Chemistry Reference
In-Depth Information
to less than 2
×
by about one order of magnitude per decade, from more than
in the 1970s, and to less than in the 1990s.
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According to Ohmi,
99
the removal of metallic and organic contaminants from the
Si surface to a very low level requires the use of a solution containing high redox poten-
tial. The metallic particles are ionized and dissolved in the solution while the organic
impurities are decomposed to
fied in order to remove particles from the surface: (1) the zeta potential of the substrate
and so on. Two requirements must be satis-
surface and particles should be in the same polarity in the solution so as to cause repul-
sive electric force between the surface and the particles, and (2) the substrate surface
or the particle surface should be slightly etched in order to lift particles off the sub-
strate surface. Also, the formation and cleaning of the oxide film in the cleaning solu-
tion and in air have a special significance in obtaining a clean silicon surface. Ohmi
pointed out that when a hydrogen-terminated silicon surface is exposed to the air in a
clean room, moisture of several tens of molecular layers is adsorbed within seconds. If
the ambience is composed only of moisture or oxygen, the Si surface will not be oxi-
dized at room temperature.
RCA Cleaning.
The RCA process, which was originally developed by Kern and
Puotinen in 1970,
884
is still the basis of the current wet cleaning process for bare or
220,461
oxide-covered silicon surfaces. Table 7.6 details a current RCA cleaning process.
The original RCA cleaning, which tends to etch
silicon substrate during the
454
process, is based on
ammonium hydroxide followed by an acidic mixture with HC1.
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In the first treatment,
a hot solution of NH
4
OH (27%), H
2
O
2
(30%) and H
2
O, with a proportion of 1:1:5 to
1:2:7 by volume of the chemical agents, known as SC1 cleaning solution, is used.
203
SC1 solution is designed to remove organic contaminants by both solvating action of
the ammonium hydroxide and the oxidizing action of the peroxide. The oxidizing action
by H
2
O
2
also results in the formation of surface silicon oxide which dissolves slowly
in the solution. It is thus very effective in particle removal as continuous oxidation and
etching of the oxide take place under the particles.
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The metal surface contaminants
two hydrogen peroxide solutions: an alkaline mixture with
