C. A. Paz De Araujo

FATIGUE AND SWITCHING IN FERROELECTRIC MEMORIES : THEORY AND EXPERIMENT

A theoretical model of fatigue in ferroelectric thin‐film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen‐deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles Ps(N) are compared with both Monte Carlo simulations for a two‐dimensional Ising model and with experimental data on small‐grain (40 nm) sol‐gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the Ps(N) curves, the theory developed explains the observed linear proportionality between switching time ts(N) and polarization Ps(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol‐gel PZT switching data is obtained.

Dielectric breakdown in high-ε films for ULSI DRAMs: II. barium-strontium titanate ceramics

Abstract We present a study of breakdown fields in BaxSr1−xTiO3 ceramics as functions of thickness, temperature, applied voltage ramp rate, electrode material and cross-sectional area. The data show evidence of Zener-like breakdown mechanisms. The d.c. leakage currents at voltages below breakdown are dominated by tunneling injection and image forces at low voltages (≤3 V), Schottky behavior at intermediate voltages V a (3 V ≤ V a ≤ 6 V; 150 kV/cm ≤ E ≤ 300 kV/cm), and Fowler-Nordheim tunneling at high voltages; it is shown that both the Schottky emission regime and the ultimate breakdown arise from inter-grain effects rather than the BST/electrode interface, in accord with the theory of Neumann and Arlt and of Waser and Klee. In contrast to PbZrxTi1−xO3 (PZT), which displays avalanche breakdown and space-charge limited d.c. leakage currents at normal 5 V silicon IC voltages, BST exhibits Schottky-dominated leakage currents in this 5 V regime, and is dominated by Fowler-Nordheim tunneling in its breakdown ...

Process optimization and characterization of device worthy sol-gel based PZT for ferroelectric memories

The purpose of this work is to develop and characterize an optimized sol-gel PZT process to be used in ferroelectric memories. A review of the sol-gel process is given, including discussions on hydrolysis under acidic and basic conditions. Application of the sol-gel process to thin films is then discussed. Topics such as removal of solvents, stresses in the thin film and how this relates to cracking are mentioned. A review of different synthesis methods of sol-gel PZT is then conducted in order to help determine a device worthy sol-gel PZT process. Methods of controlling the pore size by hydrolysis and different heat treatments at various stages during the drying and annealing cycles are then used. The quality of thin film PZT on Pt is characterized by using dispersive X-ray and X-ray diffraction analyses. Electrical results yield Pr ranging from 7.9-21.9 μC/cm2Ec ranging from 29.1 -92.3 kV/cm and switching times as fast as 56 ns, using a capacitor area of 1 × 104 μm2. However, these results depend on the...

Recent results on switching, fatigue and electrical characterization of sol-gel based PZT capacitors

AbstractRecent interest in the use of capacitors, utilizing sol-gel derived PZT for non-volatile memories and ULSI DRAM cells, has made it imperative to thoroughly characterize these devices. This paper shows state-of-the-art device data in the following regimes: (1) hysteresis versus temperature and frequency; (2) temperature dependent i-t; (3) polarization versus applied voltage; (4) I-V characteristics over temperature using triangular voltage sweep (TVS) stress; (5) static dielectric constant versus temperature and frequency; (6) leakage current; (7) time dependent breakdown under constant d.c. bias; (8) fatigue at varying temperatures; and (9) retention data. Wherever appropriate, simple models are also described to explain the measured data.

Dielectric breakdown in high-ε films for ULSI DRAMs

Abstract Dielectric breakdown has been studied in several materials intended for high-dielectric DRAMs (dynamic random access memories), emphasizing lead zirconate-titanate (PZT) and barium strontium titanate (BST) ceramics. In this paper we present our results on PZT. A second paper will deal with BST. In order to distinguish among impulse thermal breakdown, de thermal breakdown, and avalanche breakdown mechanisms, studies have been carried out as functions of temperature, electrode material and shape, frequency and duration of applied fields, and specimen size and shape. Notable in the results is the fact that maximum breakdown field varies directly with electrode work function but is uncorrelated with electrode thermal conductivity; this militates against a purely thermal breakdown interpretation and instead favors an avalanche mechanism in which the iniation step is impact ionization of Ti ions from electrons emitted from the electrodes, followed by thermal run-away. Thickness dependence also favors a...

Process Dependent Electrical Characteristics and Equivalent Circuit Model of SOL-GEL Based PZT Capacitors,

Abstract Studies of electrical properties and an equivalent circuit model is developed for ferroelectric PZT(Ti = 60%) thin film capacitors made by sol-gel spin coating with Pt electrodes. The equivalent circuit consists of two major parts: serial space charge capacitors demonstrating surface effects and parallel elements modeling the inner polycrystalline ferroelectric regions. This model is based on device physics which can demonstrate both the measured capacitance voltage characteristics and hysteresis curves. From the model fit to the data, an estimate of the space charge concentration at the surface and inner grain boundary region of 57times;1020 cm−3 and 1×1018 cm−3 respectively is made. Further electrical characterizations such as pulse switching and polarization degradation (fatigue) have also been studied. Using the equivalent circuit, other characteristics such as the switching time can be studied showing its dependence on applied voltage and capacitor area. The applied voltage dependence of fat...

Thickness dependence of D.C. leakage current in lead zirconate-titanate (PZT) memories

Abstract We have measured the voltage (field) dependence, the thickness dependence, and the temperature dependence of d.c. leakage currents in PbZr0.40Ti0.60O3. In agreement with our earlier results [Scott et al., J. Appl. Phys. 70, 382 (1991)], the voltage dependence is given by I(V) = AV + B(V - V 0)2, implying space-charge limited currents. In addition, in the present work we find that I(d) = C/d 3 at constant voltage, where d is the thickness and C is a constant dependent upon voltage and cross-sectional area. This inverse-cubic dependence of current confirms that the leakage is dominated by space charge effects.

Deposition of Ba1−xSrxTiO3 and SrTiO3 via liquid source CVD (LSCVD) for ULSI DRAMS

Abstract In 1991 we demonstrated the use of a deposition machine that injected liquid sol-gel material into a vacuum chamber at room temperature for the production of high quality lead zirconate titanate (PZT) thin films. This year we have modified the machine to produce strontium titanate and barium strontium titanate films of sufficient quality for DRAM applications.

Low temperature process for strontium bismuth tantalate thin films

Abstract As CMOS dimensions shrink so does the limitation on total thermal budget for processing. For 0.18 μm design rules, the junction depth and the salicide process requirements limit the maximum processing temperature to below 700°C, preferably down to 650°C. For FeRAMs, this results in limitation on the thermal budget available for the crystallization of ferroelectric films. SBT and SBTN films have been generally annealed at 700°C or higher. Lowering the crystallization temperature down to 650°C requires the suppression of fluorite phase in order to obtain good ferroelectric performance. We have developed a CSD based low temperature process for SBT films yielding excellent ferroelectric properties. Several materials and process parameters have been optimized to suppress the fluorite phase. These include film stoichiometry and thickness, anneal ambient and ramp rates, UV energy and precursor solvents. In this paper we present a complete 650°C process for SBT thin films, highlighting process modifications and their effect on ferroelectric performance.

Integration technology of ferroelectrics and the performance of the integrated ferroelectrics

Abstract We have successfully incorporated the ferroelectric and the high dielectric constant capacitors into integrated circuits. The GaAs MMICs with BST capacitors have been widely used for cellular phones. The BST technology is also applied to a silicon CCD delayline processor for VCRs and camcorders. With respect to the ferroelectric technology with Y1, an experimentally fabricated 256k bit FeRAM has exhibited the remarkable performance of the 100 ns and 3V operation with a 1T/1C cell configuration dedicated for the FeRAM. These integrated ferroelectrics have been achieved by controlling the ferroelectric properties in thin films and incorporating the films into GaAs and silicon devices with outstanding process technology. Furthermore, we refer to the memory cell design technology which enables the FeRAM to work below 1V. Various advantages of low-voltage and high-speed operation inherent in integrated ferroelectrics will be emphasized on the intelligent microelectronics applications toward the next m...