Chemistry Reference
In-Depth Information
boron doped materials. Also, although the alkaline etchants can be made of organic and
inorganic solutions, all of them appear to require the presence of water to etch silicon
at significant rates. In this section, the KOH etching system is taken as a model, to
which other solution systems are compared.
7.4.1. KOH Solutions
The etch rate of silicon in KOH may vary from as low as
in a concentrated solution (e.g.,
40% KOH) at high temperatures. Figure 7.13 shows that the etch rate of (100) silicon
in KOH solution at room temperature increases with concentration, peaks at about
5 M, and then decreases with further increasing concentration.
984
Agitation of the solu-
tion can significantly increase the etch rate, indicating that mass transport is involved
in the rate-determining processes. Also, the etch rate tends to decrease with etching
time; a decrease of a factor of 3 is observed over a period of 24 h.
557
A similar etch rate dependence on concentration is found for the three major sur-
faces in KOH-IPA solutions as shown in Fig. 7.14, but unlike at lower temperatures,
solution stirring has little effect on the etch rate.
206
,
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The activation energy of the
etching process is 0.56-62 eV for KOH of different concentrations indicating that the
etching is controlled by a surface process.
206
KOH solutions are often buffered with
isopropyl alcohol (IPA), which results in a decrease of etch rates.
378
,
1026
The most characteristic feature of etching in KOH solution is its anisotropic
nature, i.e., the etch rate varies with the crystal orientation of the silicon wafer. The
etch rate for the major crystal planes follows {110} > {100} > {111}.
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The exact dif-
ference between the etch rates of these planes depends on concentration, temperature,
in dilute KOH
(e.g., 0.5 M) at room temperature to as high as
and measurement procedure. For example, in 20% KOH at 100 °C the etch rates for
930:560: corresponding to a ratio of 50:30:1. The dif-
ference between the etch rates of (110) and the (100) planes and that of the (111) plane
is larger at room temperature, giving a ratio of 160:100:1 in 20% KOH. Addition of
(110): (100) : (111) are
