr range. The ion bombardment of the cathode causes secondary electrons to be emitted and sustains the plasma. The magnetic field is positioned parallel to the cathode surface and with proper orientation of the magnetic field polarity, the secondary electrons are confined within the drift loop. The
24 increased confinement forms a region of high plasma density near the cathode which in turn causes a high current low voltage discharge. Low operating pressures can be achieved due to the high plasma density and results in less gas scattering between the sputtered atoms and the background gas. The presence of reduced scattering allows an increase in the kinetic energy of the sputtered atom increasing the atom transport from the cathode to the su bstrate. The cathode can be sputtered at a high rate owing to the high discharge current and thus deposition rates of several microns/minute can be obtained. However the maximum deposition rate in a magnetron device is found to be dependent on the ability to cool the cathode. Insulating compounds are sputtered by RF sputtering as the use of DC power leads to a charge accumulation at the surface of the target and results in electrical discharging or arcing. During arcing, macro molecules of target species ar e transported to the substrate and form poor quality films. Ionization of the gas in RF sputtering is caused by the electrons which are more mobile than ions and hence respond and oscillate with the applied frequency. The deposition rate however, is reduce d in RF sputtering since the target is negative biased for only half cycle. Our laboratory at USF is equipped with a CMS 18 sputtering system which is used for depositing a variety of oxides and metals. It has three magnetron guns one is dedicated for RF sputtering while the other two are used for DC sputtering. The chamber can be pumped to as low as 10 8 T orr due to the c ryopump to the tool High quality films are obtained as there is a precise control of different process parameters. The CMS 18 s putteri ng s ystem at the lab in USF is shown in Figure 3.4.
25 Figure 3.4 Sputtering S ystem at USF 3.1.3 Sol gel Sol gel is the simplest thin film coating technique involving the hydrolysis and polycondensation of relevant molecular precursors. Liquid material c omponents are mixed in adequate proportion to form a sol. The sol is then transformed into a gel by a hydrolysis reaction. Following hydrolysis is condensation of the gel, during which there is shrinkage of the gel due to cross linkage. The gel then underg oes aging or loss of water on a solvent and any byproducts that are produced during hydrolysis and condensation. Finally t he gel is transformed by heat treatment in to a powder or film material through the process of densification To obtain a thin film the synthesized solution is spun onto a substrate and then subjected to the heating treatment. The sol gel method has been used to prepare BST thin films of different composition. The precursor solution content and formulation determines the baking tempera ture and also affects the crystallization temperature. The primary advantages of sol gel are the lower temperatures required for chemical reaction and the purity of the final product. However, the sol gel
26 process is very sensitive to any change in the prec ursor chemistry, pH and temperature which can significantly affect the final product structure. The main objective of the present research on the sol gel preparative research remains to find new precursors and precursor solutions formulation so that well c rystallized, crack free BST thin films can be obtained at lower temperatures. Ba and Sr actetates are the most commonly used Ba and Sr sources. The flow diagram to prepare BST thin films is shown in Figure 3.5. The BST sol gel solution was obtained from Mitsubishi Materials. The advantages and disadvantages of sol gel, sputtering and pulsed laser deposition techniques are given in Table 3.1 Table 3 .1 Different D eposition Techniques and T heir A dvantage s and D isadvantages Method Advantages Disadv antages Sol Gel Inexpensive, low capital investment Rapid sampling of materials Quickly produce new materials Phase control Composition Control Morphology Scalability Sputtering Uniformity Scalability Low growth Temperature Standard IC Processing Residual Stresses Point defect Concentration Pulsed Laser Deposition Non equilibrium deposition Highly stoichiometric films Quickly produce new materials Particulate formation Uniformity Morphology 3 2 Capacitor Configurations Two types of device structures namely parallel plate (vertical) and i nterdigital (planar) capacitors are available for BST varactors. The device schematic of both types of structures is shown in Figure 3.5. The structures themselves are discussed in the following sections.
27 3 2 .1 Parallel Plate Capacitors P arallel plate capacitors are very attractive for microwave and mill imeter wave applications as they have higher tunability and lower bias control voltages. Parallel plate structure is formed as a metal insulator metal structure by depositing the BST film directly on to the bottom electrode and top electrodes are deposited o top of BST. The distance between the electrodes is basically the BST film thickness and much shorter than the spacing between the fingers in interdigital structures which provides lower control voltages. The thickness of the BST can therefore be modifie d to obtain the desired control voltage and power handling capacity. The electric fields are far better confined in the film than interdigital structures and hence o ffer higher tunability 3 2 .2 Interdigital Capacitors Interdigital capacitors are simpler to fabricate compared to parallel plate structures which require more masking process steps Interdigital capacitors are fabricated by directly depositing BST films on the substrate followed by top interdigital electrode metallization. Interdigital capacit ors avoid bottom electrode issues like chemical and mechanical stability under BST film growth processing conditions and also eliminate the microstructural degradation induced in the dielectric electrode interface. They can handle much higher breakdown vol tages. However, t hey suffer from reduced tunability and higher control voltages due to large fringing effect in the air and higher spacing between the fingers. Smaller spacings between the fingers have been found to increase the tunability at lower voltage s. E asier fabrication steps in interdigital capacitor
28 make it attractive for low cost circuit applications and higher breakdown voltage makes it desirable for high power phased array antenna applications Figure 3.5 Schematic of Interdigital and Parallel Plate Capacitors 3.3 Fabrication of BST Capacitor For e lectrical characterization at RF and microwave frequencies, both interdigital and parallel plate capacitors were fabricated in coplanar waveguide (CPW) transmission line configuration, since it provides easy integration. It offers simplified bias schemes since via holes are not required in this configuration and lower dispersion loss in CPW line leads into a high quality factor. For parallel plate capacitors, BST thin fil m was deposited on a Pt/TiO 2 /SiO 2 /Si substrate. Since platinum (Pt) is promising due to its good oxidation resistan ce at higher temperatures and high conductivity it was chosen as bottom electrode. Figure 3.6 shows the parallel plate CPW capacitor structu re that was fabricated The mask for the parallel plate capacitor was designed using AUTOCAD and is shown in Figure 3.7. In t he CPW parallel plate capacitor design, the BST thin film lies below the signal electrode of the CPW line only and the ground elect rodes of the CPW line lie directly on top of the bottom electrode. Interdigital Capacitor Parallel Plate Capacitor Substrate BST Electrode
29 Figure 3.6 Schematic of the CPW Parallel Plate BST Capacitor The process flow for fabrication of BST parallel plate capacitor is shown in Figure 3.8a. As the parallel plate capacitor is fabricated in a CPW structure, the bottom electrode, in this case, platinum was not patterned. The BST film was deposited over the blanket Pt bottom electrode using suitable deposition techniques such as pulsed laser deposition sputtering or sol gel. After the deposition of BST thin film, BST film was selectively etched using lithography technique. Buffered oxide etch to DI water solution of ratio 1:1 was used for etching BST. Lift off technique was used for the patterning of t op electrode. Gold or Platinum was used as the top electrode and it was deposited using e beam evaporation. The tuning capacity of the parallel plate structure is directly proportional to the tuning capacity of the BST thin film as the electric fields are effectively confined between the top and bottom electrode. Top Electrode (Au) Bottom Electrode (Pt) BST Si SiO2 TiO2 BST Pt
30 Figure 3.7 Mask Scheme for BST Parallel Plate Capacitor Interdigital capacitors are easily fabricated than parallel plate capacitors and require fewer masking steps as shown in Figure 3.8b. The BST thin film was blanket deposited directly on the substrate using PLD, sputtering or sol gel. MgO was used as the substrate due to its high insulating properties. The top electrode, gold was then deposited using e beam evaporation and patterned using lif t off technique to form the interdigital metal fingers and signal line. The interdigital capacitor fabrication process required only one mask step and hence is highly suitable for low cost circuit applications. The elimination of the bottom electrode preve nts microstructural degradation at the dielectric electrode interface. However, the electric fields are not well confined within the BST film
31 and hence the tuning capacity of this structure is lower than the tuning capacity of the BST film. Figure 3.8 Process Flow for Fabrication of BST (a) Parallel Plate and (b) Interdigital Capacitors 3 4 Summary The BST capacitors have been fabricated as both parallel plate and interdigital capacitors. The BST film has been deposited in thi s research by different deposition techniques namely pulsed laser deposition, sputtering and sol gel. Pulsed laser deposition reproduces the stoichiometry of the target in the deposited film and can be used to deposit any material. The major drawback of PL D is particulate formation on the deposited film. BST thin film Deposition Bottom Electrode Deposition Top Electrode Deposition Lift off BST thin film deposition and Etch (b) (a) Top Electrode Deposition Lift off
32 RF magnetron sputtering is used to deposit insulating films as the application of DC power causes charge buildup on the target surface. Magnetron sputtering uses magnetic field to confine and sustain the pl asma producing high current discharge at lower bias voltages. RF sputtering however, has lower deposition rates as the negative bias in applied to the target only for the half cycle. Sol gel involv es the hydrolysis and polycondensation of relevant molecula r precursors and is by far the simplest thin film coating technique but suffers from inadequate composition and phase control. BST thin films for parallel plate and interdigital capacitors have been deposited using the above deposition techniques. Parallel plate capacitors offer higher tunability at lower control voltages but involve extensive masking steps. Interdigital capacitors, on the other hand have good ease of fabrication but lower tunability due to electric field fringing. The fabrication of both c apacitors is application driven.
33 CHAPTER FOUR STRUCTURAL CHARACTER IZATION 4.1 Experimental Matrix of Pulsed Laser Deposition By varying the deposition conditions in PLD, structural properties like roughness, crystallinity, grain size, thic kness and surface morphology can be effectively controlled. These structural properties have a strong influence on electrical properties of BST thin film and our purpose is to correlate the structural and dielectric properties of laser deposited BST thin f ilm. The deposition parameters that influence the film properties are substrate temperature, laser energy, laser frequency, oxygen pressure and distance between substrate and target. In this work, deposition conditions have been optimized to achieve the hi ghest tunability and low dielectric loss. The effect of deposition conditions o n structural properties has been discussed below. 4.1.1 XRD Analysis of PLD BST Films on Pt/TiO 2 /SiO 2 /Si XRD analysis of the pulsed laser deposited BST thin films was done in order to study the effects of temperature, oxygen pressure and substrate to target distance on the structural orientation of the thin films. The XRD spectra of the BST thin films deposited under three different conditions of temperature on Pt/TiO 2 /SiO 2 /Si are shown in Figure 4.1. The BST films were deposited at temperatures 450C, 550C and 650C at the same oxygen pressure of 350mTorr. At 450C, the BST film appears to be amorphous and BST
34 peaks were not observed. As the temperature is increased to 55 0C, the crystallization process occurs and films change from amorphous state to BST (110) oriented films As can be seen from the figure a reasonably well crystallized monophasic film is obtained at a temperature as low as 5 5 0 C. As the temperature is in creased, the BST thin films change from being single crystalline to polycrystalline. There is no preferential orientation observed in the XRD pattern and all the films deposited are polycrystalline with (110) as the major peak. As the substrate temperature is increased the intensity of (110) peak is also increased indicating the improvement in the crystallinity. Table 4.1 Intensity of A ll the Diffraction P eaks at 450C, 550C and 650C Sample No Plane Braggs Angle [ ? ] Intensity [cts] FWHM [Th.] d spacing  110 31.9376 29.37 0.2952 2.80225 B(550C) 211 56.8997 6.08 0.3542 1.61829 100 22.3725 9.38 0.2362 3.97393 110 31.9607 89.39 0.2362 2.80027 C(650C), (450mTorr) 211 56.8637 11.90 0.4320 1.61789 110 31.8696 109.0 4 0.2066 2.80807 200 46.3945 94.44 0.2160 1.95558 210 51.3305 1.44 0.5760 1.77852 D(650C), (250mTorr) 211 56.7997 17.84 0.3600 1.61956 The XRD spectra of the BST films deposited at two different oxygen pressures are shown in Figure 4.2. The BST films were deposited at oxygen pressures of 250mTorr and 450mTorr with all the other parameters maintained constant. It is seen from Figure 4.2, that both the films are polycrystalline and exhibit X Ray reflection from the BST (110) and (211) planes. At an oxygen pressure of 250 mTorr, X Ray reflection from the BST (210) plane is seen in addition to the reflections from BST (110) and (211) planes. The strongest reflection is obtained from the BST (110) plane which indicates that the
35 preferred orientation of the BST film is in (110 ) direction. At an oxygen pressure of 450mTorr, in addition to the X ray reflections from BST (110) and (211) planes, X Ray reflection from BST (100) plane is also seen. Fig ure 4. 1 X ray D iffraction P atterns Fig ure 4. 2 X ray D iffra ction P atterns S howing the E ffect of D eposition S howing the E ffect of O xygen P ressure T emperature on the C rystallinity on the C rystallinity of BST T hin F ilm s of BST T hin F ilm s As the oxygen pressure is increas ed from 250mTorr to 450mTorr at constant temperature and laser parameters, the intensities of all the reflections increase. This could be due to the increased oxygen concentration allowing denser films to be formed. The intensities of the diffraction peaks at 250mTorr and 450mTorr of oxygen pre ssure are tabulated in Table 4.1. Interplanar spacing or d spacing of BST thin films is also
36 included in the table which gives the distance between two adjacent parallel atoms with the same miller indices. The avera ge grain size of the films determined using Scherrers equation indicates that increase in substrate temperature resulted in grain growth from 5.1 to 6.37 nm. Scherrers equation is given as ? ? cos 94 0 FWHM D ? ( 4.1 ) where D is the grain size, ? is the wavelength of X ray radiation used (?=1.54060 A ) and ? is the peak position angle. 4.1.2 AFM Analysis of PLD BST Films on Pt/TiO 2 /SiO 2 /Si Atomic Force Microscopy (AFM) was used to perform the surface characterization of the BST thin films deposited at different substrate to target distances laser energ ies and oxygen pressures. All the films were deposited on Pt/TiO 2 /SiO 2 /Si at a substrate temperature of 650C. The two dimensional surface plots of BST films are shown in Figure 4.3. Since it was found out that shorted devices were obtain ed due to particulate formation films were deposited at different conditions to avoid particulate formation and to reduce the roughness. The deposition conditions, RMS roughness and average surface roughnes s of all the four films was estimated from the roughness analysis in AFM and is shown in Table 4.2. Figures 4.3 (a) and 4.3 (b) are the surface plots of BST thin films deposited at laser energies of 450mJ/cm 2 and 250mJ/cm 2 + respectively with an oxygen pre ssure of 250mTorr The substrate to target distance is 4.5 cm for the samples in (a) and (b)
37 Table 4. 2 Variation of Average Grain Size and Surface Roughness with Different Parameters Parameters Sample Oxygen (mTo rr) Distance (cm) Laser Energy (mJoules/cm2) RMS Roughness(nm) Average Roughness(nm) 4.3(a) 250 4.5 450 2 5 387 18.991 4.3(b) 250 4.5 250 20.671 17.012 4.3(c) 250 5.2 250 21.428 17.140 4.3(d) 150 5.2 250 20.113 16.400 (a) (b) (c) (d) Figure 4.3 AFM Micrographs of BST Thin Films
38 Figures 4.3(b) and 4.3(c) represent the films deposited with d ifferent target to substrate distance while other parameters are kept constant. Figure 4.3(b) is the surface plot of BST thin film when the substrate to target distance is 4.5 cm and Figure 4.3(c) is the surface plot when the substrate to target distance i s 5.2 cm. The surface plot in Figure 4.3(b) shows relatively large r grains are seen on the surface. From Figure 4.3(c), the grains are smaller in size and grains on the surface are not well defined. The films appear smoother as the substrate to target dist ance is increased. The AFM images of BST thin films deposited under oxygen pressures of 250 mTorr and 150 mTorr are shown in Figures 4.3(c) and 4.3(d ). The BST films deposited at 250 mTorr oxygen pressure has well defined grains and is polycrystalline. At reduced oxygen pressures of 150 mTorr, the BST film does not have well defined grain boundaries The reduction in roughness and particulate formation was obtained by reducing the distance between substrate and target and laser energy and it led to successf ul fabrication of devices with good tunabil ity and high breakdown voltages 4.2 Experimental Matrix of RF Sputtering By varying the deposition conditions in sputtering, structural properties can be effectively controlled. The structural properties have a strong influence on electrical properties of BST thin film s The deposition parameters that influence the film properties are the substrate temperature, RF power, Gas pressure, Argon oxygen ratio and distance between the substrate and the target. The depo sition conditions have been optimized to achieve the highest tunability and low dielectric loss. The effect of deposition conditions o n the structural properties ha s been discussed below.
39 4.2.1 XRD Analysis of S puttered BST F ilms on Pt/TiO 2 /SiO 2 /Si XRD ana lysis of the sputtered BST thin films were performed to study the effects of temperature and Ar/O 2 ratio on the structural orientation of the thin films. The XRD spectra of the BST thin films deposited under three different temperature s are shown in Figure 4.4. The BST films were deposited at temperatures 550C, 600C and 650C and at an argon pressure of 25 mTorr. All the films exhibit X Ray reflection from the BST (100), (110) and (200) planes as can be seen from Figure 4.4. However, the BST film s deposit ed at 600 C are highly polycrystalline with well defined x ray reflections as n eeded for obtain ing high tunability The effect of temperature on the crystallinity of the BST films is very dynamic. On increasing temperature from 550C to 600C, the polycrys talline nature of the film is enhanced. O n further increasing the temperature, the intensity of (10 0) and (200) peak is increased indicating the improvement in the crystallinity in (100) orientation The intensity of all the diffraction peaks at 550C, 600 C and 650C are tabulated in Table 4.3 as shown below. Table 4. 3 Intensity of A ll the Diffraction P eaks at 550C, 600C and 650C Sample No Plane Braggs Angle [?] Intensity [cts] FWHM [Th.] d spacing  A(550C) 110 31.9701 31.07 0.2362 2.79947 100 22.3143 31.81 0.4723 3.98416 110 31.8168 22.35 0.4723 2.81261 B(600C), 200 45.6065 69.59 0.3149 1.98917 100 22.3143 31.81 0.4723 3.98416 110 31.8168 22.35 0.4723 2.81261 C(650C), 200 45.5199 62.07 0.3936 1.99275
40 Figure 4.4 X ray Diffraction Patterns Showing the Effect of Deposition Temperature on the Crystallinity of Sputtered BST Thin Film s The XRD spectra of the BST films deposited at different Ar/O 2 ratios are shown in Figure 4.5. The BST films were deposited at a temperature of 600C. It is seen from Figure 4.5, that the films deposited in argon are polycrystalline. On addition of oxyge n, films change from polycrystalline to single crystalline. At an Ar/O 2 ratio of 90/10, the X Ray reflection from only BST (110) plane is visible. At an Ar/O 2 ratio of 70/30, the preferred orientation of the BST film is in (110) direction. Sputtering is a mass and temperature dependent scattering process and hence is found to have a strong impact on the film composition. The growth pressure is found to influence the film stoichiometry as well Higher oxygen pressures are found to enhance the stoichiometric growth while 650 C 600 C 550 C BST(100) BST(100) BST(100) BST(11 0 ) BST(11 0 ) BST(11 0 ) BST(200) BST(200) BST(200)
41 higher sputtering or argon pressure impedes the stoichiometry. When oxygen mixing ratio is increased, there is a considerable reduction in the deposition rate s The intensities of the diffraction peaks at different Ar/O 2 ratios are tabulated in Table 4.4. Table 4. 4 Intensity of All the Diffraction Peaks at Different Ar/O 2 Ratios Sample Plane Braggs Angle [ ?] Intensity [cts] FWHM [Th.] d spacing  100 22.3143 31.81 0.4723 3.98416 110 31.8168 22.35 0.4723 2.81261 Ar 200 45.6065 69.59 0.3149 1.98917 Ar/O 2 90/10 sccm 110 31.8358 55.54 0.2362 2.81097 110 31.7780 27.22 0.4723 2.8 1595 Ar/O 2 70/30 sccm 200 46.5031 142.37 0.2755 1.95288 Figure 4.5 X ray Diffraction Patterns Showing the Effect of Ar/O 2 Ratios on the Crystallinity of Sputtered BST Thin Film s Ar Only Ar/O 2 of 90/10 BST(100) BST( 110 ) BST( 110 ) BST( 110 ) BST(200) Ar/O 2 of 70/30
42 4.2.2 AFM Analysis of S puttered BST F ilms on Pt/TiO 2 /SiO 2 /Si Atomic Force Microscopy (AFM) was used to perform the surface characterization of the sputtered BST thin films deposited at different Ar/O 2 ratios. All the films were deposited at a substrate temperature of 600C. The two dimens ional surface plots of the BST thin films deposited at 90/10 and 70/30 Ar/O 2 ratios are shown in Figure 4.6. The BST film s deposited at 90/10 Ar/ O 2 ha ve a surface roughness of 3.4 nm while the BST film s deposited at 70/30 Ar/O 2 ratio ha ve a surface roughne ss of 7 nm. The surface roughness increases with the oxygen content as the cation mobility on the growth surface reduces with higher oxygen pressures. Also, the sputtered BST thin films have a much lower surface roughness than the pulsed laser deposited BS T thin films. (a) (b) Figure 4.6 Surface View of BST Films Sputtered at Ar/O 2 Ratios (a) 90/10 (b) 70/30
43 4.2.3 XRD Analysis of S puttered BST F ilms on MgO, Al 2 O 3 and LaAl 2 O 3 For the fabrica tion of interdigital capacitors, BST thin films were deposited on three different substrates like MgO, Al 2 O 3 and LaAl 2 O 3 using RF sputtering. A very good control over the structural properties of the BST films deposited on these substrates has been achieve d. All the films were deposited at a substrate temperature of 650 C, RF power of 150 W and Ar/O 2 ratio of 90/10 sccm T he optimized XRD results are shown in Figure 4.7. Figure 4.7 X ray Diffraction Patterns of Sputtered BST T hin Film on MgO, Al 2 O 3 and LaAl 2 O 3 B ST/MgO BST/Al 2 O 3 BST/LaAl 2 O 3
44 As shown in Figure 4.7, epitaxial films were deposited on both MgO(100) and LaAl 2 O 3 (100) substrates and they were highly crystalline, whereas polycrystalline films were deposited on alumina substrates. The intensities of the diffraction peaks of BST films on different substrate s are tabulated in Table 4.5 Table 4.5 Intensity of All the Diffraction Peaks of BST on Different Substrates Sample Plane Braggs Angle [?] Intensity [cts] FWHM [Th.] d spacing  100 21.9797 3529.52 0.1574 4.04404 MgO A 200 44.7994 14176.20 0.3542 2.02311 100 21.8089 2057.97 0.2362 4.07532 LaAl 2 O 3 B 200 44.4467 17842.50 0.3149 2.03834 110 31.5592 173.99 0.1968 2.83498 Al 2 O 3 C 200 45.3074 33.74 0.6298 2.00160 4.2.4 AFM An alysis of S puttered BST F ilms on MgO and Al 2 O 3 The two dimensional surface plots of the BST films deposited at 90/10 Ar/O 2 ratios on both MgO and Al 2 O 3 substrates are shown in Figure 4.8. The BST film deposited on MgO has a surface roughness of 1.14 nm wh ile the BST film deposited on Al 2 O 3 has a surface roughness of 3.43 nm.
45 Sol gel BST Solution 500rpm x 3sec 2000 rpm x 20 sec Drying (Hot Plate 300 400 C/min) Crystallization (Furnace/RTA 600 800 C) BST Thin Films Repetition Sol gel BST Solution 500rpm x 3sec 2000 rpm x 20 sec Drying (Hot Plate 300 400 C/min) Crystallization (Furnace/RTA 600 800 C) BST Thin Films Repetition Figure 4.8 Surface View of BST Films Sputtered on MgO and Al 2 O 3 4.3 XRD Analysis of S ol gel D eposited BST T hin F ilms BST thin films were deposited by sol gel on MgO substrate s and the process flow for the deposition is shown in Figure 4.9. Figure 4.9 Flow Diagram to Prepare BST Thin Films Using Sol gel
46 Sol gel solution were bought from Mitsubishi inc., and the steps depicted in process flow was followed. The XRD spectra of the sol gel deposited BST films show that they are epitaxial and (100) dominated and its shown in 4.10. Annealing was done at 700 C for two hours in oxygen ambience. BST(100) BST(200) BST(100) BST(200) Figure 4.10 X ray Diffraction Patterns of Sol gel Deposited BST Th in Film on MgO 4.4 Summary The deposition conditions have a significant impact on the structural and electrical properties of BST thin films. Optimization of the deposition conditions for high tunability and low dielectric loss BST thin films has been acco mplished for both parallel plate and interdigital structures. The structural characterization of the BST thin films deposited using pulsed laser deposition, sputtering and sol gel has been performed by XRD and AFM. Pulsed laser deposited BST thin films on Pt/TiO 2 /SiO 2 /Si turn polycrystalline with increase in temperature. An improvement in crystallinity has been observed with increasing oxygen content with well defined grains boundaries. High laser energy and smaller substrate to target distance cause incomp lete surface coverage with cluster formation. Sputtered BST thin films Pt/TiO 2 /SiO 2 /Si have better crystallinity at
47 higher temperatures. Changes to the Ar/O 2 ratio affect the crystallinity of the films. Increasing oxygen concentration changes the films fro m being polycrystalline to monocrystalline and increases the surface roughness of the films. BST thin films were also deposited on MgO, Al 2 O 3 and LaAl 2 O 3 substrates by sputtering. Epitaxial films were obtained on MgO and LaAl 2 O 3 substrates while polycrysta lline films were obtained on Al 2 O 3 substrate. BST thin films were deposited by sol gel deposition method on MgO substrate and were found to be epitaxial.
48 CHAPTER FIVE ELECTRICAL CHARACTER IZATION Electrical characterization of BST th in film is necessary to integrate BST capacitors in RF and microwave applications. Polarization mechanism relates the variation of dielectric constant across the frequencies and variation in dielectric loss. The total polarizability is dependent on electro nic, ionic, dipole and space charge. The objective is to study the dependence of dielectric constant (tunability) and dielectric loss tangent on the applied field in RF frequencies, since they are the important factors for the realization of device. Tuna bility is defined as the ratio of maximum dielectric constant to its minimum value and is given as follows: v o o v o T ? ? ? ? ? : 100 (%) ? ? ? (5.1) where e 0 is the dielectric constant at zero bias voltage and e v is the dielectric constant at maximum bias voltage. The quality factor and loss tangent of BST thin film is related as s p m BST R C Q Q Q ? ? ? ? ? ? tan 1 1 1 (5.2) where, R p is the parallel resistance which accounts for BST losses C is the capacitance of the dielectric, tan d is the loss tangent and Q is the total quality factor and R s is the series resistance which accounts for metallic losses The quality factor is used to characterize
49 the losses in lumped circuit elements. Quality factor can be defined as the ratio of stored energy to the average energy dissipated in the system per cycle. Dielectric loss is attributed to the damping of the polarization mode by impurities. A nother important factor to be considered is the Figure of merit. The Figure of merit is given by ? ? ? ? tan o v o k ? ? (5.3) For tunable high frequency applications, the objective is to obtain high figure of merit, which can be achieved by h igh tunability and low dielectric loss. To attain th is objective, it is very important to have accurate measurements of the devices. 5.1 Measurement Set up The BST thin film capacitors were characterized using an Agilent 4287A LCR meter connected to a Jmi cro personal probe station and 150 m pitch Ground Signal Ground (GSG) probes from GGB Industries. The LCR meter was calibrated using a Short Open Load (SOL) calibration technique using a CS 5 standard calibration substrate from GGB industries Inc in the f requency range of 1 MHz to 300 MHz The schematic of the measurement setup is depicted in Figure 5.1. The LCR meter is connected to an Agilent Bias source. The calibration procedure is verified by measuring a known 50 O load. The frequency response of the m easured load is shown in Figure 5.2. The flatness of the load standard over the entire frequency range indicates the accuracy of the calibration. It was also verified that the measured effective dielectric constant and the characteristic impedance of the C PW lines have no/minimal variation over the entire frequency range.
50 Figure 5.1 Measurement Setup Used for BST Capacitors 20 30 40 50 60 70 80 0 50 100 150 200 250 300 Frequency (MHz) Load Impedance (O) Figure 5.2 Magnitude of Z for the Load Agilent 4287A LCR Meter Bias Test Fixture Microwave Probes DUT DC supply
51 5.2 Results and Discussion 5.2.1 Pulsed Laser Deposited BST Thin Films Parallel plate capacitors in various dimensions (of different capacitance values) were fabricated using conventional lithography techniques. These devices were measured to extract the tunability and loss tangent. Capa citance, parallel resistance, series resistance and quality factor w ere measured over the frequency range of 1 MHz to 300 MHz at different bias voltage levels as mentioned previously The series resistance (R s ) accounts for the conductor losses associate d with the parallel metal plates, while the parallel resistance (R p ) accounts for the losses in the BST dielectric. The dependence of permittivity with applied bias as a function of film thickness is shown in Figure 5.3. It is found that permittivity dec reases with film thickness at zero bias voltage. It is also observed that thicker films have a higher tunability at the expense of increased bias voltage. It is also worth a mention that the roughness of the BST film tends to decrease with increasing thic kness. This leads to an improve d dielectric quality of the film Hence tuning ratio for thicker films is increased A trade off for higher tuning and dielectric strength is the increase in loss tangent of the films as the thickness increases. Figure 5.4 sh ows the variation of quality factor with bias voltages at a particular frequency and this trend continues in the entire frequency range The breakdown voltage of the laser deposited BST thin films were 1.02 MV/cm.
52 30 50 70 90 110 130 150 170 -6 -4 -2 0 2 4 6 Bias Voltage(V) Dielectric Constant 75 nm 65 nm 55 nm Figure 5.3 V ariation of Permittivity with Applied Bias for Different Thicknesses 0 5 10 15 20 25 30 35 40 45 -6 -4 -2 0 2 4 6 Bias Voltage (V) Quality Factor 75 nm 65 nm Figure 5.4 Variation of Quality Factor with Applied Bias for Different Thickness es
53 A time dependent polarization manifests itself in the frequency domain as a dispersion of the permi ttivity as a function of frequency. As seen in Figure 5.5, there is small dispersion in the capacitance characteristic. It is shown in Figure 5.6 that Q factor decreases sharply beyond 10 MHz. This could be attributed to the high conductor losses of the to p and bottom electrodes which are about 0.12 m thick. 40.0 90.0 140.0 190.0 240.0 290.0 340.0 0 50 100 150 200 250 300 350 Frequency (MHz) Capacitance (pF) 0 Voltage 3 Voltage 5 Voltage Figure 5.5 Variation of Capacitance with Frequency for Different Bias Voltages 0 5 10 15 20 25 30 35 40 0 50 100 150 200 250 300 350 Frequency (MHz) Quality Factor Figure 5.6 Variation of Q Factor with Frequency
54 Computed tunab ility and loss tangent values of various PLD films with different thickness are summarized in Table 5.1. Table 5.1 Electrical Characteristics of Pulsed Laser Deposited BST Thin Films Sample Tunability Loss Tangent BST thickness Film A 2.4 : 1 0. 0 24 75 n m Film B 2.2 : 1 0. 0 244 65 nm Film C 1.8 : 1 0. 0 25 55 nm The devices fabricated initially by PLD were shorted. It was mainly due to particulate formation observed in the films which reduced the dielectric strength. Particulate formation was avoided b y increasing the distance between the substrate and target and reducing the laser energy It has also been verified by atomic force microscopy that particulate formation was tremendously reduced It has been reported that post deposition annealing of top e lectrode reduces t he dielectric losses and minimize s the contamination at the top Pt/BST interface Annealing at 550 C for 30 minutes in air was carried out for the BST capacitor s and it proved to be necessary to obtain a symmetric response for both posit ive and negative applied bias. The optimum conditions for depositing BST thin films using pulsed laser deposition as observed are tabulated in Table 5.2 below.
55 Table 5.2 Optimum Conditions for PLD BST Thin Films Deposition Parameters Values Substrat e Temperature 650C Distance between substrate and target 52 mm O 2 pressure 250 mTorr Laser Energy 250mJ Laser Frequency 10 Hz BST thickness 75 nm Top electrode Thickness (Pt) 120 nm Annealing Conditions 550 C, 30 minutes in air 5.2.2 RF Sputtered BST Thin Films The dependence of permittivity on applied bias as a function of Ar/O 2 ratio is shown in Figure 5.7. It is found that high field dependent permittivity was achieved at an Ar/O 2 ratio of 90/10. Tunability was achieve d in films that were highly crystalline. The roughness of the sputtered BST film s was less. The breakdown voltage of the sputtered BST thin film was 2.1 MV/cm. This improve d t he dielectric quality of the film by having a higher breakdown voltage compared to the films dep osited by pulsed laser deposit ion There was not significant change in roughness with two different Ar/O 2 ratio, but there was a change in tunability. The change in tunability might be due to change in stoichiometry of the BST thin film s as the film compo sition can be changed by
56 increas ing the oxygen flow. Figure 5.8 shows the variation of quality factor with bias voltages at a particular frequency and this trend continues for the entire frequency range 50 70 90 110 130 150 170 190 210 -6 -4 -2 0 2 4 6 Bias Voltage(V) Dielectric Constant 90/10 -Ar/O2 Ratio 70/30 -Ar/O2 Ratio Figure 5.7 Tunability of Sputtered BST Thin Films at Different Ar/O 2 Ratios 0 10 20 30 40 50 60 70 -6 -4 -2 0 2 4 6 Bias Voltage (V) Quality Factor 90/10Ar/O2 Ratio 70/30Ar/O2 ratio Figure 5.8 Variation of Q Factor with Applied Bias for Ar/O 2 Ratios
57 Deposition rate decreases with reduced argon flow and increased oxygen flow because of the relative decrease of argon species contri buting to the sputter yield. It agrees with the literature that dielectric constant decreases with decrease in thickness. Films with 90/10 ratio had a higher tunability (2.8:1) than with 70/30 ratio (1.5:1). Figure 5.8 shows the variation of quality factor with bias voltages. The capacitance across the frequency at different bias voltages is shown in Figure 5.9. The typical quality factor characteristics across the frequencies at zero bias voltages are presented in Figure 5.10. Loss tangent of the sputtered BST thin films were 0.01. 15.0 35.0 55.0 75.0 95.0 115.0 135.0 0 50 100 150 200 250 Frequency (MHz) Capacitance (pF) 0 Voltage 3 Voltage 5 Voltage Figure 5.9 Variation of Capacitance with Frequency for Different Bias Voltages
58 0 10 20 30 40 50 60 70 80 0 20 40 60 80 100 120 140 160 180 200 Frequency (MHz) Quality Factor Figure 5.10 Variation of Q Factor with Frequency Tunability was observed only in crystalline films and smooth surface ro ughness on sputtered BST thin film s. It shows that sputtered films had good dielectric strength compared to pulsed laser deposited films. The optimized deposition conditions are summarized in Table 5. 3 below. Table 5. 3 Optimum Conditions for Sputtered BST Thin Films Deposition Parameters Values Substrate Temperature 650C RF power 150 W Total pressure 25 mTorr Ar/O2 Ratio 90/10 sccm Laser Frequency 10 Hz BST thickness 75 nm Top electrode Thickness (Au) 120 nm
59 5.3 Summary BST parallel plate capacitors were fabricated and the tunability and loss tangent of the BST capacitor was extracted with capa citance, parallel resistance, series resistance and quality factor measured over the frequency range of 1 MHz to 300 MHz at different bias volt age levels. For pulsed laser deposited BST thin films, thicker films had an increased permittivity at zero bias voltage and higher tunability at the expense of increased bias voltage. However loss tangent of the thicker films was also found to increase Th e capacitance characteristic was observed to have s mall or negligible dispersion while Q factor decrease d sharply beyond 10 MHz and is attributed to the high conductor losses Particulate formation during t he initial PLD of BST thin films r esulted in short circuited devices and was alleviated by depositions at increased substrate to target distance and reduced laser energy. Symmetric positive and negative bias response was obtained with annealing the BST capa citor The dependence of permittivity with applie d bias as a function of Ar/O 2 ratio was evaluated for sputtered BST thin films. H igh field dependent permittivity was achieved at higher argon to oxygen concentration and highly crystalline films only were found to have t unability The lower roughness of t he sputtered BST film resulted in films having a higher breakdown voltage than the pulsed laser deposited film.
60 CHAPTER SIX INTERDIGITAL CAPACIT OR 6.1 Design of Interdigital Capacitor The Interdigital capacitors were designed in a coplanar waveguide topolo gy. The focus of this part of this research is to design and investigat e an Interdigital capacitor using BST thin films A n electromagnetic field simulator (ADS Momentum) was used to accurately predict the behavior of the capacitor from 1 15GHz. In the designs presented, the coplanar waveguide (CPW) transmission line used as the feed line designed for a 50O characteristic impedance. The center conductor and slot width of the CPW line was designed to be 120 m and 53m respectively using the Linecalc f eature in ADS The ground plane width was chosen to be 250m. A thru reflect line (TRL) calibration technique wa s used for on wafer measurements of the capacitors The calibration standards include a thru line (1000m long), reflect lines and two delay lin es (4250 and 9000m long). 6.1.1 Capacitor Simulation Simulations we re performed from 1 15 G Hz using Agilents ADS Momentum The interdigital capacitors are simulated on an MgO substrate, 500m thick with an er of 9.8 and a loss tangent of 0.001 BST t hin film of 0.5m thick with er of 250 was defined as the dielectric layer. A conductor thickness of 0.7 m and was used for the simulation Figure 6.1 shows the momentum layout for a n interdigital capacitor. The variation in S
61 parameters for tunability of 2:1 was simulated by assigning er values of 250 and 125. The purpose of the interdigital capacitor design is to extract the dielectric constant of BST thin film at different bias voltages. The number of fingers, finger width, and finger length of the inte rdigital capacitor was arbitrarily chosen based on literature survey [ 43 ,44 ]. Figure 6.1 Layout of an Interdigital Capacitor with CPW T opology The simulated response of the interdigital capacitor is shown in Figure 6.2, Figure 6.3 and Figure 6.4. For BS T film thickness of 0.5 m, finger length of 200 m, finger spacing of 25m, and finger width of 37m, change in dielectric constant of BST thin film was observed which causes the change in capacitance ad hence a variation in S parameters as shown in Figure 6.2, 6.3 and 6.4. The v alue of the capacitance can be extracted from S parameters using the input impedance of the device.
62 in Z j C ? 1 ? (6.1) -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 1 3 5 7 9 11 13 15 Frequency (GHz) S11(dB) BST( er -250 ) BST (er -125) Figur e 6. 2 EM Simulation Plots of S 11 dB as a Function of Frequency -25.00 -20.00 -15.00 -10.00 -5.00 0.00 1 3 5 7 9 11 13 15 Frequency (GHz) S21(dB) BST( er -250 ) BST (er -125) Figure 6.3 EM Simulation Plots of S21 (dB) as a Function of Frequency
63 -60.00 -40.00 -20.00 0.00 20.00 40.00 60.00 80.00 100.00 1 3 5 7 9 11 13 15 Frequency (GHz) Phase of S21, deg BST( er -250 ) BST (er -125) Figure 6. 4 EM Simulation Plots of S21 P hase (deg) as a Function of Frequency 6.2 Capacitor Measurements Both MgO and alumina are considered as substrate for interdigital capacitors, due to their insulating and low loss properties. BST thin film of 100 nm thickness was deposited by sputtering on both substrates and gold of 700nm thick was deposited as top electrode and l ift off was done to define the top electrodes. The mask that was designed for the top electrode and TRL calibration standards is shown in Figure 6.5. The measured S parameter response is shown in Figure 6.6, Figure 6.7 and Figure 6.8. For the finger spa cing of 25 m and BST thickness of 0.1m, poor tunability was observed in the measurements. The dielectric constant of BST is controlled by the effective electric field inside the BST material. Very high spacing between the fingers compared to the dielectri c thickness causes the lower electric field in the BST thin film which leads to poor tunability. Figure 6.6, Figure 6.7 and Figure 6.8 depicts the microwave performance of the designed interdigital capacitors on 0.1m BST thin film.
64 Figure 6. 5 Mask Sc heme for BST Interdigital Capacitor
65 Figure 6. 6 Measured R esponse of S 1 1 (d B ) as a Function of Frequency Figure 6. 7 Measured R esponse of S 21 (d B ) as a Function of Frequency
66 Figure 6. 8 Measured Response of S21 P hase (deg) as a Function of Frequency By increasing the thickness of BST in the interdigital capacitor for fixed finger spacing, tunability can be increased and is shown through simulation. For the deposition techniques like sol gel which produces relatively thick film, the design that is shown a bove can be used. It should also be noted that dielectric constant increases with respect to the thickness. To obtain the tunability and to reduce the required DC bias, spacing between adjacent fingers should be reduced as small as the lithography techniqu e permits. 6.3 Summary Design of i nterdigital capacitors from 1 15GHz in a coplanar waveguide topology was achieved using an electromagnetic field simulator (ADS Momentum ) The variation in S parameters for tunability of 2:1 was simulated by assigning e r values of 250 and 125 to the dielectric. Poor tunability was observed in the measured BST capacitor
67 due to its very low thickness of BST compared to finger width. Depositing thicker film will lead to good tunability and reduced DC bias can be achieved by reducing the finger spacing to the lithography limits.
68 CHAPTER SEVEN CONCLUSIONS AND FUTU RE WORK 7 .1 Conclusions The main contribution of this work wa s to characterize pulsed laser deposited BST thin films for microwave applications RF sputter deposition was also investigated in this research. The material properties of the deposited films were characterized using metrology tools such as profilometry, AFM and XRD analysis. The diffraction patterns from XRD analysis are attributed to cubic (perovskite) crystal system Parallel plate capacitors and Interdigital capacitors were fabricated and measured in the frequency range of 1 MHz to 300 MHz and 40 MHz to 15 GHz respectively. Dielectric constant, dielectric loss, tunability was extra cted from the measured data. The deposition conditions to achieve high tunability and low dielectric loss were optimized for both the deposition techniques. The correlation between optimized structural and dielectric properties of thin films was analyzed. Specific c onclusions based on each of the deposition methods are given below. 7 .1.1 Pulsed Laser Deposited BST T hin F ilms ? When the substrate temperature is increased beyond 550 C, the BST thin films change from being monocrystalline to polycrystalline.
69 ? Particulate formation was observed in BST thin films and led to shorted devices. By increasing the distance between substrate and target and reducing th e laser energy, particulate formation was avoided and reduced roughness was observed by atomic force mi croscopy. ? Annealing at 550 C for 30 minutes in air was carried out for the whole BST capacitor and it proved to be necessary to obtain a symmetric response for both positive and negative applied bias. ? The deposition conditions to achieve high tunability a nd low dielectric loss were optimized and tunability of 2.4: 1 and loss tangent of 0. 025 was achieved at 2 0 MHz. ? Decrease in dielectric constant from 160.8 to 127.8 at zero bias voltage was observed when BST thin film thickness was reduced from 75 nm to 55 nm. 7 1. 2 RF Sputtered BST T hin F ilms ? When the substrate temperature is increased beyond 550 C, the BST thin films change from being monocrystalline to polycrystalline. ? Films were highly uniform compared to pulsed laser deposited films and average rough ness was 3 nm. Breakdown voltage of sputtered BST thin films was 2 MV/cm whereas PLD BST thin films was 1.02 MV/cm and it shows that sputtered BST thin films had good dielectric strength. ? Since passing oxygen along with argon during sputtering changes the BST thin film composition, change in tunability is observed. Deposition rate decreases
70 with reduced argon flow and increased oxygen flow because of the relative decrease of argon species co ntributing to the sputter yield. ? The effect of argon oxygen ratio in tunability was studied and Ar/O 2 ratio with 90/10 produced the highest tunability. The deposition conditions to achieve high tunability and low dielectric loss were optimized and tunability of 2.8: 1 and loss tangent of 0. 01 was achieved in frequency ra nge of 10 MHz to 200 MHz. ? Structural characterization of sputtered BST thin films on MgO, Al 2 O 3 and LaAl 2 O 3 was achieved. Interdigital capacitors were fabricated but tunability was not observed due to larger spacing (37 m) between the fingers for a BST fi lm thickness of 100 nm. The large ratio of finger spacing to the BST thickness causes the fields to disperse in to the underlying MgO or Alumina substrate, which decreases the effect of BST thin film. The deposition conditions, structural properties and el ectrical properties of BST capacitors fabricated by PLD and sputtering are summarized in Table 7.1.
71 Table 7.1 Summary of BST Capacitors Fabricated Parameters of Interest Pulsed Laser Deposited BST Films Sputtered BST Films Temperature 450C, 550C 650C 550C, 600C, 650C Oxygen Pressure 150mTorr, 250mTorr, 450mTorr Ar/O2 Ratio 70/30 90/10 Crystallinity polycrystalline Highly polycrystalline Surface Roughness High 16n m 19nm Low 3nm 7nm Tunability 1.8:1 2.4:1 corresponding to 55nm 75nm 1.5:1 2.8:1 corresponding to High tunability at 90/10 Loss Tangent 0. 025 0.0 1 Breakdown Voltage 1.02 MV/cm 2.1 MV/cm 7 1. 3 Sol gel D eposited BST T hin F ilms ? Structural cha racterization of sol gel deposited BST thin films on MgO and Si/SiO2/TiO2/Pt was achieved. This method is inexpensive and can be a suitable method to produce thick BST thin films. Microwave characterization of BST thin films will be achieved. 7 .2 Future Wo rk Effect of doping in tunability and loss tangent can be studied, as it has shown improvement in electrical properties. The effect of bottom electrode thickness can be studied as it has been shown that thicker electrodes contribute higher Q factors. Stoic hiometry of sputtered BST thin films can be varied by different Ar/O 2 ratio and correlation between stoichiometry and dielectric property can be studied. It has been
72 shown that roughness of sol gel deposited BST thin films by chemical mechanical planarizat ion. Since roughness has a great effect in electrical characteristics effect of chemical mechanical planarization in tunability and loss tangent can be studied for sol gel deposited thin films. High frequency characterization of BST varactors can be achie ved by reducing the device area in the range of 5 10 m 2 BST capacitors can be integrated in phase shifters, filters and antennas.
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