Chemistry Reference
In-Depth Information
Formation of hillocks can be reduced by controlling solution composition, tem-
perature and agitation, electrode potential, surfactants, pressurizing, and so on.
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In general, increasing silicon solubility and reducing gas formation results in the reduc-
tion of hillock formation. It has been found that addition of an oxidizing agent such as
hydrogen peroxide, which effectively inhibits the formation of hydrogen bubbles,
inhibits the formation of hillocks.
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Pressurizing with oxygen is also found to reduce
the formation of hillocks.
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It was suggested that the reaction of oxygen with the hydro-
gen atoms on the surface occurs before the formation of bubbles. Other practices such
as increasing solution temperature, solution stirring, and decreasing the concentration
of dissolved silicon in the solution, are effective in reducing the possibility of forma-
tion of solid silicates on the surface and thus the formation of hillocks.
7.7.3. Origins of Roughness
Roughness developed in a cleaning or etching solution is a result of uneven dis-
solution across the crystal surface. Many factors in an etching process may cause the
uneven distribution of the dissolution rate at both micro and macro scales. One partic-
ular example is the roughness associated with the formation of hillocks. In general, any
process that causes temporary or permanent surface inhomogeneity will result in pref-
erential dissolution of some areas relative to other areas.
The surface roughness that develops on the etched surface in KOH is attributed
to the masking by hydrogen bubbles resulting from the etching reactions.
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The
surface area that is masked by a hydrogen bubble is not etched until the bubble grows
to a certain size and leaves the surface. The roughness, depending on the amount of
preferential etching of the masked and unmasked areas, increases with the size of hydro-
gen bubbles. Increasing convection and temperature increases the rate of reaction but
decreases the hydrogen bubble dwell time, leading to the formation of more and smaller
bubbles.
The precipitate of etching products on the surface is a common cause of surface
roughness. Residues tend to form on the etched surface in KOH of low concentra-
tions.
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Varied roughness in ethanolamine-based etchants is associated with the for-
mation of precipitates.
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Staining may occur during etching in
hydrazine solutions, resulting in the formation of various kinds of surface fea-
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Stain is also found to form in etching HF solutions with
and as oxidants. Highly boron doped material is found to be more susceptible to stain
formation.
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The composition of stain has been identified to be
tures.
Roughness of etched surfaces can be attributed to several causes: (1) native oxide,
(2) residuals left from the cleaning process, (3) defects in the crystalline structure, (4)
hydrogen bubbles, (5) metal deposits (6) anisotropic etching and (7) formation of pores.
They may be grouped into two categories, process-dependent and material-dependent,
as shown in Fig. 7.57. The process-dependent causes are the deposition of metal or
nonmetal particles (Fig. 7.57a,b) or bubbles (Fig. 7.57c) which provide masking to the
deposited areas. The difference in the masking effect between a metal and a nonmetal
particle is that metal can actively participate in the dissolution reaction by a galvanic
action. The material-dependent causes, such as preferential dissolution at lattice defects
