Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 317

Rate-Limiting Process. Etching anisotropicity arising from differences in the
atomic arrangement on the surface of different orientations necessarily indicates that
the process is controlled by surface reactions that depend on lattice structure. Because
anisotropic etching essentially results from surface-controlled reactions, any shift
from surface-controlled processes to nonsurface processes, either within the silicon
semiconductor or in the electrolyte, will result in a decrease of anisotropicity. This is
in general agreement with the experimental data. Although mass transport may be rate
limiting for the etching of the fast etching planes in some solutions, surface-controlled
processes are the rate-limiting steps in the etching of the (111) plane in all anisotropic
etchants.
The rate-controlling process can also be evaluated by the apparent activation
energy as shown in Table 7.2 and Fig. 7.38. The activation energy for the various
anisotropic etchants seems to indicate an increase of etch rate ratio with increasing
activation energy, the highest ratio and activation energy being in KOH. In general, the
apparent activation energy, as determined from the dependence of rate on the recipro-
cal of temperature, is generally 0.13-0.26 eV for diffusion-limited reactions whereas
it is 0.44-0.87 eV for electrochemical reactions. 289 For given etching conditions the
activation energy corresponds closely to etch rate, and is larger for crystal planes with
slower etch rate. 206 The larger the etch rate is, the larger the diffusion effect. It is noted
that high activation energies, indicating surface-controlled reaction process, are found
in solutions with high HF concentrations which are isotropic etchants.
However, the rate-controlling surface process involved in solutions is prin-
cipally associated with the surface carrier concentration and not with the lattice struc-
ture. An important difference between isotropic etching and anisotropic etching, when
both are controlled by surface processes, is that the former is not lattice structure
sensitive whereas the latter is.
Passivation Models. The models based on surface passivation suggest that a
passive layer, similar to the silicon oxide formed under an anodic potential, exists on