Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 36

Without stirring
With stirring
FIGuRE 3.1
The etch profile, with and without stirring, using an isotropic wet chemical etchant.
nitride. Silicon nitride has a much lower etch rate compared to silicon diox-
ide and therefore is more frequently used.
The etch rate of some isotropic wet etchant solution mixtures are depen-
dent on the dopant concentration of the substrate material. For example, the
commonly used mixture of HC 2 H 3 O 2 :HNO 3 :HF in the ratio of 8:3:1 will etch
highly doped silicon (> 5 x 10 18 atoms/cm 3 ) at a rate of 50 to 200 microns/
hour, but will etch lightly doped silicon material at a rate 150 times less.
Nevertheless, the etch-rate selectivity with respect to dopant concentration
is highly dependent on solution mixture.
The much more widely used wet etchants for silicon micromachining are
anisotropic wet etchants. Anisotropic wet etching involves the immersion
of the substrate into a chemical solution where the etch rate is dependent on
the crystallographic orientation of the substrate. The fact that etching varies
according to silicon crystal planes is attributed to the different bond configu-
rations and atomic densities that the different planes expose to the etchant
solution. Wet anisotropic chemical etching is typically described in terms of
etch rates according to the various normal crystallographic places, usually
<100>, <110>, and <111>. In general, silicon anisotropic etching etches more
slowly along the <111> planes than all the other planes in the lattice, and the
difference in etch rate between the various lattice directions can be as high
as 1000 to 1. It is thought that the reason for the slower etch rate of the <111>
planes is that these planes have the highest density of exposed silicon atoms
in the etchant solution, as well as 3 silicon bonds below the plane, thereby
leading to some amount of chemical shielding of the surface.
The ability to delineate the different crystal planes of the silicon lattice in
anisotropic wet chemical etching provides a higher-resolution etch capability