Biomedical Engineering Reference
In-Depth Information
Figure 4.28
The samples after development process (See Plate 7)
4.2.3.6 Oxide Layer Etching
The conventional etchant for SiO
2
is BOE and comprises a mixture of HF (one part by
volume)
2
and ammonium fluoride, NH
4
F (six or seven parts by volume). With an etch
rate of approximately 1000 min
-1
at room temperature, BOE is a selective etchant for
oxide. It does not etch silicon, so the removal of an oxide layer from a silicon crystal is
self-limiting. Nevertheless, HF attacks photoresist to some extent, although the addition of
ammonium fluoride reduces this effect. When end-point detection is required over silicon,
we can make use of the wetting properties of silicon and of oxide. Oxide is hydrophilic
and easily absorbs water. Silicon, on the other hand, is hydrophobic and repels water.
Therefore, a completely etched silicon substrate, dipped in water, will shed the water
instantly when removed. By contrast, a substrate with even a very thin layer of oxide on
the surface will remain wet. In order to protect the back side of the samples, a special
tape was used. Using a solution of BOE (7:1), 2 min was enough to etch the oxide layer.
After rinsing in DI water, another inspection by microscope was undertaken. If remnants
of SiO
2
were observed, the BOE etching could be repeated. The photoresist layer, as well
as the protective tapes, was then removed in acetone.
4.2.3.7 Tetramethylammonium Hydroxide, TMAH Etching
One of the conventional anisotropic wet etchants for silicon is (TMAH). The solution used
for this work was TMAH 25% from Moses Lake Industries. As shown in Figure 4.29, a
2
HF with pH = 1 is extremely dangerous and must be treated cautiously. Before working with the chemicals,
reading the associated MSDS is highly recommended.
