Biomedical Engineering Reference
In-Depth Information
10
1
10
0
10
-1
KOH
Concentration
10%
24%
42%
57%
10
-2
<100> silicon
60° C
10
-3
cm
-3
10
17
10
18
10
19
10
20
Boron Concentration
FIGuRE 3.4
Etch rate plotted versus boron concentration for <100> oriented silicon wafer at different
etchant concentrations.
boron atoms per cm
3
. One of the problems with boron etch stops is that the
surface of the silicon will be so highly doped that it may not be useful. For
example, the material at the concentrations required for a good etch stop
would not be useful for making a piezoresistive device.
The other etch stop method used in silicon bulk micromachining is the elec-
trochemical etch stop [15-19]. Figure 3.5 is an illustration of a three-terminal
electrochemical etch setup. Electrochemical etching of silicon using an aniso-
tropic etchant is useful since it provides very good dimensional control (e.g.,
diaphragm thickness is reproducible) and can make diaphragms with lightly
doped material, which is required for high-quality piezoresistive devices.
The disadvantage of electrochemical etch stops is that they require special
fixturing to each wafer in order to make electrical contacts, and an electronic
control system is needed to control and apply the correct voltage potential to
the wafer during the etch.
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