Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 315

The etch rate in the vicinity of {111} crystal planes is extremely sensitive to small
angular misalignments as shown in Fig. 7.36. 206,334,1001 Deviation by 1 °C from the (111)
plane leads to an increase of etch rate by a factor of 5 in EDP and TMAH and by a
factor of about 2 in KOH. Due to the sensitivity of etch rate to small deviation from
the (111) planes, etch rate ratios may be much reduced for slightly misoriented (111)
samples relative to those of perfectly oriented (111) material. The etch rate ratio of
different planes, by measuring the etch depth of a groove bounded by (111) planes
relative to the lateral etching, is very sensitive to misalignment of the mask. Accord-
ing to Kendall, 184,478 mask misalignment is responsible for the large variations some-
times reported in the literature on etch rate ratios under identical conditions. It is shown
that due to the increased etch rate on misoriented (111) surface for a misorientation of
half a degree in the alignment of the mask the etch rate ratio of {110}: {111} in 44%
KOH at 85 °C changes from above 400 to 70. To explain this effect, Kendall 184 sug-
gested that the major etching that occurs on a slightly misoriented (111) plane is due
to etching of the ledges on the misoriented surface.