Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 35

A widely used bulk micromachining technique is chemical wet etching,
which involves the immersion of a substrate into a solution of reactive chemi-
cal that will etch exposed regions of the substrate at measurable rates [9].
Chemical wet etching is popular in MEMS because it can provide a very high
etch rate and selectivity. Furthermore, the etch rates and selectivity can be
modified by altering the chemical composition of the etch solution, adjusting
the etch solution temperature, modifying the dopant concentration of the
substrate, and controlling which crystallographic planes of the substrate are
exposed to the etchant solution.
The basic mechanism of chemical wet etching involves reactant transport
to the surface of the substrate, followed by reaction at the substrate surface
between the etchant solution and the substrate material, and then transport
of the reaction products from the substrate. If the transport of reactants to
the surface of the substrate or transport of reaction products away from the
substrate surface are the rate-determining steps, then etching is defined as
“diffusion limited” and the etch rate can be increased by stirring the solu-
tion. If the surface reaction is the rate-determining step, then etching is
“reaction-rate limited” and etch rate is very dependent on etch solution tem-
perature, etch solution composition, and substrate material. In practice, it is
usually preferred that the process is reaction-rate limited since this gives
more repeatability and higher etch rate.
There are two general types of chemical wet etching in bulk micromachin-
ing: isotropic wet etching and anisotropic wet etching [9-12]. In isotropic wet
etching, the etch rate is not dependent on the crystallographic orientation of
the substrate, and the etching ideally proceeds in all directions at equal rates.
The most common isotropic etchant for silicon is a solution of HNO 3 , HF, and
HC 2 H 3 O 2 . The reaction is given by
HNO 2 + HNO 3 + H 2 O → 2HNO 2 + 2OH - + 2H +
Holes and (OH) - ions are supplied by HNO 3 when it combines with H 2 O
and trace concentrations of HNO 2 . Note that the reaction is autocatalytic
because of the regeneration of HNO 2 . Increasing the concentration moves
the reaction toward diffusion-limited etch-rate dependence, and etching
can be controlled by stirring. Increasing HF concentration or temperature
increases the surface reaction rate. In theory, lateral etching under the mask-
ing layer etches at the same rate as the etch rate in the normal direction.
However, in practice lateral etching is usually much slower without stir-
ring, and consequently isotropic wet etching is almost always performed
with vigorous stirring of the etchant solution. Figure 3.1 illustrates the pro-
file of the etch using an isotopic wet etchant with and without stirring the
solution.
Any etching process requires a masking material to be used, preferably
with a high selectivity relative to the substrate material. Common masking
materials for isotropic wet silicon etching include silicon dioxide and silicon