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bonds, with X being a doping element that is more resistant to chemical attack than the
Si-Si bond in alkaline solutions, is a rather plausible explanation for the reduced etch
rate of highly doped materials. The increased number of such bonds can then account
for the gradual reduction of etch rate with increasing doping concentration.
7.6. ANISOTROPIC ETCHING
7.6.1. Sensitivity of Etch Rates to Crystal Orientation
Anisotropic etching, i.e., different dissolution rates on different crystal planes, is
a characteristic feature of silicon etching in alkaline solutions. Strictly speaking, the
etch rate of silicon always depends, to a variable extent, on crystal orientation in all
etching solutions, acidic or alkaline. However, the etch rate difference for different
planes is very small in acidic HF solutions relative to those in alkaline solutions. Figure
7.32 shows the etch rate ratios of (100)/(111) and (110)/(111) planes in various solu-
tions. In HF-based solutions the ratios are close to 1, varying only slightly with differ-
ent solution compositions. Exceptions, however, exist. For example, in solution
with addition of copper ions, the etch rate ratio of (100)/(111) is found to be as high as
1024
Addition of silver or gold ions does not have such an effect. Because etching in
solution with metal ion oxidants is accompanied by deposition of the metal on the
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