Analysis of Interaction of Surfactant Molecules with Si(hkl) Planes on the Basis of Anisotropic Etching in Alkaline Solutions

Abstract

In silicon anisotropic etching, tensioactive compounds are often employed as additives to etching solutions in order to improve the morphology of etched surfaces or reduce etching rates of some crystallographic planes. The reduction is advantageous in case of deep etching of complex micro electromechanical systems (MEMS) and micro-opto-electromechanical systems (MOEMS) structures since it enables minimizing of convex corners undercut. The tensioactive compounds like alcohols or typical surfactants reduce etching rates of the planes like (110), (331), (221), causing simultaneously significant improvement of their roughness [1]. A direct assessment of interaction of such compounds with the etched surface is extremely complex as it requires in situ study of etching process in concentrated alkaline solutions. Regular employment of surfactants as the additives, modifying the etching process points out that surface tension of etching solutions may be an important factor characterizing the etching process. The latest studies show that the reduction of surface tension of etching solutions connected with gathering of surfactant molecules at the solution–air interface is directly associated with reduction of wetting angle of the solid by the solution and that the phenomena at the solid–liquid interface are similar to the ones occurring in the solution surface [2]. Studying of surface tension of etching solutions may be an excellent tool for the analysis of the processes occurring on the surface of etched Si.