Chemistry Reference
In-Depth Information
This model assumes a rather complicated process, with the involvement of two
intermediates, and Si(I), as well as one to complete the step of forming
It is not convincing that such a multibody reaction step can significantly
compete with the much simpler step to form Si(III) in reaction (5.17) which involves
only
5.12.3. Models for the Reaction Mechanism in Alkaline Solutions
The mechanisms of the electrochemical reactions of silicon electrodes in
alkaline solutions at OCP have been investigated in many studies due to their impor-
tance in the etching processes in micromachining. An important issue involving the
reaction mechanisms has been whether the etching process at OCP is of chemical or
electrochemical nature, that is, whether charge transfer processes associated with silicon
dissolution and hydrogen evolution involve the carriers in the electrode.
The experimental results in support of the chemical mechanism are (1) there is
n-
109
little difference between the etch rates of
Si and
p-
Si at OCP
and (2) the etch rate
207,259,403
On
is essentially independent of the carrier concentration up to about
the other hand, the experimental results supporting the electrochemical mechanism are
(1) the etch rate varies with electrode potential with the maximum near OCP, and the
etch rates of
n
and
p
types are only similar near OCP, differing significantly at anodic
and cathodic potentials as shown in Fig. 7.15; and (2) the
i-V
curves for
n-
Si and
p-
Si, although identical in terms of the chemical nature, are different in terms of carrier
involvement, and the values of the characteristic potentials such as OCP and
are dif-
ferent for
n
-Si and
p-
Si and for different doping levels.
According to Seidel
et al.,
206
the dissolution at OCP is an electrochemical process
with concurrent anodic dissolution of silicon and reduction of hydrogen ions. The oxi-
dation of silicon gives out electrons which are consumed for the reduction of hydro-
gen. Both
and
are the active species in that
is involved in silicon
dissolution and
in hydrogen evolution:
The reaction scheme is supported by the fourth power dependence of the etch rate on
and concentrations observed experimentally (see Chapter 7 on etching of silicon).
On the other hand, Palik
et al
.
114,379,984
suggested that silicon etching in KOH solu-
tions at OCP is of chemical nature and the etching reactions involve
attack on
Si-Si bonds to form Si-H and Si-OH bonds followed by
attack to form
and
Since the etching product is found to be silicate
the overall reac-
tion was suggested to follow
which has a ratio of of 1/2/0. The individual steps were proposed by Palik
et al.
to follow the scheme illustrated in Fig. 5.65.
