L. D. McMillan

Quantitative measurement of space‐charge effects in lead zirconate‐titanate memories

By combining Auger data on the width of an oxygen depletion layer near the Pt electrodes with a modified Langmuir–Child law for the leakage current: I(V) = aV + bV2, we deduce parameters related to the space‐charge density and field in 210‐nm‐thick PbZr1−xTixO3 memories. The results are compared with the space charge fields inferred by Okazaki (∼10 kV/cm for PZT), which involve measuring the switching speeds ts(E) for positive and negative voltages. Differences in the voltage dependencies of the leakage current are found after fatigue and are related to specific electrochemical processes involving oxygen deposition on electrode surfaces.

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 ...

A 256 kb nonvolatile ferroelectric memory at 3 V and 100 ns

One of the most important features for ferroelectric material is fast write at low voltage. This feature is used in a 256 kb nonvolatile memory that operates at 3 V power supply with a read/write time of 100 ns. Active current is 3 mA at 200 ns cycle time at 3 V for battery operation. The cell consists of 1 transistor and 1 capacitor per bit (1T1C) permitting a high level of integration. For low-voltage low-power operation, use is made of a preset reference-cell circuit, wordline boost circuits with a ferroelectric boosting capacitor and a divided-cell plate circuit.<<ETX>>

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.

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.

Effects of scaling the film thickness on the ferroelectric properties of SrBi2Ta2O9 ultra thin films

We have investigated the effect of reducing the thickness of strontium bismuth tantalate film to as low as 25 nm on its ferroelectric characteristics. A degradation of ferroelectric properties such as significant reduction in remanent polarization is generally observed with reduction in film thickness, in particular below 100 nm. This has been overcome by using a modified deposition process sequence and a crystallization technique based completely on the rapid thermal annealing process. The resulting ultrathin films show good remanent polarization, low-voltage saturation, low leakage current, high breakdown strength, and good endurance. These films demonstrate the potential for scaling and are excellent candidates for several generations of ferroelectric random access memory applications.

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.