Diagram for TMAH etching. The top ones are the masks, and
the bottom ones give the etched results. In case a, a big square opening
whose edge is along the (110) direction is used. The sidewalls are the
slowest etching planes-(111) planes and the bottom is a (100) plane. In
case b, the mask is similar to that of case a, just the size is smaller, so
those (111) planes meet together at the bottom, thus forming a pyramid
shape. In case c, a rotated square is used, and now the edge is along
the (100) direction. In this situation, the vertical (100) planes appear as
sidewalls, and the (111) planes expose on the corners.