Michael C. Scott

Structure and Device Characteristics of SrBi 2 Ta 2 O 9 -Based Nonvolatile Random-Access Memories

Recently there has been a paradigm shift in nonvolatile computer memories from silicon-technology-based EEPROMs (electrically erasable, programmable read-only memories) to devices in which the stored information is coded into + and − polarizations in thin-film ferroelectric capacitors. Such devices have read and erase/rewrite speeds of the order of 1–35 ns, many orders of magnitude faster than the erase/rewrite speeds of the best EEPROMs (Table I). However, fundamental questions concerning their lifetimes had delayed full commercialization. Because ferroelectrics normally have extremely large dielectric constants, their use as nonswitching capacitors in dynamic random-access memories (DRAMs) is also rapidly evolving. The majority of studies to date have emphasized lead zirconate titanate (PZT)-based capacitors for nonvolatile ferroelectric random-access memories (NVFRAMs) and barium strontium titanate-based capacitor DRAMs (see Table II).

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 ...

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.

Characterization of self-patterned SrBi2Ta2O9 thin films from photo-sensitive solutions

Abstract Self-patterned SrBi2Ta2O9 thin films were successfully fabricated from photo-sensitive solutions by means of UV irradiation through photo masks. After conventional baking and wet etching the films were annealed. The photo-sensitive SrBi2Ta2O9 solutions give high resolution negative-pattern of the mask image down to 1 μm line width by deep UV irradiation at 900 mJ/cm2. The capacitor characteristics of the 210 nm thick films fabricated on the Pt/Ti/SiO2/Si substrates by this process showed 2Pr values of 17 μC/cm2, 2Ec of 89 kV/cm, and leakage current densities of 5×10−9 A/cm2 at 5 V. The films showed no fatigue after 1×1011 switching cycles.

Microstructure-induced Schottky barrier effects in barium strontium titanate (BST) thin films for 16 and 64 Mbit (DRAM cells)

The current-voltage and capacitance-voltage characteristics of fine-grained, ceramic BST thin films for ULSI (ultralarge-scale integrated) DRAM (dynamic random-access memory) applications are examined. A model and pertinent data for integrated BST capacitors used in 16/64-Mb DRAM cells are presented. The results confirm in detail the conclusion of R. Waser and M. Klee (1991, 1992) that conduction in ceramic SrTiO/sub 3/ is Schottky-dominated and that the Schottky barriers arise from depletion regions at grain boundaries and not only at the ferroelectric/electrode interface.<<ETX>>