Page 35 - MEMS Mechanical Sensors

24 Materials and Fabrication Techniques

Table 2.5 Comparison of Commonly Used Silicon Anisotropic Etchants
Etchant Etches Aluminum Etches Oxide Silicon Surface Advantages Disadvantages
KOH Yes Yes Very good Easy to use and Etches aluminum
dispose of and oxide
EDP Yes (but some No Good Does not etch Hazardous,
formulations do oxide difficult to use, not
not etch aluminum) clean-room
compatible
TMAH Yes No Good Clean-room and IC Etches aluminum
process compatible
TMAH:Si No No Poor Does not etch Poor surface finish
aluminum

Etching silicon in these etchants results in three-dimensional structures bounded
principally by {111} planes, but also by other planes. The simplest structures are
made in {100} silicon. Illustrations showing the anisotropic etch property and the
structures that are formed can be seen in Figure 2.11. The intersection between a
{111} plane and the {100} surface of the silicon is in a {110} direction. Four such
planes intersect the surface, such that the lines of intersection on the silicon surface
are at right angles to each other. Each set of planes is inclined at an angle of 54.7 with
respect to the surface. Etching is usually done through a window in a masking layer,
and if the edge of the window is parallel to the intersection between a set of {111}
planes and the surface, then the {111} facet that reaches the surface at this edge is

(a)

(b)

(c)

(d)

(e)

(f)
Figure 2.11 Illustration of wet anisotropic etching in {100} silicon showing plain views on the
left-hand side and cross-sectional views on the right-hand side: (a) square opening in mask with
the silicon etched for a relatively short time; (b) square opening in mask with the silicon etched
until inverted pyramid forms; (c) rectangular opening in mask with the silicon etched for a
relatively short time; (d) rectangular opening in mask with the silicon etched until V-groove is
formed; (e) sequence showing undercutting to form a cantilever beam in the masking material;
and (f) etching through arbitrarily shaped opening in mask.