A method for tapered deep reactive ion etching using a modified Bosch process

Abstract

This paper presents a method for etching tapered sidewalls in silicon using deep reactive ion etching. The method is based on consecutive switching between anisotropic etching using the Bosch process and isotropic dry etching. By controlling the etch depths of the anisotropic and isotropic etch sessions, the sidewall angle can be controlled over a relatively large range. Tapered sidewalls are useful in microfabrication processes such as metal coating of 3D structures (e.g. for electrical connections or vias), mold tool fabrication or as a tool to compensate for reentrant etching. The method was tested and characterized by etching basic test structures in silicon wafers. Based on the investigated anisotropic and isotropic etch depths the sidewall angle could be varied between 0° (straight vertical) and 36°. The sidewall angle was well predicted by a model using the etch depths as parameters. Due to the alternating etch procedure a scalloping pattern is generated on the sidewalls. By frequent switching and short etch sessions this scalloping can be reduced to less than 1 µm. The process represents an easy method to tailor the sidewall angle in deep etching of silicon. The etch scheme is run in a single etch system and can be implemented in ICP systems of most manufactures. The method can also be used in conjunction with the standard Bosch process as demonstrated herein, where the method was applied to compensate for reentrant etching of high out-of-plane mesa-structures.