Model for a multiple-step deep Si etch process

Abstract

A multiple-step deep Si etch process involving separate etching and polymerization steps is often employed for fabrication of microelectromechanical systems, microfluidics devices, and other assorted deep structures in Si. An integrated plasma equipment-feature evolution model for this multiple-step deep Si etch process is described in this article. In the two-dimensional plasma equipment model, the etching (SF6/O2) and polymerization [octafluorocyclobutane(c-C4F8)] chemistries are separately simulated assuming steady-state conditions. The outputs of the equipment simulations are combined in a string-based feature profile evolution model to simulate the multiple-step deep Si etch process. In the plasma equipment models, detailed gas phase plasma chemistries including electron impact processes, ion–molecule reactions, and neutral chemistry have been considered for both the etching and polymerization gas mixtures. The plasma–surface interaction mechanisms in the feature profile evolution model are based on qualitative information available in literature and the correlation of modeling results with experimental data. Under the relevant operating conditions, F is assumed to be the primary Si etchant, film deposition in c-C4F8 is due to sticking of C, CF2, and C2F4 under ion bombardment, and the polymer is etched by energetic ions through physical sputtering. It is demonstrated that predictions of the resulting model are in close agreement with experiments. The validated model is used to understand the dynamics of the multiple-step deep Si etch process and how etching characteristics can be controlled using a variety of process parameters. Etching characteristics have been found to be quite sensitive to gas pressure, coil power, bias power, and relative step time during both etching and polymerization processes. The Si etch rate and feature sidewall angle are coupled to each other over a wide range of operating conditions.