Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 120

locally melted by radio-frequency heating. Crystal growth starts from the end with the seed. Following
the crystal growth process, the silicon rod is then sawed and polished into wafers.

4.1.2.1 Wet etching
Wet etching is referred to as an etching process of solid materials in a chemical solution. During these
processes, the substrate is dipped in the solution or the solution is sprayed on the substrate. Wet-etching
processes are mostly isotropic, independent of crystalline orientation. However, as discussed later in
this section, etching of single-crystalline silicon in KOH depends on the crystalline orientation. For the
fabrication of micromixers, wet etching is often used for making the microchannel network.
Isotropic etching has drawbacks in fabrication of precise lateral structures because of underetching. If
the etch solution is well stirred, the isotropic etch front has almost a spherical form. If a microchannel is
fabricated with isotropic etching, the channel width also depends on the channel depth and needs to be
considered in the mask design. The major advantages of wet etching include the high selectivity,
a relatively planar etching surface, a high repeatability, and the controllable etch rate. Isotropic wet
etching is often used for removing thin layers or thinning a film. Due to possible bubble formation on the
etched front, well-stirred etch solution is crucial for the process quality. Isotropic wet etching of silicon
can be achieved using a mixture of acids such as hydrofluoric HF and nitric acids HNO 3 . The etching
process consists of two steps: oxidation of silicon by nitric acids and dissolution of formed silicon oxide
by hydrofluoric. The disadvantage of isotropic etching is its controllability; very small structures cannot
be fabricated by this technique. Tabl e 4 .2 lists some common recipes for isotropic wet etching.
Anisotropic etching of silicon is usually achieved with KOH. The etch rate of KOH in single-
crystalline silicon depends on the crystalline orientation. The (111) plane of the silicon crystal has two
orders of magnitude slower etch rate than the (100) plane; so, single-crystalline structures etched in
KOH are defined by the (111) plane. Because of this, the etching process in KOH is also called
anisotropic etching. Since this technology is a wet-etching process, the equipment is simple and easy to
implement. Because KOH is a source of mobile ion contamination, anisotropic etching in KOH is not
compatible with standard CMOS processes. The problem of metal ions can be avoided by using
a barrier layer or a metal-ion free etchant such as TMAH. While the standard CMOS process allows the
fabrication of a number of microsensors, more complex devices can be fabricated by the combination
between the standard CMOS process and an additional micromachining process. The additional
process can precede (pre-CMOS), follow (post-CMOS), or be between the steps (intermediate-CMOS)
of the standard CMOS process.

Table 4.2 Recipes for Isotropic Wet Etching (After [3] )

Material Etchants Selective To

Si HF, HNO 3 ,CH 3 COOH SiO 2

Si KOH SiO 2

SiO 2 NH 4 ,HF Si

SiO 2 HF, NHO 3 ,H 2 O Si

SiO 2 H 3 PO 4 , NHO 3 ,H 2 O Si

Si 3 N 4 H 3 PO 4 SiO 2

Al H 3 PO 4 , HNO 3 ,H 2 O SiO 2