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Osiander / MEMS and microstructures in Aerospace applications DK3181_c003 Final Proof page 41 1.9.2005 8:59pm

MEMS Fabrication 41

100 µm shafts

75 µm thick
nickel
gears

FIGURE 3.3 Assembled LIGA fabricated mechanism. (Courtesy: Sandia National Labora-
tories.)

practical interest, a number of different aspects of the etch processes need to be
considered:
. Masking
. Etch selectivity due to crystallographic orientation or materials
. Etch stop and endpoint detection

3.4.1 WET ETCHING

Wet etching is purely a chemical process that can be isotropic in amorphous
materials such as silicon dioxide and directional in crystalline materials such as
silicon. Contaminants and particulates in this type of process are purely a function
of the chemical purity or of chemical system cleanliness. Agitation of the wet
chemical bath is frequently used to aid the movement of reactants and by-products
to and from the surface. Agitation will also aid the uniformity of etch, since the by-
products may be in the form of solids or gases that must be removed. A modern,
wet-chemical bench will usually have agitation, temperature, and time controls as
well as filtration to remove particulates.
The etching of silicon dioxide (SiO 2 ) is a common wet-etch process employed
in a surface micromachining release etch or etch of isotropic features. This may be
done with water to HF mixture in the ratio of a 6:1 by volume.

SiO 2 þ 6HF ! H 2 þ SiF 6 þ 2H 2 O (3:1)

Since HF is consumed in this reaction, the concentration will decrease as the etch
proceeds, which would require that more HF be added to maintain concentration.
Alternatively, a buffering agent could be used to help maintain the concentration
and pH in this reaction. Equation (3.2) shows the chemical reaction that would
enable NH 4 F to be used as a buffering agent in the HF etches.

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