Etching around corners In etching of mono silicon, why does the etching process around the corners differ from the etching process around the flat edges? What brings about this difference and is there some way that MEMS engineers could quantify the process around the corners?
What happens is that convex (outside) corners get severely undercut in wet etching. On a &lt;100> cut wafer, the edges leading up to the corner reveal &lt;111> planes which are resistant to etching, but right at the corner, where the two planes intersect, etching reveals a &lt;110> plane, which is etched much more quickly. The difference in etch rates has to do with the number of "dangling" bonds—faces with fewer Si-Si bonds at the surface etch faster. The reason why concave corners don't undercut, despite having the same &lt;111> plane intersection is that there are no dangling bonds in an inside corner so it doesn't etch any faster than the &lt;111> planes.

It should also be noted that adding a small amount of surfactant to the etchant can dramatically decrease undercutting by adsorption to the corners.

This (fortunately open-access) article explains the phenomenon quite nicely.