Chemistry Reference
In-Depth Information
It can now be summarized regarding surface condition that (1) the etch rate of
perfect (111) surfaces is negligibly small; (2) the etching of (111) surfaces is due to the
etching along the edges of (111) steps; (3) the amount of (111) steps depends on the
degree of misorientation from the perfect (111) plane; (4) all crystallographic planes
can be considered to be composed of (111) terraces and steps at a microscopic level.
Thus, the etch rate of a surface with an angle from the (111) surface can then be cor-
related with that at (111) steps as shown in Fig. 7.41. The vertical etch rate,
which
is the nominal planar etch rate measured in experiments, is
where
a
is the step height,
b
the terrace width, and
the step etch rate. For a small
it becomes
Thus, the etch rate of a surface is proportional to the angle of deviation from the (111)
surface, which agrees with experimental observation (Fig. 7.36). For a given surface
with a dimension of
l
, the above equation can also be expressed as
where
n
is the number of steps over the length
l
. Thus, the larger is the misaligment
from the (111) surface, the more the terraces and steps and thus the larger the etch rate.
This means that the surfaces with large angles to the (111) surface have higher etch
rates relative to the (111) surface. Essentially, this implies that etch does not proceed
perpendicular to any surface at the atomic scale but only at (111) steps in a direction
parallel to the (111) terrace.
In the case of a perfectly oriented (111) surface, such as the sidewalls of an etched
cavity on a (100) wafer, there may still be a definite etch rate due to the fact that steps
may be generated from surface defects such as vacancies and dislocations. The etch
rate of the perfectly oriented (111) surface can be viewed as being limited by the rate
of step generation, i.e., a nucleation process.
