Substrate Trenching Mechanism during Plasma and Magnetically Enhanced Polysilicon Etching

Abstract

Plasma and magnetically enhanced reactive ion etching processes with chlorine gas have been developed for subhalf micron polysilicon gate electrode tching (1). In this paper, gate oxide trenching during polysilicon gate etching using chlorine-based reactive ion etching (RIE) and magnetically enhanced reactive ion etching (MERIE) processes i studied in detail. The trenching mechanism was found to be a direct result of off-angle ion bombardment and reflection from the par-tially etched polysilicon sidewall during plasma etching. The off-angle ion bombardment is caused by the elastic ollision between ions and neutrals as ions move across the plasma sheath. Monte Carlo simulation confirms the angular distribu-tion of ions under most plasma processing conditions. A two-step MERIE process, first with high etch rate and high ani-sotropy and second with high selectivity to the gate oxide, with low ion energy bombardment is also developed to mini-mize the trenching effect during polysilicon-gate etching. For high density multimegabit DRAM fabrication, the polysilieon gate electrode must have subha]f micron di-mensions and vertical profiles. The etch process used to fabricate this tructure must achieve these objectives while maintaining high selectivity to the underlying ate