Effects of etch products and surface oxidation on profile evolution during electron cyclotron resonance plasma etching of poly-Si

Abstract

Evolution of etched profiles has been numerically investigated during electron cyclotron resonance (ECR) Cl2/O2 plasma etching of polycrystalline Si. The calculations included the processes of chemically enhanced ion etching and passivation layer formation, which are important in such low-pressure, high-density plasma etching environments. The sidewall passivation was modeled by taking account of the redeposition of etch products on sidewalls of etched features. In addition, the effect of surface oxidation was also included in the model. Etched profiles were then simulated to examine effects of the neutral-to-ion flux ratio onto the substrate, sticking coefficient of etch products, and additional incident fluxes of etch products and oxygen atoms. Numerical results indicated that in typical ECR plasma etching environments, where the gas pressure is P0≲1 mTorr and the ion current density is Ji∼10 mA/cm2 onto the substrate, the chlorinated surface coverage α is microscopically nonuniform on sidewalls and bottom surfaces of etched features: α≲0.3 at the bottom and α≊1 on the sidewalls. This microscopic nonuniformity in coverage was found to lead to inversely tapered etched profiles without any sidewall passivation. These profiles changed to vertical and further tapered, when simultaneous redeposition of etch products were taken into account with their sticking coefficients Sp≳0.1. Furthermore, it was shown that in the presence of oxidation, the sidewall surfaces are easily oxidized, and the lateral etching is effectively reduced during overetch.