Chemistry Reference
In-Depth Information
hydrazine-water mixture gives 8 curved faces with the center of the faces as (100)
planes and the corners point in the <111> directions.
542
In RbOH solution the polyhe-
dron has 14 vertices and 24 curved faces.
249
Using masks of different shapes and orientations and controlling the planes to
disappear and develop during etching, various etched features such as cavity or pyramid
can be obtained from anisotropic etching.
513
Cavities and mesas etched on silicon in
alkaline etchants are bounded by sidewalls which represent the slowest etching (111)
planes of the crystal. The exact geometry of cavities etched through a mask opening is
a function of the orientation of the wafer, the geometry of the opening, its alignment
relative to the wafer's crystal axes, and the duration of etching.
531
Figure 7.43 illus-
trates the shape of three cavities anisotropically etched on (100) surfaces through open-
ings of different geometry defined by the masks. After sufficient etching time, the
intersection of the cavity to the surface is a rectangle bounded by four (111) planes
which enclose the opening defined by the mask.
The final rectangular cavity bounded by four (111) planes independent of the
initial opening shape can be explained with the (111) step etching model illustrated in
Fig. 7.41. All of the (111) steps that intercept the etching (100) surface and are exposed
within the opening defined by the mask are etched in a direction parallel to the (111)
planes. The etching along these planes will continue until perfect (111) planes, which
define the walls of the etched cavity, are reached and all of the exposed (111) steps
vanish.
The etched cavities on (100) wafer bounded by the convergent {111} planes have
an angle of 54.74° with respect to the surface plane. The dimensions of the resulting
square pyramidal cavity, as shown in Fig. 7.44, are given by
