The most common and easily processable materials system for undercut.
The result is a pyramid shaped hole instead of a hole. The classic example of this is the 111> crystal plane sidewalls that appear when etching a hole in a 100> silicon wafer in a chemical such as potassium hydroxide (KOH). Anisotropic etching in contrast to isotropic etching means different etch rates in different directions in the material.Anisotropic KOH Etching of Silicon Isopropyl alcohol is added to improve selectivity As etch selectivity over SiO 2 is less than 500 at various concentrations of KOH, SiO 2 etch mask is not adequate for long etches Si 3 N 4 is an effective masking film for KOH etchant Ratio of Etch Rate of Si in KOH + H 2 O : = 400 : 1 : [111.
