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

Abstract

We present a study of breakdown fields in Ba_xSr_1−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 PbZr_xTi_1−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 regime (ca. 120 MV/m). The reason for these differences in two otherwise similar perovskite titanates is due, we believe, to the columnar grain structure in PZT thin films, where the grains extend from electrode to electrode; this eliminates Zener breakdown across grain boundaries in PZT films. But it permits large space charge currents along PZT grain boundaries. The role of double injection (of electrons and holes) is discussed, and comparisons are made with PZT leakage current characteristics.

Keywords:

• breakdown
• barium strontium titanate
• thin films
• dram