Anodic Behavior - Electrochemistry of Silicon and Its Oxide - page 209

concentration. It has generally been found that the frequency of oscillation increases
with increasing etch rate as shown in Fig. 5.52, or equivalently, with increasing current
such that where is on the order of and depends on poten-
tial. 544,937 The fact that oscillation is not observed in concentrated fluoride solutions, in
which is large, may be because at the large oxide dissolution rates the frequency is
too high for oscillation to be observed. The dependence of frequency on potential
is related to the frequency. 1136 For low-frequency oscillation the frequency tends
to decrease with potential, 860,951,1136 and for high-frequency oscillation the opposite can
be seen. 1136
Although the period of oscillation depends on solution composition, it is found
that the product of the period and the current is only a function of potential; it is essen-
tially constant over a large set of the solution compositions corresponding to an oscil-
lation current span of about two orders of magnitude as shown in Fig. 5.53. 951 This
result indicates that, at a given potential, a constant amount of silicon is dissolved during
one period of oscillation independent of electrolyte composition.
Damped or sustained oscillation amplitude can be regulated by adding a series
resistance into the current flowing path as shown in Fig. 5.54. 693 The series resistance
may be associated with the electrolyte, the electrode, or the back contact. It is particu-
larly significant in concentrated fluoride solution in which the anodic current is high
and the critical resistance for such a crossover to occur is small. Thus, depending on
the electrochemical system the true intrinsic interface behavior appears when such
series resistance is zero. This finding also implies that some of the differences among
spontaneous, perturbed, or damped oscillation reported in different studies may be due
to the effect of the series resistance existing in the system.