Chapter 22 - Wet Etching of Silicon

Abstract

The use of an alkaline or acidic solution (the etchant) to chemically remove material from the surface of a sample is wet etching and this chapter explains the different types of wet etching. Although the etch rate anisotropy leads to faceting, the description of anisotropic etching as the propagation of a collection of crystallographic planes is an oversimplification. The important microscopic processes are briefly explained with relevant examples, which include pit nucleation and step flow, micromasking, diffusion, etc. The removal of the surface atoms during wet etching of silicon is a complex process that involves both chemical and electrochemical reactions. This chapter also provides an overview of the typical surface morphologies that can be observed during/after wet etching on different surface orientations, and an unified explanation of the mechanisms that may be responsible for it. The items used to characterize a silicon wafer like orientation issues, wafer dimensions, surface quality, and bulk features affect the etching result and thus silicon wafer plays a key role in wet chemical etching. This chapter gives detailed data on examples of wet etching, popular wet etchants, temperature dependence of the etch rate, concentration dependence of the etch rate and other variables affecting etch-rate values, etc. It also tries to give some data on the conversion between different measures of concentration.