Etching around 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.