Hardmask charging during Cl2 plasma etching of silicon

Feature profile evolution simulations of plasma etching rely to first order on the accurate prediction of ion fluxes to all points on the evolving surface. Previous experimental and theoretical work strongly suggests that dielectric charging effects play a key role in a type of anomolous feature evolution known as ''notchings.'' This involves charging of newly exposed gate dielectric material, subsequent ion trajectory bending, and notch formation due to localized ion Aux enhancement. Few researchers, however, have considered charging of masking dielectrics (e.g., SiO2 hardmasks) and its associated effects on feature evolution, even though such charging is likely to occur in modem high-density plasma etching systems. In this article, we develop a combined reactor- and feature-scale model of Cl-2 plasma etching of crystalline silicon, allowing for the possibility of hardmask charging and ion trajectory deflection. We show via comparison of simulation results to cross-sectional scanning electron micrographs of silicon trenches that these charging effects can explain the formation of wide, "triangular," microtrenches seen when etching silicon at 2 mTorr pressure and low rf-bias power. Furthermore, the model correctly predicts the disappearance of these microtrenches when the rf-bias power is raised. (C) 1999 American Vacuum Society. [S0734-2101(99)00906-9].