Chemistry Reference
In-Depth Information
scopically smooth surface the microroughness is mainly determined by the wafer pol-
ishing and cleaning processes that are used to prepare the surface for further processes.
As-received wafers generally have very small microroughness, typically 0.15-0.4 nm
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Any cleaning process may alter the microroughness.
Cleaning with the RCA process is found to increase the microroughness of as-
received wafers from 0.2nm (rms) to about 0.4nm.
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The SC1 cleaning step is found
to be mainly responsible for the increase in microroughness after the RCA cleaning.
The acid cleaning and water rinse steps have relatively much less effect. The rough-
ness tends to increase with increasing the relative concentration of NH
4
OH in the SC1
solution and is related to the etch rate of the solution.
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Modifications to each of the
steps used in the RCA cleaning can also affect the microroughness.
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However, the effects of surface cleaning factors on microroughness reported in differ-
ent studies are not consistent, probably due to the minute differences in the cleaning
conditions and measurement procedure.
The microroughness of the silicon surface in various cleaning solutions gener-
ally increases with time, that is, with material removal.
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Thus, the small rough-
ness observed on the surface treated in cleaning solutions is mainly due to the small
amount of material etched during the cleaning process. Much larger surface roughness
is observed on the etched surface, where generally a large quantity of material is
removed.
Etching in HF-containing solutions is particularly important as they are involved
in almost all cleaning processes for silicon surfaces. It is often the last cleaning step in
surface preparation for further processing. The silicon surface that is treated with HF
solution is terminated by hydrogen (see Chapter 2). The atomic level flatness is related
to the concentrations of monohydride, dihydride, and trihydride.
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On a (111)
surface monohydrides tend to form on the perfect lattice while dihydrides and trihy-
drides tend to form at steps and defects. An atomically flat (111) surface is dominated
by monohydrides and straight step is dominated by dihydrides.
It has been reported that silicon surfaces, both (111) and (100) orientations, after
being treated in HF solutions are rough at an atomic scale and have surface features
such as steps, kinks, and defects.
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The microroughness of silicon surfaces treated
in HF solutions varies with pH. Results from slightly misoriented (111) surface show
that at pH < 3 the (111) terraces are rough. At pH 6.6 the surface is characterized
by flat (111) terraces and straight steps, indicating that the atomic-scale defects on the
terraces and the corner and kink atoms on the steps are preferentially removed. At
pH > 7.8 the surface becomes rougher due to the formation of multiple steps. The for-
mation of multiple steps with increasing pH is attributed to the faster etching rate which
causes larger fluctuations of terrace length. Such variation of surface atomic-scale fea-
tures with pH shows the importance of solution composition in determining the surface
microroughness.
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pH may have different effects in a different solution composi-
tion. For example, in a solution for cleaning oxide, the smoothest surface
is obtained at pH 0.5.
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In solutions the slightly misoriented (111) surface lacks
well-defined terraces at pH 4 but has well-defined terraces at pH 8.
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In another study
it is found that the Si(111) surface treated in BHF is atomically flat and is less flat in
HF.
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Also, water rinse significantly flattens the HF-treated (111) surface but is less
effective for the (100) surface.
(rms).
