Chemistry Reference
In-Depth Information
noble metals are all below the conduction band edge of silicon indicating that these
metals can be deposited electrolessly as well as cathodically.
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The driving force can
be altered by changing electrode potential, metal concentration, and complexing agents
in the solution. Also, illumination increases the driving force for both
n
and
p
types of
materials due to the negative shift of the quasi Fermi level on illumination.
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Electroless deposition of noble metals, even at trace levels, on silicon is practi-
cally always possible due to the large driving force. For example, Table 6.2 shows that
the critical concentration of copper in solution at which the driving force is zero is
extremely low.
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The critical concentrations are even lower for Au, Ag, and Pt because
they have move positive standard potentials than copper.
Figure 6.9 shows the
i-V
curve of copper deposition in a
solution.
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The onset of reduction of occurs at with a diffusion-
limited peak at The current increases again at a more negative potential,
about -0.8V, due to hydrogen evolution at the copper deposits which catalyzes the
hydrogen evolution reaction. On the reverse scan in Fig. 6.9 a current of about
is due to the continuous growth of the copper clusters. The reverse scan
does not show an anodic peak indicating that the rate of anodic dissolution of the copper
deposits is low due to the large barrier height, about 0.6 eV, existing at the Si/Cu
interface. Although copper deposition dominates the cathodic reaction, hydrogen
