Fabrication of vertical mirrors using plasma etch and KOH:IPA polishing

Abstract

A new approach to etch structures with vertical sidewalls in Si is presented in this paper. This process reduces the loading effect in deep reactive ion etching (DRIE) and maintains a uniform etch profile and etch rate throughout the wafer. Shallow areas were patterned under the regions to be removed. The wafer was then bonded to a pyrex wafer which acts as a handle wafer and as a package lid for packaging MEMS devices. Uniform width narrow channels encompassing these shallow patterns were then subjected to long through wafer DRIE. These narrow channels maintain uniform etch rates while patterning structures with various fill factors on the same wafer. Various structures were etched with vertical side walls across the 550 µm thick Si wafer. Average side wall angles of 89.8° were obtained with just 0.3° variation across the 4'' Si wafer. The process showed resistance to slight variations in DRIE parameters with a negligible effect on the sidewall profile. Additionally, the verticality of the structures was improved to 90.08° by performing KOH:IPA wet etching on the plasma etched surfaces. Once characterized, the same processes can be used for various shape/size structures. These etched vertical mirrors were used to assemble a corner cube retroreflector.