Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Barium Strontium Titanate (BST) was deposited on Pt/ZrO2 / SiO2/Si substrates using liquid source metal organic chemical vapor deposition. A stoichiometry series was deposited with various GrII/Ti ratios (0.658 to 1.022) and a temperature series was deposited at 550 to 700°C. The thin films were characterized using transmission electron microscopy. Both series of samples contained cubic perovskite BST and an amorphous phase. The grain size increased and the volume fraction of amorphous phase decreased with increasing deposition temperature. The electrical and microstructural properties improved as the GrII/Ti ratio approached 1 and deteriorated beyond 1. This research demonstrates that BST thin films are a strong candidate for future MOS transistor gate insulator applications.

Sensors is one of the major areas of current research. Thin film micro/nano sensors are gaining attention worldwide, as there is necessity of miniaturization. There are varieties of sensors available by utilizing different materials in bulk and thin film form for measuring parameters like temperature, pressure, flow, humidity etc. Apart from these, there are various sensors available to measure impact force. Impact sensor offers potential application possibilities in robotics, aerospace, structural & mechanical engineering and related areas. Many physical principles have been explored for the realization of impact sensor. The present thesis reports the efforts made in developing impact sensor using piezoelectric thin film. The necessary brief background information on impact sensors is presented in Chapter 1. This includes the description of available literature on impact sensors and their probable applications. In Chapter 2, a review of the various techniques such as thin film deposition techniques, film thickness measurement techniques, thin film characterization techniques, used in our work are explained in detail. Chapter 3 explains the direct and indirect methods of characterization used for confirming the piezoelectric property of zinc oxide thin films. The detailed experimental work carried out in realizing the impact sensor using piezoelectric thin films is presented in chapter 4. This includes design of the sensor, calibration setup used & the procedure followed and results obtained. Finally, we present the summary of the work carried out in the thesis, conclusions arrived at and the scope for carrying out further work in the direction of making the sensor more efficient.

No description available.

Man’s desire to replicate/mimic the nature’s creation provided an impetus and inspiration to the rapid advancements and progress made in the sensors and actuators technology. A normal human being has five basic sensory organs, which helps and guides him in performing the routine tasks. This underlines the importance of basic sensory organs in a human life. In a similar fashion, sensors and actuators are of paramount importance for most of the science and engineering applications. The aim of the present thesis work is to explore the application of piezoelectric ZnO thin films deposited on a flexible substrate for the development of sensors and actuators. Detailed study was performed on the suitability of three different flexible substrates namely Phynox, Kapton and Mylar. However, Phynox alloy substrate was found to be a suitable substrate material for the above mentioned applications. Sputtering technique was chosen for the deposition of ZnO thin films on to Phynox substrate. The necessary process parameters were optimized to achieve good quality piezoelectric thin films. In the present work, sensors have been developed by utilizing the direct piezoelectric effect of ZnO thin films deposited on Phynox alloy substrate. These includes a flow sensor for gas flow rate measurement, impact sensor for non-destructive material discrimination study and a Thin Film Sensor Array (TFSA) for monitoring the impact events. On the other hand, using the converse piezoelectric effect of ZnO thin films, actuators have also been developed. These include a thin film micro actuator and a Thin Film Micro Vibrator (TFMV) for vibration testing of micro devices. The thesis is divided into following seven chapters. Chapter 1: This chapter gives a general introduction about sensors and actuators, piezoelectric thin films, flexible substrates, thin film deposition processes and characterization techniques. A brief literature survey of different applications of piezoelectric thin films deposited on various flexible substrates in device development is presented. Chapter 2: A novel flexible metal alloy (Phynox) and its properties along with its applications are discussed in this chapter. ZnO thin films were deposited on Phynox substrate by Rf reactive magnetron sputtering technique. The sputtering process parameters such as: Ar:O2 gas ratio, substrate temperature and RF power were optimized for the deposition of good quality piezoelectric ZnO thin films. The deposited ZnO thin films were characterized using XRD, SEM, AFM and d31 coefficient measurement techniques. Chapter 3: It reports on the comparative study of properties of piezoelectric ZnO thin films deposited on three different types of flexible substrates. The substrate materials employed were a metal alloy (Phynox), polyimide (Kapton), and polyester (Mylar). Piezoelectric ZnO thin films deposited on these flexible substrates were characterized by XRD, SEM, AFM and d31 coefficient measurement techniques. A vibration sensing test was also performed for the confirmation of good piezoelectric property. Compared to the polymer flexible substrates, the metal alloy flexible substrate (Phynox) was found to be more suitable for integrating ZnO thin film for sensing applications. Chapter 4: The development of a novel gas flow sensor for the flow rate measurement in the range of L min-1 is presented in this chapter. The sensing element is a Phynox alloy cantilever integrated with piezoelectric ZnO thin film. A detailed theoretical analysis of the experimental set–up showing the relationship between output voltage generated and force at a particular flow rate has been discussed. The flow sensor is calibrated using an in-house developed testing set-up. Chapter 5: This chapter is divided into two sections. Section 5.1 reports on the development of a novel packaged piezoelectric thin film impact sensor and its application in non-destructive material discrimination studies. Different materials (Iron, Glass, Wood and Plastic) were successfully discriminated by using the developed impact sensor. The output response of impact sensor showed good linearity and repeatability. The impact sensor is sensitive, reliable and cost-effective. Section 5.2 reports on the development of a Thin Film Senor Array (TFSA) for monitoring the location and magnitude of the impact force. The fabricated TFSA consists of evenly distributed ZnO thin film sensor array. Chapter 6: It consists of two sections. Section 6.1 reports on the fabrication of micro actuator using piezoelectric ZnO thin film integrated with flexible Phynox substrate. A suitable concave Perspex mounting was designed for the actuator element. The actuator element was excited at different frequencies for the supply voltages of 2V, 5V and 8V. The developed micro actuator has the potential to be used as a micro pump for pumping nano liters to micro liters of fluids. Section 6.2 reports the design and development of a portable ready to use Thin Film Micro Vibrator (TFMV). The TFMV is capable of providing the vibration amplitude in the range of nanometer to micrometer. A thin silicon diaphragm was used as a test specimen for its vibration testing studies using the developed TFMV. The TFMV is light-weight and have internal battery, hence no external power supply is required for its functioning. Chapter 7: The first section summarizes the salient features of the work presented in this thesis. In the second section the scope for carrying out the further work is given.

The present research work mainly focuses on the fabrication and characterization of single and multilayer thin films based on Bismuth Vanadate (BVO) and Bismuth Titanate (BTO). The multi-target laser ablation technique was used to fabricate single layer thin films of BVO, BVN and BTO; and multilayers composed of BVO and BTO in different structures. The fabricated thin films exhibited dense microstructure and a sharp interface with the substrate. The lattice strain, surface roughness and grain size could be varied as functions of composition and individual layer thickness in different structure fabricated. The optical properties were studied by spectroscopic ellipsometry and optical transmission spectra. The various models that were used for ellipsometric data analysis gave an excellent fitting to the experimental data. The optical constants were determined through multilayer analyses of the films. The band gap of these films was studied by spectroscopic ellipsometry and optical transmission. The optical studies carried out on BVO-BTO bilayer indicated the presence of an interfacial layer in between the BVO and BTO layer, whose refractive index was different from that of the individual layers and is attributed to different nature of the interfacial layer. The ferroelectric nature of BVO films was confirmed by P-E hysteresis loop studies under different applied fields and at various probing frequencies. The same was corroborated via the C-V measurements of these BVO films which exhibited butterfly shaped C-V characteristics. Fatigue studies in these films indicated that the switchable polarization is essentially constant through 105 cycles, after which it starts increasing probably due to the ionic conduction in BVO thin films. The dielectric response of undoped and Nb doped BVO as well as BVBT ML thin films were studied over a wide range of temperatures. The BVO films exhibited remarkable dielectric dispersion at low frequencies especially in the high temperature regime. Further, the frequency and temperature dependence of the dielectric, impedance, modulus and conductivity spectra of these films were investigated in detail. The ac conductivity was found to obey well the double power law in case of ML, indicating the different contributions to the conductivity, the low frequency conductivity being due to the short range translational hopping and the high frequency conductivity is due to the localized or reorientational hopping motion. DC leakage conduction in BVO, BVN and BVBT ML thin films was studied over a wide range of temperatures and applied electric fields. The experimental data were analyzed in light of different models to investigate the dc conduction mechanism in these films which were broadly classified into electrode limited and bulk limited conduction processes. In the case of BVO thin films the dc leakage current exhibited an ohmic nature at low electric fields followed by an onset of the space charge limited conduction (>1). Further in case of BVN films, three distinct regions were observed in I-V characteristics signifying different types of conduction processes in these films. In case of BVBT ML thin films, bulk limited PF mechanism was found to determine the conduction behavior at moderate electric fields. At higher electric fields, a trap filled region was observed which was followed by SCL conduction at higher fields. Therefore the present observation indicates the presence of more than one bulk limited conduction process in BVBT ML thin films. BVO thin films exhibiting good structure and dense morphology were successfully prepared on p-type Si by chemical solution decomposition technique. The C-V characteristics were evaluated for Au/BVO/Si MFS structure which showed a typical high frequency feature of a conventional MFIS structure.

All solid state Li-ion batteries (thin film micro batteries) have become inevitable for miniaturized devices and sensors as power sources. Fabrication of electrode materials for batteries in thin film form has been carried out with the existing technologies used in semiconductor industry. In the present thesis, radio frequency (RF) sputtering has been chosen for deposition of cathode material (ceramic oxides) thin films because of several advantages such as precise thickness control and deposition of compound thin films with equivalent composition. Conventional sputtering involves fabrication of thin film using custom made pellet according to the specification of sputter gun. However several issues such as target breaking are inevitable with the pellet sputtering. To forfend the issues, powder sputtering has been implemented for the deposition of various thin film cathodes in an economically feasible approach. Optimization of various process parameters during film deposition of cathode materials LiCoO2, Li2MnO3, LiNixMnyO4, mixed oxide cathodes of LiMn2O4, LiCoO2 and TiO2 etc., have been executed successfully by the present approach to achieve optimum electrochemical performance. Thereafter the optimized process parameters would be useful for selection of cathode layers for micro battery fabrication. Chapter 1 gives a brief introduction to the Li ion and thin film solid state batteries. It also highlights the advantages of powder sputtering compared to conventional pellet sputtering. In Chapter 2, the materials used and methods employed for the fabrication of thin film electrodes and analytical characterizations have been discussed. In chapter 3, implementation of powder sputtering for the deposition of LiCoO2 thin films has been discussed. X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS) and electrochemical investigations have been carried out and promising results have been achieved. Charge discharge studies delivered a discharge capacity of 64 µAh µm-1 cm-2 in the first cycle in the potential range 3.0-4.2 V vs. Li/Li+. The possible causes for the moderate cycle life performance have been discussed. Systematic investigations for RF power optimization for the deposition of Li2-xMnO3-y thin films have been carried out. Galvanostatic charge discharge studies delivered a highest discharge capacity of 139 µAh µm-1cm-2 in the potential window 2.0-3.5 V. Thereafter, effect of LMO film thickness on electrochemical performance has been studied in the thickness range 70 nm to 300 nm. Films of lower thickness delivered higher discharge capacity with good cycle life than the thicker films. These details are discussed in chapter 4. In Chapter 5, fabrication and electrochemical performance of LiNixMnyO4 thin films are presented. LMO thin films have been deposited on nickel coated stainless steel substrates. The as deposited films were annealed at 500 °C in ambient conditions. Nickel diffuses in to LMO film and results in LiNixMnyO4 (LMNO) film. These films were further characterized. Electrochemical studies were conducted up to higher potential 4.4 V resulted in discharge capacities of the order of 55 µAh µm-1cm-2. In chapter 6, electrochemical investigations of mixed oxide thin films of LiCoO2 and LiMn2O4 have been carried out. Electrochemical investigations have been carried out in the potential window 2.0–4.3 V and a discharge capacity of 24 µAh µm-1cm-2 has been achieved. In continuation, TiO2 powder was added to the former composition and the deposited films were characterized for electrochemical performance. The potential window as well as the discharge capacity enhanced after TiO2 doping. Electrochemical characterization has been carried out in the potential window 1.4–4.5 V, and a discharge capacity of 135 µAh µm-1cm-2 has been achieved. Finally chapter 7 gives overall conclusions and future directions to the continuation of the work.

Chalcogenide glasses are a class of covalent amorphous semiconductors with interesting properties. The presence of short-range order and the pinned Fermi level are the two important properties that make them suitable for many applications. With flash memory technology reaching the scaling limit as per Moore’s law, alternate materials and techniques are being researched at for realizing next generation non-volatile memories. Two such possibilities that are being looked at are Phase Change Memory (PCM) and Programmable Metallization Cell (PMC) both of which make use of chalcogenide materials. This thesis starts with a survey of the work done so far in realizing PCMs in reality. For chalcogenides to be used as a main memory or as a replacement to FLASH technology, the electrical switching parameters like switching voltage, programming current, ON state and OFF state resistances, switching time and optical parameters like band gap are to be considered. A survey on the work done in this regard has revealed that various parameters such as chemical composition of the PC material, nature of additives used to enhance the performance of PCM, topological thresholds (Rigidity Percolation Threshold and Chemical Threshold), device geometry, thickness of the active volume, etc., influence the electrical switching parameters. This has motivated to further investigate the material and experimental parameters that affect switching and also to explore the possibility of multi level switching. In this thesis work, the feasibility of using two chalcogenide systems namely Si15Te85-xGex and Ge15Te85-xAgx in the form of amorphous thin films for PCM application is explored. In the process, electrical switching experiments have been carried out on thin films belonging to these systems and the results obtained are found to exhibit some interesting anomalies. Further experiments and analysis have been carried out to understand these anomalies. Finally, the dynamics of electrical switching has been investigated and presented for amorphous Si15Te85-xGex thin films. From these studies, it is also seen that multi state switching/multiple resistance levels of the material can be achieved by current controlled switching, the mechanisms of which have been further probed using XRD analysis and AFM studies. In addition, investigations have been carried out on the electrical switching behavior of amorphous Ge15Te85-xAgx thin film devices and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter one of the thesis, gives a brief introduction to the limitations in existing memory technology and the alternative memory technologies that are being researched, based on which it can be inferred that PCM is a promising candidate for the next generation non volatile memory. This chapter also discusses the principle of using PCM to store data, realization of PCM using chalcogenides, the material properties to be considered in designing PCM, the trade offs in the process of design and the current trends in PCM technology. Chapter two provides a brief review of the electrical switching phenomenon observed in various bulk chalcogenide glasses, as electrical switching is the underlying principle behind the working of a PCM. In the process of designing a memory, many parameters like read/write operation speed, data retentivity and life, etc., have to be optimized for which a thorough understanding on the dependence of electrical switching mechanism on various material parameters is essential. In this chapter, the dependence of electrical switching on parameters like network topological thresholds and electrical and thermal properties of the material is discussed. Doping is an efficient way of controlling the electrical parameters of chalcogenides. The nature of dopant also influences switching parameters and this also is briefly discussed. Chapter three provides a brief introduction to the different experimental techniques used for the thesis work such as bulk chalcogenide glass preparation, preparation of thin amorphous films, measurement of film thickness, confirmation of amorphous nature of the films using X-Ray Diffraction (XRD), electrical switching experiments using a custom made setup, crystallization study using XRD and Atomic Force Microscopy (AFM) and optical band gap studies using UV-Vis spectrometer. Vt is an important parameter in the design of a PCM. Chapter four discusses the dependence of Switching voltage, Vt, on input energy. It is already established that the Vt is influenced by the composition of the base glass, nature of dopants, thickness of films and by the ambient temperature. Based on the results of electrical switching experiments in Si15Te74Ge11 amorphous thin films a comprehensive analysis has been done to understand the kinetics of electrical switching. Chapter five discusses a current controlled crystallization technique that can be used to realize multi-bit storage with a single layer of chalcogenide material. In case of PCM, data is stored as structural information; the memory cell in the amorphous state is read as data ‘0’ and the memory cell in crystalline state is read as data ‘1’. This is accomplished through the process of electrical switching. In order to increase the memory density or storage density, multi-bit storage is being probed at by having multiple layers of chalcogenide material. However, with this technique, the problems of inter-diffusion between different layers cannot be ruled out. In this thesis work, a current controlled crystallization technique has been used to achieve multiple stable resistance states in Si15Te75Ge10 thin films. Chapter six discusses the mechanism behind multi state switching exhibited by certain compositions of Si15Te85-xGex thin films. Crystallization studies on certain Si15Te85-xGex films have been carried out using XRD and AFM to understand the phenomenon of multiple states. The results of these experiments and analysis are presented in this chapter. Chapter seven discusses the results of electrical switching experiments and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter eight gives the conclusion and scope for future work.

The research in permanent magnet thin films focuses on the search of new materials and methods to increase the prevalent data storage limit. In the recent past the work towards the application of these films to micro systems have also gained momentum. Materials like samarium cobalt with better magnetic properties and temperature stability are considered to be suitable in this regard. The essential requirement in miniaturization of these films is to deposit them on silicon substrates that can alleviate the micro fabrication process. In this work, an effort has been made to deposit SmCo films with better magnetic properties on silicon substrates. In the deposition of SmCo, the composition of the deposited films and the structural evolution are found to play an important role in determining the magnetic properties. Proper control over these parameters is essential in controlling the magnetic properties of the deposited films. SmCo being a two component material the composition of the films is dependent on the nature of the source and the transport of the material species from source to substrate. On the other hand, structural evolution is dependent on the energetical considerations between the SmCo lattice and substrate lattice. This most often is dominated by the lattice match between the condensing lattice and the substrate lattice. As such Si does not provide good lattice match to SmCo lattice. Hence suitable underlayers are essential in the deposition of these films. Materials like W, Cu, Mo and Cr were used as underlayers. Out of all these Cr is found to provide good lattice match and adhesion to SmCo lattice. Sputtering being the common deposition tool, SmCo could be sputtered either from the elemental targets of Sm and Co or from the compound target of SmCo5. Sputtering of elemental targets of Sm and Co provides the flexibility of varying the composition whereas sputtering from the SmCo alloy target provides to flexibility of controlling the structural evolution by different process parameters. In this work two different techniques namely Facing Target Sputtering (FTS) and Ion Beam Sputter Deposition (IBSD) were followed in depositing SmCo films. In FTS technique, SmCo films were directly deposited on silicon substrates by simultaneous sputtering of samarium and cobalt targets facing each other. This sputtering geometry enabled to achieve films with a wide composition range of 55 – 95 at. % of cobalt in single deposition. The resulting composition variation and material property variation were investigated in terms of process parameters like pressure, temperature, SubstrateTarget Distance (STD) and InterTarget Distance (ITD). The composition distribution of the films was found to be dependent on the thermalisation distances and the mean free path available during the transport. To explain the process and the composition variation, a simulation model based on Monte Carlo method has been employed. The simulated composition variation trends were in good agreement with that of the experimental observations. IBSD, known for its controlled deposition, was employed to deposit both Cr (as an underlayer) and SmCo films. Cr with close epitaxial match with SmCo induces structural evolution in deposited films. The initial growth conditions were found to play a dominant role in the structural evolution of these Cr films. Hence, initial growth conditions were modified by means of oblique incidence and preferential orientation of (200) plane was obtained. With three different angles of incidence, three different surface orientations of Cr films were achieved. These films were then used as structural templates in the deposition of SmCo films. The influence of parameters like composition, impurities, film thickness, beam energy, ion flux, annealing, angles of incidence and underlayer properties on the structural and magnetic properties of SmCo was studied. The structural evolution of SmCo has been found to depend on the structural orientation of Cr underlayers. This followed the structural relation of SmCo(100)||Cr(110)||Si(100) and SmCo(110)||Cr(100)||Si(100). A mixed surface plane orientation was observed in the case of mixed orientation Cr template. The magnetic coercivities were found to increase from 50 Oe to 5000 Oe with the change in the structure of the deposited films.

NiTiCu is a shape memory alloy that regains its original shape after large amount of shape changing deformation when heated above a critical temperature called reverse martensitic trans-formation temperature( Af). When external load is applied on the sample in twinned martensite phase at low temperature, it deforms by detwinning, accommodating large amount of strains. When it is heated above Af, the shape recovers by transformation of the martensite to austenite phase. However, the amount of shape recovery degrades over time due to internal factors such as precipitates, residual strains and thermal history as well as external factors such as stresses. Severe localized stresses induced by contacts result in plastic deformation that affect the reverse martensitic transformation and hence the shape recovery. In this work, we study how varying levels of contact stresses induced in NiTiCu thin film affect its shape recovery. NiTiCu thin films of six different compositions are deposited on Si(100) wafer by co-sputtering from elemental targets. After deposition, the films are annealed at 500 C for 4 h to make them crystalline. The composition of the films varied linearly with applied power to the targets. Uniformity in composition over a 4 inch substrate area is achieved by substrate rotation. All the films show ne grain microstructure after annealing. The subsurface of the Ni-rich films is columnar. Ni-rich films have annealing cracks and the crack width increases with Ni composition in the films. The roughness of as-deposited films is found to be more for Ni-rich films compared to Ti-rich films. The roughness of the Ni-rich and Ti-rich films increased after annealing. From the X-ray diffraction studies, it was observed that the films are nanocrystalline. Indentation is carried out using a Berkovich diamond indenter with spherical apex, at nine different locations with loads ranging from 0.25 mN to 25 mN. A predefined array is chosen for indentation such that the larger indents act as a guide to precisely locate minute indents generated at lower loads, with residual depth as small as 10 nm, for imaging in high-resolution microscopes like Scanning Electron Microscope as well as in Atomic Force Microscope . In Ti60 (a Ti-rich) lm, the residual indents generated at loads greater than 10 mN show radial cracks originating at corners. Average crack length increases with the maximum load used for generating the indent. Sequential sectioning of Ti48 (a Ni-rich) lm using Focused Ion Beam microscope, revealed that the cracks originate at the lm-substrate interface and reach the surface. In Ti48 lm, residual indents do not show any indentation cracks. The indentation stresses are accommodated by breaking of the columnar structure and the voids between them. Delamination of the film from the substrate is observed on either sides of the indent in both the Ti60 and Ti48 films. The hardness of the films is high at low loads and decrease as the load increases. The deformation by indentation at lower loads is mainly due to detwinning as only the apex of the indenter, which is nearly spherical, is in contact with the sample and the resulting stresses are low. As the load increases, the deformation starts getting accommodated through dislocations along with detwinning as the stress beneath the indenter increases. Spherical cavity model extended to SMA shows that inner hemisphere near the tip contains dislocations where stresses are very high, surrounded by detwinned region with stresses that are relatively low. When the sample is heated above reverse martensitic transformation temperature to induce shape recovery in the indents, only the detwinned region recovers to the original shape. Recovery ratio, quantification of shape recovery, is calculated from the depth of the indents before and after heating. Recovery ratio in Ti60 films is found to be large at low loads and decreases with increase in load. The decrease in shape recovery in Ti60 is attributed to the increase in the amount of plastic deformation at the expense of detwinning. Three-dimensional mapping of the surfaces shows that the recovery ratio is high at the apex of the indent at the maximum depth and reduces towards the edges of the indent. There is no evident recovery in Ti48 films. The shape recovery of SMAs can be achieved by Joule heating. When electric current is passed through the material, it heats up by Joule heating because of the intrinsic resistivity. The resistivity and hence the resistance would get effected by the dislocation based plastic deformation induced by the contact. This might result in shape recovery through resistive heating. Towards understanding this, the effect of contact stresses on electrical contact resistance is studied. Experimental setup is designed, developed and calibrated for studying the variation of electrical contact resistance of the NiTiCu thin films as a function of load. Electrical contact resistance is found to decrease with increase in applied load. Contact stresses in sub-micron NiTiCu thin films are simulated by carrying out nanoindentation at different loads. The recovery ratio is high when the stresses induced by the contact is less, at lower loads. The shape recovery ratio is reduced when the induced contact stresses in-creases. There is no shape recovery at the sharp edges of the indentation where contact stresses are very high. Hence, by carefully designing the features to reduce the stress concentrations, the performance of the device can be improved.