Page 33 - MEMS Mechanical Sensors

22 Materials and Fabrication Techniques

processes give anisotropic etch profiles, which are extremely important for submi-
crometer semiconductor fabrication. By combining chemical and physical processes
in a dry etch process, the optimum conditions for any particular process can be
obtained.
The most common type of etching adapted for MEMS is deep etching into the
silicon substrate; and this is often referred to as bulk micromachining. This bulk
micromachining can be done either in a wet or dry process, and in each case it can be
either isotropic or anisotropic. Other MEMS-specific etching is done on quartz or
glass, using HF-based solutions or ammonium fluoride.

2.3.3.1 Silicon Wet Isotropic Etching
The most widely used isotropic etchant is a mixture of HNO ,HF, and CH COOH,
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and this system proceeds by oxidation followed by dissolution of the oxide. Since the
oxide is removed in the etch, masking materials such as silicon nitride, silicon car-
bide, or gold have to be used. The etch rate, surface roughness, and the geometrical
aspects at the edges and corners of features depend on the precise composition of the
etchant. All of these properties are difficult to control and even very small changes in
temperature, agitation, and composition can cause large changes in the etch proper-
ties. Thus, the usefulness of this etchant is severely restricted. Etch rates as high as
1,000 µm/min have been reported, so the etch may be useful for removing large
quantities of bulk silicon where precise definition is not required. Another poten-
tially useful property is the dependence of the etch rate on the silicon dopant concen-
tration. A solution of HF:HNO3:CH COOH mixed in a 1:3:8 ratio etches silicon
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doped at 10 cm 15 times faster than silicon doped at 10 cm , both for n- and
p-type silicon. This provides an alternative etch stop to the usual etch stop method
using anisotropic etchants described in the following section.
2.3.3.2 Silicon Wet Anisotropic Etching
There are many chemicals and mixtures that etch silicon anisotropically including
the alkali metal hydroxides, simple and quarternary ammonium hydroxides, ethyle-
nediamine mixed with pyrochatechol (EDP), hydrazine, and amine gallates. Many
of these are still the subject of research and in practice only KOH, tetra methyl
ammonium hydroxide (TMAH), and EDP are regularly used in MEMS manufactur-
ing. The common properties of these etchants are that the etch rate is dependent on
the crystal plane and that they selectively etch n-type or lightly p-doped silicon com-
pared to heavily p-doped silicon. Without exception, the slowest etching planes are
the {111} planes, but the fastest etching planes depend on the precise composition of
the etchant. The other planes of interest are the {100} and {110} planes, which,
although not the fastest etching planes, etch at a much faster rate than the {111}
planes. Relative etch rates of 400 between the {100} and {111} planes are typical, for
example, with KOH etching [10]. For all these etchants the etch rate drops signifi-
cantly for heavily p-doped silicon. This property can be used to create etch stop lay-
ers, making it possible to fabricate a variety of structures, in which the structure is
formed from the heavily doped material. The level of boron doping required for an
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etch stop layer is of the order of 5×10 cm and the etch rate selectivity is of the