THE STUDY OF THE MICROSTRUCTURE AND TUNABILITY OF Ba(SnxTi1-x)O3 THIN FILMS

ABSTRACT

Barium tin titanate Ba(Sn_xTi_1?x)O₃ (BTS, x = 0.05,0.1,0.15,0.2,0.25) thin films have been prepared by sol-gel technique on Pt/Ti/SiO₂/Si substrates. The influences of Sn content on the evolution of the phase and microstructure of the thin films were examined. Dielectric properties of the thin films were investigated as a function of frequency and direct current electric field. The result shows that the Sn content strongly influences the microstructure and the dielectric properties of the films. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

Conduction mechanism in antiferrolectric PbZrO3 thin films—analysis of charge carrier trapping phenomenon

Abstract

Antiferroelectric compositions, such as PbZrO₃, are attractive candidates in charge storage devices and actuator/transducer applications in MEMs technology. Thin films of PbZrO₃were deposited on Pt coated Si substrates by a pulsed excimer ablation process. The process of field induced ferroelectric phase switching involves the domain wall reorientation in the polycrystalline thin films. The presence of grain boundaries and various defects in the polycrystalline thin films acts as the pinning sources for the various domain walls. These defects capture the charge carriers in the presence of external applied field and hinders further switching of the dipoles in the domains, thereby increases the response times and threshold voltages for the devices operations. Understanding of the trapping phenomenon in these films is very essential. Using Lampert's theory of space charge limited conduction both shallow and deep trap energies were estimated approximately from charge transport analysis.     

Antiferroelectric lead zirconate thin films by excimer laser ablation

Abstract

Utilization of antiferroelectric thin films was proposed for high charge storage capacitors and transducer applications. The volume changes that are associated with the AFE? FE and FE ? AFE phases are high enough to use them in MEMs device technology. Lead zirconate was the first identified antiferroelectric compound with a reported dielectric phase transition temperature of ～ 230°C. In this article, deposition of lead zirconate thin films by a pulsed excimer laser ablation technique is reported. The antiferroelectric nature of the lead zirconate thin films were confirmed by the presence of double hysteresis loop in polarization vs. applied electric field response as well as double butterfly behavior in capacitance vs. voltage characteristics. The variations in the polarization hysteresis with temperature were elucidated in detail. The switching times between the field induced FE and AFE phases (backward switching) were studied at various applied electric fields.     

Studies on piezo coefficients of PLD and sol-gel grown PZT thin film for devices

Abstract

The effect of electrical (DC) contact poling processes on the ferroelectric and piezoelectric properties of sol–gel and pulsed laser-deposited PZT thin films has been investigated as a function of the poling field, temperature and time. The remnant polarization and piezoelectric coefficient are found to increase with and saturate at dc-poling field of 100- 300?kV/cm, temperature of 100–120?°C and poling time of 10–20?min. as compared with un-poled PZT thin films. The P-E hysteresis loops of poled PLD grown PZT films shows sharper switching behaviour as compared to imbalanced loop in sol-gel grown films. An improvement of piezoelectric and ferroelectric properties of sol–gel PZT thin films is found, as compared to those deposited using pulsed laser deposition (PLD), indicating that a poling process is required for sol–gel PZT thin films.     

SWITCHING TRANSIENTS IN KRAMER DRIVE

Abstract

This paper deals with the torque, speed and current transients of a Kramer drive consequent to switching. Various starting conditions have been considered and torque, speed and current transients are computed in each case. Effect of D.C. motor field current, load torque, system inertia and filter inductance of these transients have been discussed. Experimental results are also presented     

Characteristics of lithium niobate based capacitors and transistors

Abstract

The electrical properties of thin film (<1000 Å) capacitor devices of lithium niobate grown on silicon and platinum and of thicker film metal-ferroelectric-semiconductor field effect transistors (MFSFET) with lithium niobate as the gate material were measured. Dielectric constants of the thin films on silicon were as high as 27, while those for films on platinum were as high as 49. The MFSFET structures showed good FET properties, and demonstrated a channel current modulation consistent with switching of the ferroelectric gate by pulsing.     

Advances in processing and properties of perovskite thin-films for FRAMs,DRAMs, and decoupling capacitors

Abstract

Highly oriented, dense, and crack-free ferroelectric and paraelectric thin-films on three inch diameter Pt/Ti/Si₃N₄/Si (100) substrates were obtained by polymeric sol-gel processing. Ferroelectric PZT thin-films were fabricated at temperatures as low as 550°C within 15 minutes by rapid thermal annealing. The films heat treated at 700°C for 5 minutes were single grain thick and exhibited P_r, P_sp, and E_c in the ranges of 29–32 μC/cm², 44–53 μC/cm², and 50–60 kV/cm, respectively, and high speed switching times below 5 ns on 30×30 μm² electrodes. A switching time of 2.7 ns was observed on 19×19 μm² area electrodes at a field of 200 kV/cm. Results of low and high field characterization on paraelectric PLT thin-films which were conventionally heat treated indicated that it has an excellent potential for use in ULSI DRAMs and as decoupling capacitors. These films showed a high charge storage density (15 μC/cm²) and a low leakage current (0.5 μA/cm²) at a field of 200 kV/cm. Also, the charging time for a capacitor area of 1 μm² at 200 kV/cm was estimated to be 0.10 ns.     

EFFECT OF SILICON CARBIDE PASSIVATION FILM ON THE RESONANCE CHARACTERISTICS OF CANTILEVER

ABSTRACT

We have used silicon carbide (SiC) thin films as an insulating material of the PZT micro cantilevers for electrical and biological passivation. The use of SiC thin films as a passivation layer of the PZT microcantilevers is also seemingly viable to insure the high mass sensitivity as well as the stable passivation. In this study, we report the effect of SiC passivation layer on the performance of the PZT microcantilevers. The micromachined PZT microcantilevers having a structure of SiN_x/Ta/Pt/PZT/Pt were fabricated through MEMS processes. In order to improve the mass sensitivity and the passivation, SiC thin films of the high elasticity material were deposited on the cantilever using plasma enhanced chemical vapor deposition (PECVD) at the temperature of 400°C. Plane-strain modulus of SiC thin film was measured by nanoindentation. We observed that SiC thin films showed higher Young's modulus than Si and SiO₂. Before and after the deposition of SiC thin films, the end-tip deflection and the resonant frequency change of microcantilevers were measured by a confocal microscope and an impedance analyzer. It was confirmed that end-tip deflection of microcantilever was reduced by 13～18% through the deposition of SiC thin films, indicating the stress relaxation of the microcantilevers.     

Comparison of sputtered SBTN and PLZT thin film capacitors for FRAM applications

Abstract

Thin films of the solid solution SrBi₂Ta₂O₉-SrBi₂Nb₂O₉ (SBTN) and Ca and Sr doped (Pb, La)(Zr, Ti)O₃ (PLZT) were prepared by conventional, single target, rf-magnetron sputtering. The films were deposited onto Pt coated substrates with a diameter of 6 inches. The electrical properties were characterized by hysteresis and pulsed field switching measurements. A ferroelectric performance comparison between two types of capacitors will be presented. The recent improvement in PLZT capacitor processing have greatly extended fatigue resistance beyond what is usually observed and reported for SBTN capacitors.     

EFFECT OF ANNEALING ON THE ELECTRICAL PROPERTIES OF SrTiO3 THIN FILMS PRODUCED BY ION BEAM SPUTTERING

ABSTRACT

Thin film capacitors with SrTiO₃ (STO) as dielectric and Pt as electrode material have been prepared by ion beam sputtering. The as-deposited film is amorphous and exhibits a crystallization temperature around 321°C as proved by X-ray diffraction. The effect of post annealing on the crystalline quality of the films was systematically studied by x-ray diffraction and Atomic Force microscopy (AFM). The temperature and frequency dependent dielectric properties were measured from 30°C to 200°C and 0.01 Hz to 10⁵ Hz, respectively. The influence of the microstructure of SrTiO₃ thin films on their electrical properties was investigated through an extensive characterization. The electrical properties of SrTiO₃ films appear to be strongly depending on the annealing temperatures. The capacitance voltage (C-V) characteristics reveal an improvement of capacitance density with increasing the annealing temperature.     

EFFECTS OF PZT BUFFER LAYERS ON PZFNT FERROELECTRIC THIN FILMS

ABSTRACT

PZFNT thin films were fabricated on 5-inch Pt/Ti/SiO₂/Si and PZT/Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering method and rapid thermal annealing process. By investigating the two thin films at various annealing temperatures, the results show that the annealing temperature of PZFNT thin films without PZT buffer layers is about 730°C, which is higher than that of PZFNT films with PZT buffer layers. By use of PZT buffer layers, the annealing temperature of PZFNT films is decreased greatly, and the dielectric and ferroelectric properties are improved. In the optimum process, the thin films with PZT buffer layers have a dielectric constant of 1199 and dielectric loss of 3.0% at 1 KHz. The remanent polarization and coercive field of the thin films are 21.1 μC/cm² and 53.5 KV/cm respectively. The films have the significant potential for FRAM and pyroelectric infrared detectors.     

Low temperature preparation of ferroelectric SrBi2Ta2O9 thin films by a modified RF magnetron sputtering technique

Abstract

Bi–layered ferroelectric SrBi₂Ta₂O₉ (SBT) films were successfully prepared on Pt/Ti/SiO₂/Si substrates at 650°C by a modified rf magnetron sputtering technique. The SBT films annealed for 1 h in O₂ (760 torr) and again for 30 min in O₂ (5 torr) at 650°C show a average grain size of about 49 nm. The SBT films annealed at 65 0°C have a remanent polarization (P_r) of 6.0 μC/cm² and coercive field (E_c) of 36 kV/cm at an excitation voltage of 5 V. The films showed fatigue–free characteristics up to 4.0 × 10¹⁰ switching cycles under 5 V bipolar pulse. The retention characteristics of SBT films looked very promosing up to 1.0 × 10⁵ s.     

Optically addressed ferroelectric memory and its applications

Abstract

Thin films of perovskite titanates can be composition-tailored to exhibit ferroelectric, pyroelectric, or piezoelectric properties, in varying degree of combinations, and thereby modulating their response when illuminated with light. This paper reviews the potential applications of photoresponse from lead zirconate titanate thin films. In ferroelectric materials, such as lead zirconate titanate (PZT), for example, the photoresponse shows a clear dependence on remanent polarization. The main highlight of the paper is a review of the concept of optically addressed ferroelectric memory and identification of its high impact applications. Incidence of energetic laser beam pulses gives rise to two different classes of phenomena: first, a thermally triggered piezoelectric/pyroelectric response and second, an optoelectronic response. Optimizing the device geometry and selecting the illumination characteristics, one can easily control the dominating mechanism in a device. The optoelectronic effect emerges at a relatively lower incident beam power (≤2 mW/μm²) and can be as fast as the duration of the pulse (≤10ns), and therefore, is directly relevant as a non-destructive read-out signal from optically addressed ferroelectric memory. The optical NDRO signal offers itself as a unique tool that allows a non-destructive ‘probe’ for the capacitor, without causing any polarization switching in it, the characteristic artifact of the electrical destructive readout (DRO) measurement technique. Furthermore, the polarization dependent photoresponse could also be exploited as a non-destructive evaluation tool for mapping the domains in ferroelectric films, to generate fundamental understanding of the domain dynamics (generation, growth, movement, etc., under applied field, illumination, and/or temperature) in such thin films. In addition, with a high fidelity analog nature of the remanent polarization, the highly parallel, high speed photoresponse output from such optically addressed memories may be ideally suited for high performance computing applications especially involving image processing, high speed communication and parallel processing with architectures, such as large scale artificial neural networks.     

A LINEAR TRANSFER FIELD MACHINE FOR ASYNCHRONOUS MODE OF OPERATION

ABSTRACT

For the first time, this paper describes a working model of a novel linear transfer field machine along with an equivalent circuit analysis method for the asynchronous mode of operation In the sinusoidal steady-state. The machine is found to have a thrust-speed characteristic like unto that of the flat linear induction machine with a conducting sheet secondary. Analysis of performance is made for values of speed across the range from standstill to synchronous speed. Laboratory measured data are compared with calculated values for standstill operation.     

PERFORMANCE STUDY OF A CHOPPER-CONTROLLED INDUCTION MOTOR

Abstract

In this paper, the speed of an induction motor is controlled using a chopper circuit on the rotor side. Analysis of transient, including switching power supply and changing in duty cycle of chopper, and steady-state performance of the chopper-controlled induction motor are predicted-Analysis and simulation consider the induction motor equations from the rotor side where a diode bridge is connected. Experimental results are obtained to verify predicted behaviour based on this digital simulation.     

LAGRANGE MULTIPLIERS FOR THE INTERMEDIATE METALLIC REGIONS IN THE COMPUTATION OF THE POTENTIAL DISTRIBUTION OF INSULATOR CHAINS BY FEM

ABSTRACT

It is very important to predict the behaviour of the high voltage insulators at the operating conditions from the point of improving of the design and practical selection of the insulator type. Therefore, it is necessary to compute the potential and electric field distributions and to determine the critical regions forced much more by the electric field. In case the insulator surface is clean or the pollution film at the surface is dry, a little capacitive current flows and the solution of the problem results in a capacitive potential distribution.

In the numerical computational methods of potential distribution of insulator chains, it requires to insert into the resulting equation system as boundary conditions of the intermediate metallic zones that connect two units that have the same potential at every point. In this work, the potential distribution of clean insulator chains have been computed using the Finite Element Method (FEM). The metallic zones between the units have been taken into account as equipotential surfaces by Lagrange Multipliers. The computed results were compared with experimental results obtained by using the Conducting Paper Method and good agreement was observed.     

BIPOLAR RESISTIVE SWITCHING IN Au/TiO2/Pt THIN FILM STRUCTURES

ABSTRACT

The conditions and physical mechanisms of electroforming and subsequent resistive switching in Au/TiO₂/Pt thin film structures were investigated. It was concluded that the electroforming, being a current-limited electric breakdown of the TiO₂ films, resulted in a considerable increase of the oxygen vacancy concentration in the bulk and on the surface of the films. The resistive switching implemented by short voltage pulses of different polarities is proposed to be due to the change of the Schottky barrier height at the Pt/TiO₂ interface as a result of the current-induced variation of the occupancy by electrons of the surface states in the band gap of TiO₂.     

Pulsed laser deposition (PLD) of oriented bismuth titanate films for integrated electronic applications

Abstract

In this paper we describe recent successes of growth of epitaxial bismuth titanate (BTO) films by pulsed laser deposition (PLD) suitable for electro-optic and electrical switching device structures, and fabrication of an improved gate structure for a ferroelectric memory FET (FEMFET). TEM and x-ray results indicate that excellent crystalline quality BTO films were achieved on LaAlO₃. Polarization switching was demonstrated for BTO capacitors with epitaxial superconducting YBa₂Cu₃O₇ as the lower electrode. Using an SiO₂ buffer layer, a BTO/Si structure was fabricated and direct charge modulation in the Si by polarization reversal in the BTO was demonstrated.     

STRUCTURE AND PROPERTIES OF EPITAXIAL THIN FILMS OF Bi2FeCrO6: A MULTIFERROIC MATERIAL POSTULATED BY AB-INITIO COMPUTATION

ABSTRACT

Experimental results on Bi₂FeCrO₆ (BFCO) epitaxial films deposited by laser ablation on SrTiO₃ substrates are presented. It has been theoretically predicted using first-principles density functional theory that BFCO is ferrimagnetic (with a magnetic moment of 2μ_B per formula unit) and ferroelectric (with a polarization of ～ 80 μ C/cm² at 0K). The crystal structure investigated using X-ray diffraction shows that the films are epitaxial with a high degree of crystallinity. Chemical analysis carried out by X-ray Microanalysis and X-ray Photoelectron Spectroscopy indicates the correct cationic stoichiometry in the BFCO layer, namely (Bi:Fe:Cr = 2:1:1). Cross-section high-resolution transmission electron microscopy images together with selected area electron diffraction confirm the crystalline quality of the epitaxial BFCO films with no identifiable foreign phase or inclusion. The multiferroic character of BFCO is proven by piezoresponse force microscopy (PFM) and magnetic measurements showing that the films exhibit ferroelectric and magnetic hysteresis at room temperature. The local piezoelectric measurements show the presence of ferroelectric domains and their switching at the sub-micron scale.     

CALCULATION OF THE PULL-OUT TORQUE OF HYBRID STEPPING MOTORS IN THE HALF-STEP MODE

ABSTRACT

A method is described for predicting the pull-out torque/speed characteristics of hybrid stepping motors operating in the half-step excitation mode. The analysis assumes initially a linear magnetic circuit in order to derive an expression for the pull-out torque. Subsequently, the important effect of magnetic non-linearity is introduced into the calculation method by appropriate adjustment of the flux-linkage coefficients using an iterative computational routine. A comparison of experimental and predicted results is presented for a commercial 1.8°(200 step/revolution) stepping motor