Chemistry Reference
In-Depth Information
6.3.2. Morphology
The morphology of metal deposits is determined by the kinetics of the formation
process, which in general occurs in two stages: nucleation of deposits and growths of
the deposit islands to form a continuous film followed by thickening of the film. On a
semiconductor, once a continuous film is formed the semiconductor is isolated from
the solution and subsequent growth depends on the properties of the metal deposit.
848
Thus, the effect of the substrate on the morphology occurs predominantly in the nucle-
ation and growth of islands before the formation of continuous film. According to
Oskam
et al
.,
848
the deposition of continuous metal films onto a semiconductor surface
can be difficult. The interaction energy between metal adatoms and the semiconductor
surface is generally small and deposition usually follows the 3D island growth mech-
anism. In order to obtain a continuous deposit layer the density of nuclei must be suf-
ficiently high for the islands to merge during the deposition. Once the nuclei are formed,
further growth into a continuous film requires a non-diffusion-controlled condition to
enhance the ratio of lateral growth to vertical growth. Thus, to deposit a continuous
film requires an initial large current to produce a high density of nuclei followed by
growth at relatively low current.
Metal deposition on the silicon surface may follow an instantaneous or a
progressive nucleation process followed by a diffusion-limited growth of the nuclei. The
growth of nuclei can be either kinetically limited, diffusion limited, or under a mixed
control. The current transients measured by Oskam
et al
.
848,1167
at various potentials of
cathodic deposition of copper, platinum, and gold indicate that the deposition of these
metals at relatively low metal concentrations proceeds via a nucleation followed by
diffusion-limited growth. At high concentrations and large cathodic overpotentials the
deposition may proceed through instantaneous nucleation. The nucleation sites become
saturated after a short time, a fraction of a second, depending on the potential. Gold
