Chemistry Reference
In-Depth Information
in the nonaqueous electrolyte.
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Initially, the water is molecularly adsorbed on the
silicon surface, then slowly oxidizes the surface silicon atoms to form oxide. In organic
solvents, adsorbed residual water can remain on the silicon surface for hours before
oxidation takes place.
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The oxidation may be uniform or in islands depending on the
composition of the electrolytes.
The reported experimental results indicate that the formation of native oxide in
both air and water has a strong electrochemical component because (1) the oxide growth
rate depends on doping concentration; (2) the growth rate increases with the presence
of oxidants such as dissolved oxygen or hydrogen peroxide in the solution; and (3) the
growth rate is increased by an anodic polarization, and by the presence of a submono-
layer of metal atoms. This electrochemical nature has not been considered in the exist-
ing models on the mechanism of formation of native oxide.
2.5. HYDROPHOBIC AND HYDROPHILIC SURFACES
The surface of silicon can be hydrophilic or hydrophobic depending on the
surface condition. It has been reported that a hydrophilic surface is characterized by
OH groups and a hydrophobic surface is covered with Si-H, or Si-F groups.
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Thus, a hydrogen-terminated surface is hydrophobic whereas an oxide- or hydroxide-
covered surface, which tends to be terminated by OH, is, in general, hydrophilic. A
surface covered with a substantial amount of carbon bonded to oxygen is hydrophobic.
Hydrophilic and hydrophobic surfaces can be generated by treatment in certain
solutions. As shown in Table 2.7, silicon surfaces with a termination of different species,
giving hydrophilic or hydrophobic characters, can be obtained using different cleaning
processes.
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Hydrophilic surfaces can be generated by SC1 solution cleaning followed
by DI water rinse; hydrophobic surface can be generated by dipping in dilute HF solu-
tion followed by DI water rinse.
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Treatment with alcohols can also modify the
hydrophilic and hydrophobic property.
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The surface polarized at potentials more neg-
ative than the passivation potential is hydrophobic whereas the surface after polarized
at potentials positive of the passivation potential is hydrophilic.
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Also, a hydrophilic
or hydrophobic surface tends to change during storage in air.
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The aging effect is asso-
ciated with a loss of hydroxyl groups on hydrophilic surface or the appearance of OH
groups on hydrophobic surface.
Upon immersion in aqueous HF solutions a clean silicon surface becomes
hydrophobic very rapidly due to the adsorption of hydrogen ions. On the other hand,
a silicon oxide remains hydrophilic when exposed to such echants and a silicon sub-
strate covered with an oxide film remains wetted by the etching solution until the oxide
film is completely dissolved. This change of wetting characteristics can be used as a
criterion for determining the end point in etching of an oxide film. As shown in Fig.
2.23, the surface hydrophobisation can be classified into three regimes: in regime I
oxide etches at a constant rate; regime II it is hydrophilic and oxide etches with non-
linear time dependence; and regime III surface becomes hydrophobic. There is also a
short time of hydrophobicity initially when the oxide becomes wet (regime w).
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On the other hand, lack of wettability by dilute HF solution is not necessarily an
indication of a bare Si surface. A silicon oxide can also be hydrophobic when its surface
