Stress Anisotropy in Silicon Oxide Films

Abstract

The residual stress in films formed by vacuum sublimation of bulk silicon monoxide has been studied as a function of the angle of incidence of the evaporant. Using a source temperature of 1350–1400°C and a system total pressure of ∼2×10⁻⁶ Torr, a pronounced dependence of stress on angle of incidence and a distinct stress anisotropy was observed. The stress in the x direction (i.e., perpendicular to the direction of the evaporant irrespective of substrate orientation) increases from 4×10⁸ dyn/cm² for normal incidence to 14×10⁸ dyn/cm² for an incident angle of ∼50° and then decreases to 4×10⁸ dyn/cm² for an incident angle of 80°. The stress in the y direction (i.e., perpendicular to the x direction) is peaked at ∼30° at a value of 13×10⁸ dyn/cm² and then decreases to ∼0.5×10⁸ dyn/cm² at 80°. Films formed at normal incidence are quite stable and not susceptible to the effects of water vapor. However, films formed by depositing at grazing incident angles (>30°) are very unstable and invariably buckle and rupture when exposed to atmospheres of water vapor. Substrate temperature, film thickness, and inert residual gases were found to have little effect on the stress and stability of films deposited at an angle of incidence of 45°. When a lower source temperature (−4 Torr) of water vapor is present during deposition, the resulting highly oxidized films show a compressive stress (∼2×10⁸ dyn/cm²) and very little dependence on the angle of incidence. These films are stable when exposed to higher partial pressures of water vapor.