Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 310

Mechanism
The mechanism of boron-induced etch rate reduction has been extensively inves-
tigated since its first observation in the late 1960s. 207 Several models have been pro-
posed attributing the phenomenon to increased lattice distortion and defect density, or
surface passivation, or electron deficiency. As described below, although each of these
models provides an explanation of some aspects of the phenomenon, a coherent account
for the underlying mechanism is still lacking.
In an earlier investigation, it was suggested that etch rate reduction is caused by
the increased lattice distortion from high doping. Later studies indicated that the
reduced etch rate of high-boron-doped material is not likely to result from lattice
distortion or stress. According to Seidel et al. 207 who measured the etch rate on stress-
compensated boron/germanium co-doped samples, neither lattice strain nor crystal
defects induced by the high boron concentration play a significant role in the etch stop
phenomenon. In one study, the etch rate of highly doped samples actually increased
with increasing lattice defect density. 251 This is the opposite of what one would expect
were etch rate reduction the result of increased lattice distortion. In another study, the
etch rate of highly doped silicon significantly increased with sputtering damage whereas
that of lightly doped silicon changed very little, indicating that lattice damage does not
cause etch rate reduction. 386
The shortened electron life model is mainly based on the observation that the
decrease of etch rate with boron concentration exhibits an inverse fourth power depen-
dence on the boron concentration. 207,251,386 Raley et al. 386 postulated that etching in KOH
is a corrosion process and that the etch rate reduction at high boron doping levels is
due to the decreased electron concentration required for the reduction of hydrogen in
the etching process described by the following reactions:
The rate of reduction reaction is a fourth-order reaction with respect to electron
concentration:
The surface concentration of electrons, n, which are generated by the oxidation reac-
tion is determined by Auger recombination and is related to the hole concentration by
where h is the hole concentration and A a constant. Thus,
Seidel et al . 207 proposed a similar model, ascribing the reduced etch rate of highly
doped material to the reduced electron concentration which is caused by degeneracy