Chemistry Reference
In-Depth Information
The surface concentration of a metal ion depends on (1) the adsorption site density,
(2) the attraction between surface sites and metal ion, (3) the concentration of
the metal ion, and (4) the concentrations of other metal ions and hydrogen ion in the
solution.
When silicon wafers are exposed to a clean room atmosphere, organic molecules
in the air can rapidly be adsorbed onto the surface.
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The amount of contamination,
on the order of (equivalent to a surface density of assuming a mol-
ecular weight of l00g/mol), for alcohol, butyl acetate, ethylene glycol, amide, phtha-
late, and others, increases with exposure time. The organic species that have low boiling
points tend to be adsorbed immediately and are later replaced by those with higher
boiling points. Organic molecules have been found to bond covalently via Si-C
bonds.
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The amount of adsorption decreases with gradual formation of the native
oxide in air.
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The initial concentration of organic species follows the rate of adsorp-
tion, but the final concentration is determined by the desorption rate.
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In alcohols,
freshly prepared H-Si surface may be terminated by methoxy groups.
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UV radiation
has been found to decompose the organic contaminants and its use during cleaning
reduces surface carbon content to a very low level.
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2.4. NATIVE OXIDE
Native oxide is the oxide that grows spontaneously on a clean silicon surface
exposed in an ambient environment such as air or solutions. The formation of native
oxide, which is inert in most solutions, passivates the silicon surface allowing the
silicon to be used as an electronic material in processing and application environments.
The presence of native oxide degrades the low-temperature growth of high-quality
epitaxial Si film or the precise control of the thickness and quality of very thin gate
oxides.
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In electrochemical experiments in particular, native oxide is part of the
initial condition for a silicon surface and affects the electrode properties. It is com-
monly removed by dipping in a HF solution, which produces a hydrogen-terminated
surface, prior to the experiment.
2.4.1. In Air
The surface of silicon in air is always covered with a very thin oxide film. Table
2.10 shows that the thickness of native oxide formed on the surface of silicon after
several days in air varies from 5 to 20 Å depending on preparation conditions. Such a
large variation in the thickness of native oxide indicates the great sensitivity of the
surface reactivity to minute variations of environmental and material conditions.
The rate of formation of native oxide in air depends on the initial condition of
the surface and the cleanness of the air. The formation of oxide on a perfectly clean
silicon surface, e.g., a cleaved surface, is relatively fast, reaching 5-7 Å within a few
minutes after exposure to air.
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The formation of native oxide on the surface after being
