Chemistry Reference
In-Depth Information
and holes, at the semiconductor surface; chemical species such as and
near the surface in the solution; and active surface silicon atoms which are favorable
for reaction and removal. Unlike the other two species, charge carriers may or may not
be involved depending on whether the reaction is of electrochemical nature. The con-
centrations of each of these species are determined by different processes such as dif-
fusion, migration, adsorption, and solvation. In particular, the concentration of surface
active silicon atoms depends on crystal orientation, which is the third aspect to be
discussed below.
The anisotropic etching in alkaline solutions occurs on a bare surface of silicon
free of oxide. Formation of an oxide film will mask the crystallographic differences
among the surfaces of different orientations and thus result in isotropic etching. Also,
experimental results indicate that although isotropic etching may occur under either
diffusion control or activation control, anisotropic etching can only occur by processes
that are surface controlled at least for the slow etching surfaces. Anisotropic etching
does not occur when the etch rates of all of the surfaces, including those with the slowest
etching rates, are controlled by supply of the solution species. Whether charge carriers
are involved in etching reactions is also important. Charge carriers are involved in
the isotropic etching in HF solutions but not in the anisotropic etching in KOH
solutions.
The third aspect in the etching mechanism concerns the nature and concentration
of the active surface silicon atoms. Because the surface atom stability depends on the
number of back bonds, it can be proposed that the probability of atom removal from a
perfect (111) surface lattice is very small and etching on the (111) surface proceeds
only at lattice inhomogeneities such as steps, kinks, and vacancies. Thus, a perfect (111)
surface will have an extremely low etch rate and the etch rate of real (111) surfaces is
determined by the etch rate at the steps (including other surface lattice defects) and the
density of steps.
This is in agreement with experimental findings. The etch rate of the (111) surface
is extremely sensitive to a minute misorientation of the specimen (Fig. 7.36); a devia-
tion of less than 1° can cause a several-fold increase in etch rate.
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A consequence
of misalignment from the (111) surface is the appearance of terraces and steps. Accord-
ing to STM imaging on a slightly misoriented (111) surface (0.7°), the higher etch
rate is principally due to the preferential dissolution at the terrace edges in alkaline
solutions.
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