The Crystal Structure of Silicon
Since TMAH is an anisotropic etchant, the structure etched in TMAH are usually
formed by several special planes of silicon. In order to get a good understanding
of TMAH etching, we should first know the crystal structure of silicon.
The crystal structure of silicon is that of the diamond type with lattice constant
a = 5.43A˙
. The structure is face centered cubic(FCC) but with two atoms in the
unit cell. The following figure shows the atom lattice of silicon and some facets of
the silicon crystal.
The commercially available silicon wafers are (100) and (110) wafers, depending
on the orientation of the wafer surface. In research, the mostly used ones are (100)
wafers. On this type of wafer, the surface is (100) so (111) planes make an angle of
54.74o
to the wafer surface.