Photosensitive etch protection coating for silicon wet-etch applications

Abstract

A spin-on polymeric material has been developed to replace the silicon nitride mask used in the MEMS industry for silicon wet-etch processing. Built-in photosensitivity eliminates the need for additional photoresists in the system. The process consists of applying an organosilane-based primer layer onto a silicon wafer, followed by spin coating the photosensitive layer. After a soft bake, the coating is imaged by exposing it to ultraviolet light. After a post-exposure bake, the coating is developed by a solvent. After a final bake, the prepared wafer is then etched in a hot concentrated alkaline solution to complete the pattern transfer. The polymer-coated area remains protected with insignificant and controllable undercut after extended hours of wet etching. Etch protection performance was characterized as a ratio of undercut (u) to etch depth (h). The polymeric mask allows silicon substrates to be etched anisotropically in the same way as silicon nitride masks although more undercut occurs when KOH or NaOH are used as etchants. With use of tetramethylammonium hydroxide (TMAH) as an etchant, a consistent 1-2% undercut ratio (u/h×100%) was obtained. The effects of various parameters such as use of different etchants and the effects of etchant concentration and delayed processing on undercut ratio are investigated.