VTechWorks Repository :: Browsing by Author "Desu, Seshu B."

Browsing by Author "Desu, Seshu B."

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Anisotropic etching for silicon micromachining
Hobbs, Neil Townsend (Virginia Tech, 1994-05-05)
Silicon micromachining is the collective name for several processes by which three dimensional structures may be constructed from or on silicon wafers. One of these processes is anisotropic etching, which utilizes etchants such as KOH and ethylene diamine pyrocatechol (EDP) to fabricate structures from the wafer bulk. This project is a study of the use of KOH to anisotropically etch (lOO)-oriented silicon wafers. The thesis provides a thorough review of the theory and principles of anisotropic etching as applied to (100) wafers, followed by a few examples which serve to illustrate the theory. Next, the thesis describes the development and experimental verification of a standardized procedure by which anisotropic etching may be reliably performed in a typical research laboratory environment. After the development of this procedure, several more etching experiments were performed to compare the effects of various modifications of the etching process. Multi-step etching processes were demonstrated, as well as simultaneous doublesided etching using two different masks. The advantages and limitations of both methods are addressed in this thesis. A comparison of experiments performed at different etchant temperatures indicates that high temperatures (800 C) produces reasonably good results at a very high etch rate, while lower temperatures (500 C) are more suited to high-precision structures since they produce smoother, higher-quality surfaces.
Characterization and elimination of dry etching damaged layer in Pt/Pb(Zr0.53Ti0.47)O-3/Pt ferroelectric capacitor
Lee, J. K.; Kim, T. Y.; Chung, Ilsub; Desu, Seshu B. (AIP Publishing, 1999-07-01)
The damage of Pb(Zr0.53Ti0.47)O-3 thin film due to dry etching process was characterized in terms of the microstructure and electrical properties. The damaged layer seems to be amorphous and the thickness of the damaged layer is about 10 nm. The existence of such a layer in Pt/Pb(Zr0.53Ti0.47)O-3/Pt ferroelectric capacitor tends to increase the coercive voltage and the leakage current. The damaged layer was not fully reverted to perovskite phase by the thermal annealing. With the wet cleaning treatment, however, the damaged layer was successfully removed thereby revealing significantly improved electrical properties. (C) 1999 American Institute of Physics. [S0003-6951(99)00529-X].
Characterization and patterned polymer films from a novel self-assembly process
Liu, Yanjing (Virginia Tech, 1996-05-03)
The layer-by-layer molecular-level manipulation of ionic polymer have been utilized to fabricate ultrathin multilayer films (SAMp). In this process, monolayers of polycations and polyanions are sequentially adsorbed onto a substrate surface by alternately dipping the substrate into aqueous solutions of poly(vinylamine) backbone azo (PDYE), poly(sodium 4-styrenesulfonate) (PSS), and poly(al1ylamine hydrochloride) (PAH). The ionic attractions developed between the oppositely charged polymers promote strong interlayer adhesion and a uniform and linear multilayer deposition process. UV/Vis absorbance, contact angle, and ellipsometry measurements revealed that in all cases the bilayer deposition process was linear and highly producible from layer to layer and film thickness of up to 1 µm can be easily obtained by repeating the deposition process. The typical thickness of bilayer film depend on the solution concentration. Contact angle and UV/Vis spectroscopy measurements demonstrated that the deposition time for a full monolayer coverage of azo dye and PAH was about 20 seconds. Our results showed that the mechanical stability of these SAMp films was remarkable, and SAMp films can only be removed from the substrate by scraping. SAMp films are stable in the common organic solvents and even in the high acidic media (6M HCl aqueous solution). The conformation of these films are thermally stable at high temperature. In an attempt to develop patterned surfaces of sulfonate and thiol functionality, close-packed, well-ordered (3-mercaptopropyl)trimethoxysilane (MPS) monolayer were formed on the surfaces of single crystal silicon, quartz, and glass by allowing hydrolyzed silane to self-assemble from a dilute hydrocarbon solution. The films of MPS were irradiated with an ozone-producing UV light source results in efficient conversion of the surface-localized thiol groups to sulfonated groups, a complete photo-oxidation of the thiol surface was obtained and characterized by x-ray photoelectron spectroscopy (XPS) and contact angle measurements. Sulfonated self-assembled films can be used as good organic templates for the deposition of SAMp films and for micropatterning of organic surfaces based on our results. Such results significantly extend the application of SAMp films since the sulfonate-functionalized surface can be introduced into the surfaces of aromatic polymers, metals, ceramics, semiconductors, and plastics. So that the process of SAMp deposition can be carried out onto many different substrate surfaces. The novel self-assembly technique combined with photolithography was used to develop three different methods of micropatterning fabrication in an attempt to achieve the goal of full-color flat-panel display. The characteristic of distinguishing our methods from the existed ones is that the patterning is done first and then the vertical multilayers were built-up on the patterned areas. Moreover, in this process, SAMp films were used as active species. Scanning Electron Microscopy (SEM) was employed to confirm the patterning technique. In order to block the further growth of the second film type on the sites of first film type, several molecules with inert function groups were tried. UV/Vis absorbance and contact angle measurements revealed that dodecyltrimethylammonium bromide (DTAB) atop the PAH/PSS SAMp film could prevent further adsorption of the ionic polymers.
Characterization of Ferroelectric Films by Spectroscopic Ellipsometry
Dickerson, Bryan Douglas Jr. (Virginia Tech, 2003-07-18)
Process dependent microstructural effects in ferroelectric SrBi2Ta2O9 (SBT) thin films were characterized and distinguished from material dependent optical properties using a systematic multi-layer modeling technique. Variable angle spectroscopic ellipsometry (VASE) models were developed by sequentially testing Bruggeman effective-media approximation (EMA) layers designed to simulate microstructural effects such as surface roughness, porosity, secondary phases, and substrate interaction. Cross-sectional analysis by atomic force microscopy (AFM), transmission and scanning electron microscopy (TEM) and (SEM) guided and confirmed the structure of multi-layer models for films produced by pulsed laser deposition (PLD), metal-organic chemical vapor decomposition (MOCVD), and metal-organic deposition (MOD). VASE was used to estimated the volume percentage of second phase Bi2O3 in SBT thin films made by MOD. Since Bi₂O₃ was 10 orders of magnitude more conductive than SBT, second phase Bi₂O₃ produced elevated leakage currents. Equivalent circuits and percolation theory were applied to predict leakage current based on Bi₂O₃ content and connectivity. The complex role of excess Bi2O3 in the crystallization of SBT was reviewed from a processing perspective. VASE helped clarify the nature of the interaction between SBT films and Si substrates. When SBT was deposited by MOD and annealed on Si substrates, the measured capacitance was reduced from that of SBT on Pt due mainly to the formation of amorphous SiO₂ near the SBT/Si interface. VASE showed that the thickness and roughness of the SiO₂ reaction layer increased with annealing temperature, in agreement with TEM measurements. Unlike PZT, SBT crystallization was not controlled by substrate interaction.
Chemical vapor deposition of β-SiC thin films on Si(100) in a hot wall reactor
Chiu, Chienchia (Virginia Tech, 1994-01-12)
A systematic method was developed for the deposition of β-SiC thin films on Si(100) substrates in a hot wall reactor, using low pressure chemical vapor deposition (LPCVD). Due to poor adhesion resulting from lattice mismatch and difference in thermal expansion coefficients between the (SiC films and the Si(100) substrates, the feasibility of forming a SiC buffer layer on the Si(100) surface before beginning the chemical vapor deposition (CVD) process was investigated. The SiC buffer layers were formed with either a smooth or porous morphology. A nonporous Si(100) substrate with a 35Å thick SiC buffer layer was formed when the Si surface was heated at 1050°C in an atmosphere of C₂H₂ and H₂. A porous surface was obtained when the Si substrate was heated at 1000°C in C₂H₂ alone. The porous defects were correlated to the out—diffusion of Si in the carburizing process. On smooth Si(100) substrates, polycrystalline and stoichiometric β-SiC thin films with the (111) planes paralleling the Si(100) substrates were grown from a CH₃SiCl₃ (MTS)—H₂ mixture at 1050°C. At high H₂/MTS ratios and/or low deposition pressures, no etching on the Si substrates of the β-SiC films was observed, resulting in a smooth topography. Degradation in film morphology, changes in the preferred orientation, and etching of the Si substrates were observed at higher pressures, temperatures, and H₂/MTS ratios. The etching of the Si substrate was due to the out—diffusion of Si atoms from the substrate and the presence of Cl—containing radicals, which resulted from the decomposition of MTS molecules before arriving at the substrates. A model of the deposition mechanism is proposed which predicts the deposition rates in a hot wall CVD reactor and agrees very well with the experimental data. On the Si(100) substrate with a porous topography, epitaxial β-SiC(100) thin films were grown from MTS—H₂ at 1150°C. The crystallinity of the deposited films was influenced by the deposition time. With increasing deposition time, rotational β-SiC(100) crystals and polycrystalline β-SiC with a highly preferred orientation of (100) and/or (111) were obtained. At a lower temperature of 1100°C, poor morphology and polycrystalline β-SiC thin films were observed. Finally, a new approach to the calculation of the local equilibrium CVD phase diagrams, which represent the most stable phases above the substrates in a hot wall reactor, for SiC deposition from the MTS—H₂ gas mixture by coupling the depletion effects to the equilibrium thermodynamic computer code SOLGASMIX—PV. The calculated CVD phase diagrams were also compared with experimental and the literature data. Although the local equilibrium CVD phase diagrams predicted the deposition of single phase SiC better than established CVD phase diagrams, the experimental regions for depositing single phase SiC are larger than those calculated from local CVD phase diagrams. This may be because of the high linear velocity of the gas flux under low pressure and the polarity of the Si—containing intermediate species.
Chemically modified Ta₂O₅ thin films for dynamic random access memory (DRAM) applications
Desu, Chandra S. (Virginia Tech, 1998-07-17)
Increasing demand for high-density memories has necessitated the search for new materials with higher dielectric constants to satisfy the minimum charge storage density requirements. Several materials such as Ta₂O₅, BST¹, BBT² are being investigated to replace the currently used Si based oxide/nitride dielectrics. Among the materials under investigation, Ta₂O₅ is one of the most promising, especially from the fab compatibility point of view. Ta₂O₅ thin films offer a six-fold increase in dielectric constant compared to conventional dielectrics. However, the significant improvement in dielectric constant is offset by higher leakage currents compared to conventional dielectrics. Improvement in both, dielectric and insulating properties is required for the successful integration of Ta₂O₅ thin films into devices. In the current research work, it was demonstrated that by chemically modifying the tantalum pentoxide matrix, significant improvements in its electrical properties can be achieved which would enable the fabrication of a reliable high-density memory device. In the present work, the effects of Al addition on Ta₂O₅ thin films were systematically studied. The structural and electrical properties of these chemically modified thin films were investigated in detail to establish their potential for device applications. The effects on dielectric and insulating characteristics due to incorporation of Al in Ta₂O₅ matrix were studied in capacitor configuration. A metallorganic solution decomposition (MOSD) technique was used to deposit thin films onto Pt coated Si(100) substrates. The capacitors were fabricated by sputter depositing Pt electrodes on the top surface of the films. The dielectric and insulating properties of pure and modified Ta₂O₅ thin films and their dependence on film composition, processing temperature, and the thickness were discussed and an attempt was made to provide theoretical understanding for the experimental observations. The dielectric and insulating properties of Ta₂O₅ were found to be significantly modified by addition of Al. It was observed that Al addition has decreased the leakage currents approximately by an order of magnitude and improved thermal and bias stability characteristics of Ta₂O₅ capacitors. For example, the leakage currents in crystalline pure Ta₂O₅ thin films were found to be 4.5 x 10⁷ A/cm² in a 1MV/cm dc field which decreased to 3.4 x 10⁸ A/cm² for 10% Al modified Ta₂O₅ thin films. A typical dielectric constant of 42.5 was obtained for 10% Al modified Ta₂O₅ thin films. This is significantly higher compared to the commonly reported dielectric constant of 25 to 35 for Ta₂O₅ thin films. This enhancement was attributed to strong (100) orientation exhibited by both pure and modified Ta₂O₅ thin films. The high dielectric constant, low dielectric loss, low leakage currents and low temperature coefficient of capacitance suggest the suitability of Al modified Ta₂O₅ as a capacitor dielectric for future generation DRAM applications. ¹Barium strontium titanate, ²Barium bismuth tantalate
The chemistry of metalorganic chemical vapor deposition from a copper alkoxide precursor
Young, Valerie Lynne Vandigrifft (Virginia Tech, 1992)
The chemistry of chemical vapor deposition from copper (II) dimethylaminoethoxide onto single crystal strontium titanate has been studied by in situ infrared analysis of the vapor phase in the reactor, and by simultaneous mass spectrometer analysis of the reactor outlet gas. Species condensed from the reactor outlet gas in a liquid nitrogen trap were analyzed by proton nuclear magnetic resonance. Chemical information was also obtained by Auger electron and X-ray photoelectron spectrometer analysis of the deposited films. Deposition chemistry was studied with respect to deposition temperature, presence of ultraviolet light, and presence of a reactive gas cofeed. The goal was to determine the reaction pathway and relate it to deposited film composition. In a reduced pressure helium atmosphere, copper dimethylaminoethoxide deposits clean, conductive films of copper metal at 200°C. The ligands are eliminated by two interdependent reactions: β-hydride elimination produces dimethylaminoethanol, while reductive elimination produces dimethylaminoethanol. The minimum deposition temperature is 150°C. At substrate temperatures near 250°C some ligand fragmentation occurs, in addition to the clean elimination pathway, leading to carbon contamination of the deposited films. The deposition chemistry of copper dimethylaminoethoxide is not affected by irradiation with ultraviolet light of wavelengths between 360 nm and 600 nm. The ultraviolet light source was a Spectronics B-100 UV lamp. A light source with higher power might affect deposition chemistry. At a substrate temperature of 200°C in the presence of oxygen, dimethylaminoethanol and dimethylaminoethanol are not detected as products. Decomposition involves extensive ligand fragmentation, producing small amines and carbonyl species, carbon monoxide, and carbon dioxide. Films are free of carbon and nitrogen, because the ligand fragments are volatile and stable. Films are a mixture of copper metal and copper (I) oxide. Optimization of oxygen concentration in the reactor could lead to deposition of a pure copper oxide.
Comparison of lead zirconate titanate thin films on ruthenium oxide and platinum electrodes
Bursill, Les A.; Reaney, Ian M.; Vijay, Dilip P.; Desu, Seshu B. (American Institute of Physics, 1994-02-01)
High-resolution and bright- and dark-field transmission electron microscopy are used to characterize and compare the interface structures and microstructure of PZT/RuO2/SiO2/Si and PZT/Pt/Ti/SiO2/Si ferroelectric thin films, with a view to understanding the improved fatigue characteristics of PZT thin films with RuO2 electrodes. The RuO2/PZT interface consists of a curved pseudoperiodic minimal surface. The interface is chemically sharp with virtually no intermixing of RuO2 and PZT, as evidenced by the atomic resolution images as well as energy dispersive x-ray analysis. A nanocrystalline pyrochlore phase Pb2ZrTiO7-x, x not equal 1, was found on the top surface of the PZT layer. The PZT/Pt/Ti/SiO2/Si thin film was well crystallized and showed sharp interfaces throughout. Possible reasons for the improved fatigue characteristics of PZT/RuO2/SiO2/Si thin films are discussed.
Degradation in lead zirconate titanate thin film capacitors for non-volatile memory applications
Bhattacharya, Mayukh (Virginia Tech, 1994-07-05)
A study of the degradation of ferroelectric properties in Lead Zirconate Titanate (PZT) thin film capacitors is presented in this work. Metal- Ferroelectric - Metal capacitors were prepared by sputtering and metal organic decomposition (MOD) techniques. Samples with several different film thicknesses were considered in this study. Depolarization, leading to imprint has been studied at various temperatures. Changes in the dielectric properties of the capacitors as a function of the number of fatigue cycles is presented. Impedance and modulus spectroscopic techniques have been applied to study the effect of degradation on the ferroelectric thin film. It has been shown that with accurate low frequency impedance measurement equipment, new insight can be gained on the mechanisms of degradation in ferroelectric capacitors.
Dielectric properties of PFN-PFT solid solution synthesized by the molten salt method
Amanuma, Kazushi (Virginia Tech, 1991-08-09)
Various compositions of PFN-PFT solid solution were synthesized by the molten salt method at 800°C and 900°C. Higher temperature was necessary to form pure perovskite of PFT than PFN. However, those synthesized at higher temperatures tended to decompose during sintering. The particle size was not uniform and increased from PFT to PEN. The distribution of grain size in the sintered pellets was bi-modal. The grain size increased as the particle size increased. Those synthesized at 900°C had quite a different behavior from those synthesized at 800°C during sintering. Each composition of PFN-PFT system showed a broad dielectric phase transition in the different temperature range. This behavior was well described by the statistical model of compositional fluctuation.
Dry etching of layer structure oxides
(United States Patent and Trademark Office, 1999-02-23)
A method of patterning layered structure oxide thin films involving placing the layered structure thin film (with or without a mask) laid on a substrate into a chamber which is partially filled with CHC1FCF.sub.3 gas and producing a glow discharge to cause the etching of the thin film ferroelectric material. The method provides high etch rates, good etch anisotropy and good etch uniformity. For example, for SBT and SBN thin films, the etch process provides etch rates in the range of 2.5 to 17.5 nm/min depending on the etch conditions and minimal etch residues at the end of the etch process is removed easily by low temperature (250.degree. C.) baking. Also, the method provides good etch selectivity in the films and minimal surface damage.
Effect of interfacial thermal conductance and fiber orientation on the thermal diffusivity/conductivity of unidirectional fiber-reinforced ceramic matrix composites
Bhatt, Hemanshu D. (Virginia Tech, 1992-02-06)
The role of an interfacial barrier at the fiber-matrix interface in the heat conduction behavior of an uniaxial silicon carbide fiber-reinforced reaction-bonded silicon nitride and the effect of fiber orientation on the heat conduction characteristics of carbon fiber-reinforced borosilicate glass was investigated. In the study of the effect of an interfacial thermal barrier, a composite with fibers having a carbon-rich coating of about 3 J.l m was chosen as the reference material. The fiber-matrix interface was then modified by preferential oxidation of the carbon coating on the fibers, using fibers with no carbon coating and using hotisostatic-pressing (HIP) after nitridation. The formation of an interfacial gap at the interface due to thermal expansion mismatch between the fiber and the matrix in reference and HIP'd composites, and removal of carbon coating for oxidized composites, resulted in the dependence of thermal diffusivity/conductivity on the surrounding . atmosphere. This effect was attributed to gaseous heat transfer at the interface. However, no atmospheric effects were observed for composites with fibers without the carbon coating due to very strong bonding between the fiber and the matrix. HIP'ing increased the thermal diffusivity/conductivity of the composites due to densification of the matrix, crystallization of the fibers and increased physical contact at the interface. Removal of the interfacial carbon layer by preferential oxidation lowered the interfacial conductance considerably, due to decrease in the direct thermal contact between the fibers and the matrix. Interfacial contact conductance determined from measurements made in vacuum for reference and HIP'd composites increased rapidly with increasing temperature in accordance with interfacial gap closure. These observations indicate that the heat conduction behavior of all the composites investigated was strongly affected by the existence of an interfacial thermal barrier, for heat transfer transverse to the fiber direction.
Effects of alloying elements upon austenite decomposition in high strength low alloy steels
Chen, Jhewn-Kuang (Virginia Tech, 1992-10-03)
The kinetics of austenite decomposition were studied in high purity Fe-0.1 C-0.4 Mn-0.3 Si-X (concentrations in weight percent, X represents 3 Ni, 1 Cr, or 0.5 Mo) steels at temperatures between 500 and 675°C. The transformation stasis phenomenon was found in the Fe-C-Mn-Si-Mo and Fe-C-Mn-Si-Ni alloys isothermally transformed at 650°C and 675°C but not in the Fe-C-Mn-Si and Fe-C-Mn-Si-Cr alloys at any of the temperatures investigated. The occurrence of transformation stasis was explained by synergistic interactions among alloying elements. The paraequilibrium model was applied to calculate the metastable fraction of ferrite in each alloy. This fraction was shown to coincide with cessation of transformation in the Mo alloy transformed at 600°C. Transformation stasis was found in both the Ni and the Mo alloys isothermally reacted at 650°C and 675°C. The interactions among Mn, Si, and Mo as well as interactions among Mn, Si, and Ni appear to decrease the threshold concentrations for occurrence of transformation stasis in Fe-C-Mn-Si systems. The segregation of Mn and Mo to the α/γ boundary assisted by Si was suggested to enhance the drag force and led to transformation stasis. In the Ni alloy, lower driving force for ferite formation by addition of Ni could be responsible for occurrence of transformation stasis.
Electrode contacts on ferroelectric Pb(Zr x Ti1−x )O3 and SrBi2Ta2O9 thin films and their influence on fatigue properties
Lee, J. J.; Thio, C. L.; Desu, Seshu B. (American Institute of Physics, 1995-10-15)
The degradation (fatigue) of dielectric properties of ferroelectric Pb(ZrxTi1-x)O-3 (PZT) and SrBi2Ta2O9 thin films during cycling was investigated. PZT and SrBi2Ta2O9 thin films were fabricated by metalorganic decomposition and pulsed laser deposition, respectively. Samples with electrodes of platinum (Pt) and ruthenium oxide (RuO2) were studied. The interfacial capacitance (if any) at the Pt/PZT, RuO2/PZT, and Pt/SrBi2Ta2O9 interfaces was determined from the thickness dependence of low-field dielectric permittivity (epsilon(r)) measurements. It was observed that a low epsilon(r) layer existed at the Pt/PZT interface but not at the RuO2/PZT and Pt/SrBi2Ta2O9 interfaces. In the case of Pt/PZT, the capacitance of this interfacial layer decreases with increasing fatigue while the epsilon(r) of the bulk PZT film remains constant. This indicates that fatigue increases the interfacial layer thickness and/or decreases interfacial layer permittivity, but does not change the bulk properties. For the capacitors with RuO2/PZT/RuO2 and Pt/SrBi2Ta2O9/Pt structures, however, the epsilon(r) does not change with ferroelectric film thickness or fatigue cycling. This implies no interfacial layer exists at the interfaces and which can be correlated to the observed nonfatigue effect. Additionally, the equivalent energy-band diagrams of these different capacitor structures were proposed to complement the proposed fatigue mechanism. (C) 1995 American Institute of Physics.
Ferroelectric bismuth titanate films by hot wall metalorganic chemical vapor deposition
Si, Jie; Desu, Seshu B. (American Institute of Physics, 1993-06-01)
Ferroelectric bismuth titanate thin films were successfully deposited on Si, sapphire disks, and Pt/Ti/SiO2/Si substrates by hot wall metalorganic chemical vapor deposition. Triphenyl bismuth [Bi(C6H5)3] and titanium ethoxide [Ti(C2H5O)4] were used as the precursors. The deposition rates were in the range of 3.9-12.5 nm/min. The Bi/Ti ratio was easily controlled by precursor temperature, carrier gas flow rate, and deposition temperature. As-deposited films were pure Bi4Ti3O12 phase. The films were specular and showed uniform and fine grain size. Optical constants as a function of wavelength, were calculated from the film transmission characteristics in the ultraviolet-visible-near infrared (UV-VIS-NIR) region. The 550-degrees-C annealed film showed a spontaneous polarization of 26.5 muC/cm2 and a coercive field of 244.3 kV/cm.
Ferroelectric Thin Films for High Density Non-volatile Memories
Song, Yoon-Jong (Virginia Tech, 1998-08-13)
Ferroelectric random access memories (FRAM) are considered as future memories due to high speed, low cost, low power, excellent radiation hardness, nonvolatility, and good compatibility with the existing integrated circuit (IC) technology. The non-volatile FRAM devices are divided into two categories, based on reading technique: destructive readout (DRO) FRAM and non-destructive readout (NDRO) FRAM. Lead zirconate titanate (PZT) is recently considered as one of the most promising materials for DRO FRAM devices due to its excellent ferroelectric properties. There are remarkable advances in the applications of PZT thin films, but the direct integration into high density CMOS devices is restricted by high processing temperatures. Hence, it is desirable to lower processing temperature and develop novel high temperature electrode-barrier layers for achieving high density DRO FRAM devices. The NDRO FRAM devices have been developed mainly using metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure. This devices use the remanent polarization of ferroelectric films to control the surface conductivity of a silicon substrate. The problem of the NDRO FRAM is that the actual electric field applied to ferroelectric films is very small compared to the external electric field, because of the large depolarization field in the MFS structure and the high capacitance ratio of ferroelectric capacitor and SiO2 capacitor in series in the MFMIS structure. Since the typical ferroelectric films show very high dielectric constant over 400, it is desired to develop ferroelectric films with low dielectric constant and low coercive electric field. This research is primarily focused on developing low temperature processing and high temperature electrode-barrier layers for DRO FRAM application, and exploiting novel ferroelectric materials for NDRO FRAM application. The low temperature processing was achieved by a novel sol-gel processing, which takes advantage of in-situ electrode template layer, rapid heating-treatment without pyrolysis step, and molecularly modified precursors. The PZT films with various composition were also investigated as a function of Ti content. In order to study the integration issues for these PZT films, a substrate was constructed as Pt/TiN/TiSi₂/poly-Si, which represents a scheme of capacitor in high density DRO FRAM devices. The ferroelectric films were incorporated into the substrate, and their ferroelectric properties were investigated as a function of annealing temperature. Excellent ferroelectric properties were observed for the thin films processed at a low temperature of 500 °C as contacting between top Pt and bottom polysilicon. The other approach we have taken to overcome the integration problems in high density DRO FRAM devices is to develop high temperature electrode barrier layers. In this research, Pt/IrO2/Ir hybrid layers were prepared on poly-Si substrate as high temperature electrode-barriers. The PZT films fabricated on the Pt/IrO₂/Ir/poly-Si substrates exhibited good ferroelectric properties and outstanding fatigue properties after high temperature processing. It was observed from Auger electron spectroscopy (AES) profiles that the hybrid oxide electrode minimized fatigue problem by reducing the oxygen vacancies entrapment at the electrode/ferroelectric interfaces. This results indicated that Pt/IrO₂/Ir high temperature electrode-barrier layers promise to solve major problems of PZT integration into high density DRO memory devices. For the NDRO FRAM devices, Sr₂Nb₂O₇ and La₂Ti₂O₇ thin films were prepared on Pt-coated silicon, Si(100), and Pt/IrO₂/SiO₂/Si substrates by metalorganic deposition (MOD) technique. The Sr₂Nb₂O₇ and La₂Ti₂O₇ thin films showed the dielectric constant values of 48 and 46, respectively. However, no ferroelectricity was observed at room temperature, which might be attributed to extremely small grains. Extensive studies on preparation and properties of Sr₂(Ta_1-xNb_x)O₇ (STN) both in bulk and thin film form were carried out as a function of composition. The STN films exhibited small dielectric constant of around 46, irrespective of the composition.
Fiberoptic Polarization And Phase Modulator Utilizing Transparent Piezofilm with Indium Tin Oxide Electrodes
Sudarshanam, V. S.; Desu, Seshu B. (Optical Society of America, 1995-06-01)
A highly efficient optical polarization and phase modulator formed by the placement of a thin transparent piezofilm with indium tin oxide electrodes directly in the path of the output from an optical fiber is presented. Various configurations that differ in the clamping conditions, utilization of epoxy, and optical arrangement are presented. For a film thickness of 63.9 mu m, a linear phase-shifting coefficient of 0.131 rad/voltage peak (Vp) at 2 kHz and of 0.508 rad/Vp at 7.4 kHz is demonstrated. An intrinsic birefringence of 0.0328 between the directions along the stretch and its perpendicular in the plane of the film has been measured. The polarization modulation coefficient was determined to be 0.323 rad/Vp at 8.423 kHz, corresponding to a half-wave voltage of 8.353 Vp. Applications of the device involving concurrent spatiotemporal polarization and phase modulation are indicated.
Flash evaporator
(United States Patent and Trademark Office, 1999-03-23)
A device and method for flash evaporating a reagent includes an evaporation chamber that houses a dome on which evaporation occurs. The dome is solid and of high thermal conductivity and mass, and may be heated to a temperature sufficient to vaporize a specific reagent. The reagent is supplied from an external source to the dome through a nozzle, and may be supplied as a continuous stream, as a shower, and as discrete drops. A carrier gas may be introduced into the evaporation chamber and create a vortex flow therewithin. After evaporation, the gas vapor may be removed from the evaporation chamber through a regulating valve to a reaction chamber. Another embodiment of the invention includes a plurality of evaporating domes that separately receive reagent, and may receive reagents of differing composition.
High temperature electrode-barriers for ferroelectric and other capacitor structures
(United States Patent and Trademark Office, 1998-08-04)
A capacitor for use on silicon or other substrate has a multilayer electrode structure. In a preferred embodiment, a bottom electrode situated on the substrate has a bottom layer of Pt--Rh--O.sub.x, an intermediate layer of Pt--Rh, and a top layer of Pt--Rh--O.sub.x. A ferroelectric material such as PZT (or other material) is situated on the bottom electrode. A top electrode, preferably of identical composition as the bottom electrode, is situated on the opposite side of the ferroelectric from the bottom electrode.
Highly c-axis oriented Pb(Zr,Ti)O-3 thin films grown on Ir electrode barrier and their electrical properties
Lee, K. B.; Tirumala, S.; Desu, Seshu B. (AIP Publishing, 1999-03-01)
We have investigated the structural and electrical properties of sol-gel derived Pb(Zr, Ti)O-3 (PZT) thin films deposited on Ir electrode barrier (Ir/poly-Si/SiO2/Si). Owing to the interface-controlled growth, highly c-axis oriented perovskite PZT thin films were obtained for the postdeposition annealing temperature of 580 degrees C. Additionally, we found that the ferroelectric properties of IrO2/PZT/Ir/poly-Si capacitors were remarkably changed by the partial pressure of oxygen during the deposition of IrO2 top electrodes, which could be due to the enhanced reaction of IrO2 with PZT by the oxygen ion bombardments. Remanent polarization and coercive field of IrO2/PZT/Ir/poly-Si capacitor with the top electrodes deposited at P-O2 = 1 mTorr was 20 mu C/cm(2) and 30 kV/cm, respectively, and showed negligible polarization fatigue up to 10(11) switching repetitions. The leakage current density at a field of 80 kV was 5 X 10(-8) A/ cm(2). (C) 1999 American Institute of Physics. [S0003-6951(99)01510-7].

Browsing by Author "Desu, Seshu B."

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