Gate Oxide Damage in Dry Photoresist Stripping Environments

ABSTRACTThe effect of oxygen plasma stripping environments on the electrical properties of thin oxides has been studied. A barrel, parallel plate and downstream stripper are compared. Damage, measured by shifts in flatband voltage (Δ Vfb), increases in the density of interface traps (Dit), and degradation in minority carrier lifetimes, was observed in all plasma processed samples. Plasma treated wafers had 10–100 fold increases in Dit but recovered to nearly control levels after a 450 °C anneal. ΔVfb and lifetimes did not recover in all cases. Mobile ion contamination dominated in the high temperature (≥ 250 °C) downstream stripping tool. The mobile ion concentration was halved by decreasing the process temperature in the downstream etcher. Ion bombardment-induced damage appears to be most important in the parallel plate configuration. Lifetime results were poor for wafers etched in an unshielded barrel reactor and did not recover after anneal. Use of an etch tunnel improved results to control levels. Degradation of minority carrier lifetimes measured using the photoconductivity technique correlated well with changes in Vfb and Dit. This measurement provides a simple method for evaluating plasma-induced damage.