Koji Arita

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.

Graded PZT thin film capacitors with stoichimetric variation by MOD technique

Abstract By tailoring Zr/Ti ratio in Pb(Zr,Ti)O3 from 45/55 to 75/25, functionally graded PZT thin films were prepared by metal organic decomposition (MOD) technique. P-E hysteresis loops were measured, and polarization was found to shift up or down depending on the direction of the composition gradients of the PZT. This polarization offset was modeled using Poissons equation in one dimension. Switching charge was also found to be compositional gradient direction dependent.

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.

Low temperature crystallization of SrBi2Ta2O9 (SBT) films

Abstract 650°C process of SrBi2Ta2O9 (SBT) has been achieved through the use of new metal organic deposition (MOD) solution and the optimization of the deposition conditions. The sample showed a high remnant polarization (2Pr) of 14 μC/cm2 @3V, a low leakage current of 10–8 A/cm2 or less @3V, and a fatigue-free nature. We believe this processing will realize high-density FeRAM integration of SBT.

Electrical Properties of (Ba, Sr)TiO3 Films on Ru Bottom Electrodes Prepared by Electron Cyclotron Resonance Plasma Chemical Vapor Deposition at Extremely Low Temperature and Rapid Thermal Annealing

(Ba, Sr)TiO3 (BST) films were prepared on Ru bottom electrodes by electron cyclotron resonance chemical vapor deposition at extremely low temperature and rapid thermal annealing (RTA). Leakage current characteristics were improved by lowering the BST deposition temperature down to 120°C. (Ba+Sr)-rich films with a (Ba+Sr)/Ti ratio of 1.1–1.5 had lower leakage current densities than stoichiometric and Ti-rich films with a ratio of 0.8–0.9. Cross sectional transmission electron microscopy observations showed that the 120°C-deposited and 700°C-RTA-treated (Ba+Sr)-rich film had a granular structure and smooth interfaces with the electrodes. The stoichiometric and Ti-rich films had columnar structures and larger interface roughness. As a result, low leakage current density less than 10-7 A/cm2 at ±1 V were obtained for 30 nm-thick BST films with a (Ba+Sr)/Ti ratio of 1.1–1.5 by combination of 120°C deposition and 700°C RTA.

Vapour—liquid equilibrium of the non-azeotropic refrigerant mixture formed by chlorofluoromethane and 1,1,1,2-tetrafluoroethane

Abstract Isothermal vapour—liquid equilibrium for the chlorofluoromethane (HCFC22)—1,1,1,2-tetrafluoroethane (HFC134a) system was measured at 273.16, 298.16 and 323.16 K, by the static method. The data obtained were correlated with use of the Redlich—Kwong—Soave equation of state with one interaction parameter.

The future of ferroelectric memories

Since 1984, ferroelectric RAMs (FeRAMs) have been demonstrated in many applications such as smart cards and low-density memories. Prior to 1984, attempts failed because of the poor quality of thin films of complex materials. Currently, two materials compete for the large-scale integration development of FeRAMs. The first is a perovskite ceramic known as PZT (PbZr/sub 1-x/Ti/sub x/O/sub 3/). The second material is known as a layered perovskite such as SBT (SrBi/sub 2/Ta/sub 2/O/sub 9/). For low-density devices which employ thin films of either material with a thickness <300 nm operated at 3-5 V, both materials yield approximately the same results. As FeRAMs enter the deep submicron realm, the ferroelectric thin-film technology is ready to support high-density integration. SBT-based devices can be integrated as capacitors in DRAM-like 1T/1C stacked cells and flash-like FeFET cells. Experience with embedded FeRAMs is positive, so that the system-on-chip as well as stand-alone high-density devices are foreseen. The possibility of 1 V operation at a few to several tens of nanoseconds write with nonvolatility brings FeRAMs to the forefront of non-volatile memories. Scaling of capacitor areas as small as 0.04 /spl mu/m/sup 2/ is possible. With capacitor and FET technologies, FeRAMs blur the line between non-volatile memories as DRAM-like destructive read-out (DRO) devices and flash-like non-destructive read-out (NDRO) devices, which compete for the highly mobile generation of Internet devices and G-3 phones.