Etching of Silicon - Electrochemistry of Silicon and Its Oxide - page 331

7.7.2. Macroroughness
Macroroughness is a measure of the surface topographic variations at a scale
much larger than the atomic scale. The macroroughness of a silicon surface treated in
a solution can be determined by numerous factors associated with the initial surface
condition (cleanness and roughness), the crystal orientation, solution composition,
treatment procedure, and time.
In HF solutions, in which silicon etches only very slowly, a silicon surface tends
to become rough due to the formation of etch pits over a period of days to months. 1007
These pits, formed by processes similar to formation of porous silicon, tend to vary in
size with doping concentration. Similarly, roughness tends to develop in
BHF solutions due to surface pitting. 468 In solutions, in which the etch rate
is high, the roughness of etched surfaces depends on the composition of the solution.
For a given HF concentration, there is a concentration range of within which a
smooth surface is obtained. 204,331 In solutions the surface roughness is found
to decrease with decreasing HF/CrO 3 ratio. 195,196
The roughness of the etched silicon surface in KOH solutions has been investi-
gated by Palik et al . 557 The roughness varies with KOH concentration with a peak at
about 5 M as shown in Fig. 7.49 and it is lower in stirred solutions than in nonstirred
solutions at room temperature. In nonstirred solutions the roughness is parallel to the
etch rate which also shows a peak at about 5-6 M, but in the stirred solutions it remains
at a relatively small value over the whole concentration range. Kwa et al . 458 found that
the roughness of (100) etched surface decreases with KOH concentration at different
temperatures as shown in Fig. 7.50. Results similar to those in Fig. 7.50 are also
reported by Sato et al. 1002 Roughness in KOH tends to decrease with increasing tem-
perature. Cleaning with different water and HF rinses has little effect on the roughness
of the etched surfaces in KOH solution, implying that native oxide on the surface is
not important in determining the roughness. 124,557,1000
In TMAH solutions a smooth etched surface is obtained at concentrations above
22% TMAH. 1005 Addition of l-4g/liter pyrazine to 25% TMAH solution increases
slightly the etch rate and improves the surface smoothness. 594 Dissolved silicon in the
etchant results in higher roughness as shown in Fig. 7.51. 518 In the ternary etching
system of hydrazine, IPA, and water, 542 the etched surfaces vary from being highly
faceted to very flat. The faceted surfaces are made up of square pyramidal hillocks
whereas flat surfaces are marked by a cellular structure. In ethanolamine-based
etchants, the roughness of etched surface appears to be associated with the formation
of precipitates. 520
Surface roughness is generally a strong function of potential. 2,18,33,541,957,1004 For
example, it has been found that the etched p (100) surface in KOH at potentials
negative of the OCP is rough, covered with pyramidal hillocks, whereas that etched at
anodic of the OCP is mirrorlike. 541 In HF solutions the silicon surfaces etched at anodic
potentials below the current peak are extremely rough due to the formation of porous
silicon, whereas those etched at potentials above the current peak are mirrorlike (see
Chapter 8). It has been reported that the surface roughness of (111) silicon in 0.1
at decreases from 4 nm to 0.7 nm after being treated in the current oscillation
region. 494