Profile simulation model for sub-50 nm cryogenic etching of silicon using SF6/O2 inductively coupled plasma

Abstract

Cryogenic etching of silicon is a route to high aspect ratio silicon features with high mask selectivity and smooth sidewalls. These benefits have been realized even down to 10 nm half-pitch features. In this work, we develop a semi-empirical simulation model to understand mechanisms responsible for nanoscale profile evolution during plasma etching of silicon in SF6/O2 chemistry at cryogenic temperatures. The model parameters are first calibrated to the etching equipment using features from 500 nm to 2 μm. Using the calibrated model, we show the experimental finding that smaller features need more oxygen to achieve vertical anisotropic profiles. This is a consequence of two related effects: (1) the SiOxFy passivation layer sputtering yield is strongly dependent on the oxygen content at the feature sidewalls and (2) Knudsen transport within small features or higher aspect ratios depletes oxygen faster than fluorine due to the higher sticking coefficient of oxygen. The simulation was applied to 25 nm half-pitch features with excellent results.