Tatsuo Otsuki

Ferroelectric Nonvolatile Memory Technology and Its Applications

Nonvolatile memory utilizing ferroelectric material is expected to be the ultimate memory due to its theoretical low power operation and fast access. We integrated a ferroelectric thin film using a standard complementary metal-oxide-semiconductor (CMOS) process and evaluated its basic characteristics and reliability including endurance and imprint effect. The film was prepared using a spin-on sol-gel method. A ferroelectric thin film formed using liquid source misted chemical deposition (LSMCD) was found to have almost the same characteristics as those of the film formed by the sol-gel method. No effects of the ferroelectric process on the CMOS transistors were observed. Design of ferroelectric memory cells and applications of the ferroelectric nonvolatile memory have been reviewed.

Temperature-dependent current-voltage characteristics of fully processed Ba0.7Sr0.3TiO3 capacitors integrated in a silicon device

Temperature-dependent current-voltage characteristics of fully processed Ba0.7Sr0.3TiO3 thin film capacitors integrated in a charge-coupled device delay-line processor as bypass capacitors were studied. The thin film capacitors with a film thickness of 185 nm were formed by metal-organic decomposition processing. The leakage current measured after completion of the integration process was 1 to 2 orders of magnitude higher than that measured after capacitor patterning. The leakage current at low voltages ( 10 V, 500 kV/cm), the Schottky mechanism plays a dominant role in leakage current, while the Frenkel-Poole emission begins to contribute to the leakage current as the temperature is elevated.

FeRAM circuit technology for system on a chip

The ferroelectric memory (FeRAM) has a great advantage for system on a chip, since FeRAM not only supports non-volatility but also delivers a fast memory access similar to that of DRAM. To enhance the applicability of FeRAM for embedded reconfigurable hardware, three circuit technologies are discussed in this paper. Simulation and measurement data confirmed that both power consumption and memory area can be substantially reduced, making FeRAM the most promising new technology for implementing high-performance, low-power reconfigurable hardware.

Retention Characteristics of a Ferroelectric Memory Based on SrBi2(Ta, Nb)2O9

The polarization decay process in SrBi 2 (Ta, Nb) 2 O 9 capacitors and retention characteristics of a 288-bit ferroelectric memory device fabricated from SrBi 2 (Ta, Nb) 2 O 9 were studied. The remanent polarization decay at room temperature showed good linearity when plotted against logarithmic retention time over a wide range of 10 -3 -10 5 s. The distribution of times to failure of a 288-bit memory was fit to a model having a linear relationship between log(log t f ) and 1/T for the period of infant failures and to the Arrhenius model having the form log t f vs 1/T for the period of random failures, where t f is the time to failure and T is the temperature. The activation energy was found to be 0.35eV for infant failures and 1.15eV for random failures. Possible causes for the difference in activation energies are discussed.

Voltage Shift Effect on Retention Failure in Ferroelectric Memories

We investigated the origin of retention failure in ferroelectric memories (FeRAMs) with SrBi2(Ta, Nb)2O9 (SBTN) memory cell capacitors by considering the time-dependent behavior of polarization vs. voltage (P-V ) curves of the capacitors during high-temperature storage. Since the SBTN capacitors exhibited no marked decrease in the nonvolatile component of polarization even after high-temperature storage, we focused on the effect of voltage shift observed in P-V curves. We calculated bitline voltage along the storage from the P-V curves and the bitline capacitance, and successfully estimated a decrease in the bitline voltage, which is in agreement with the retention failure in FeRAMs. In addition, the calculation indicated that the lifetime limited by the retention failure in FeRAMs with SBTN capacitors at 125°C exceeds 10 years.

Time-dependent leakage current behavior of integrated Ba0.7Sr0.3TiO3 thin film capacitors during stressing

Time-dependent leakage current behavior of integrated Ba 0.7 Sr 0.3 TiO 3 capacitors accelerated by stresses in excess of operating temperature and voltage was studied. Current-voltage (J-V) studies revealed that the time-dependent leakage current behaviors are different according to the initial conduction process. When the initial leakage current of a fully processed integrated capacitor at high voltages at elevated temperatures is of the Frenkel-Poole emission type, the leakage current increases rapidly with time. The difference in the initial leakage currents is related to the difference in film growth conditions which determine the formation of defects in the films. The time-dependent increase in leakage current is ascribed to a change in the conduction mechanism from the interface-controlled Schottky type to the bulk-related space-charge-limited type due to the accumulation of oxygen vacancies near the cathode as a result of interface barrier lowering and the migration of distributed oxygen vacancies across the film.

Quantum jumps in FeRAM technology and performance

Abstract Layered perovskite ferroelectric materials (also known as “Y-1”) allowed a quantum jump in Ferroelectric Random Access Memories (FeRAMs) due to their low operating voltages and excellent endurance properties. The ability to impose technological control in the material microstructure and overall device properties has made a major impact in the commercialization of FeRAMs. In this paper, we review our new material and integration technologies, and present the performance of “state-of-the-art” devices incorporating Y-1 FeRAMs, such as a microcontroller and an RF-ID tag. These data indicate not only the great potential of the Y-1 technology but also its maturity for the production of commercial products.

SI LSI PROCESS TECHNOLOGY FOR INTEGRATING FERROELECTRIC CAPACITORS

The fabrication procedure of ferroelectric thin film capacitors onto conventional Si LSIs is investigated. Electrical characteristics of the integrated Ba1-x Srx TiO3 (BST) capacitors and the metal-oxide-semiconductor transistors embedded in the Si substrate are examined. Results of these measurements suggest the usefulness of this integration process for the fabrication of ferroelectric thin film devices, which is substantiated by the evaluation of an analog/digital IC with the integrated BST bypass capacitor.

Object recognition with luminance, rotation and location invariance

We propose a neural network based on image synthesis, histogram adaptive quantization and the discrete cosine transformation (DCT) for object recognition with luminance, rotation and location invariance. An efficient representation of the invariant features is constructed using a three-dimensional memory structure. The performance of luminance and rotation invariance is illustrated by reduced error rates in face recognition. The error rate of using a two-dimensional DCT is improved from 13.6% to 2.4% with the aid of the proposed image synthesis procedure. The 2.4% error rate is better than all previously reported results using Karhunen-Loeve (1990) transform convolution networks and eigenface models. In using the DCT, our approach also enjoys the additional advantage of greatly reduced computational complexity.

Study of Pt Bottom Electrodes using High-Temperature Sputtering for Ferroelectric Memories with SrBi2Ta2O9 (SBTO) Film

Pt bottom electrodes for SrBi2Ta2O9 (SBTO) capacitors have been investigated for use in nonvolatile memories. A Pt thin film which is sputtered at a temperature of 300°C consists of enlarged columnar grains, while a film sputtered at room temperature consists of fibrous columnar grains. The formation mechanism of enlarged grains of the Pt film sputtered at 300°C is due to the fact that the film structure changes with substrate temperature based on the structure-zone model. The Pt bottom electrode which consists of the enlarged grains results in a decrease in the Ti diffusion path, leading to effective adhesion of Pt to a SiO2 film.