Chemistry Reference
In-Depth Information
tions
592,989
and can vary with KOH concentration.
254
In KOH-propanol-water solutions
the etched corner is bounded by {331} planes.
54
In hydrazine-water solutions, {211}
planes are found to define the etched corner.
323,542
The problem of corner undercutting
can be circumvented using compensation masks.
592,707,989,1003
Etching of a corner defined by (111) planes can also be explained by the (111)
step etching model illustrated in Fig. 7.41. The etching of perfectly oriented (111)
planes is determined by the generation of steps. The corner, being the terminating line
of the two (111) planes, can serve as the site to generate (111) steps. Once a (111) step
is generated it will be etched at a rate determined by the etching conditions. The appar-
ent orientations of the two etched surfaces are determined by the rates of step genera-
tion and step etching as illustrated in Fig. 7.47. For example, a fast step etch and a slow
step generation will result in a surface with a small angle to the (111) plane.
7.7. SURFACE ROUGHNESS
Surface roughness, which appears as a series of peaks and valleys on a micro-
scopic scale, is a measure of the degree of unevenness of a solid surface. At an atomic
scale, it describes the uneven termination of the lattice at the surface relative to a per-
fectly flat reference plane. Roughness is commonly described on two levels: micror-
oughness and macroroughness. Microroughness is used to describe the surface
unevenness due to adatoms, vacancies, kinks, and steps associated with the few layers
of atoms on the surface, and macroroughness describes the surface topological varia-
tions beyond the first few atomic layers. Macroroughness usually exceeds the micro-
scopic roughness by several orders of magnitude.
1000
Figure 7.48 illustrates the two
levels of roughness. Table 7.4 shows that surface roughness, for silicon surface treated
in different solutions, may vary by many orders of magnitude.
The roughness of etched silicon surface can be measured by a number of dif-
ferent methods: profilometer,
458,557,1002
reflection electron microscopy,
631,1046
optical
microscopy,
302,303
interference microscopy,
302,303
STM,
488,494
AFM,
968,1000
SEM.
127,490
Two
methods are commonly used for quantifying surface roughness: area averaged surface
roughness which is defined as the root mean square (rms) of the height distribution on
a defined area of the surface, and valley-to-peak roughness which measures the largest
height difference between valleys and peaks on the surface of a given area.
458
Table 7.4
