Chemistry Reference
In-Depth Information
schematically illustrates the surface condition of silicon electrodes in terms of hydride
and oxide coverage and thickness in the different potential ranges.
6.3. METAL DEPOSITION
Metal deposition can occur either electrolessly at the OCP or under a cathodic bias.
Both processes are electrochemical in nature in that the deposition of metal atoms is a
reduction reaction involving charge transfer with the substrate. Although deposition under
bias provides the best quantitative control of the deposition process, it is the electroless
deposition that has received more attention for metal deposition on silicon surface, mainly
due to the simplicity of the process. In most cases, as shown in Table 6.1, the deposition
is limited to noble metals such as copper, gold, and platinum mainly due to their excellent
conductivity and chemical stability, which are important in the application of microde-
vices. Also, the electroless deposits from trace amount of metals on silicon during wet pro-
cessing steps are very important in silicon technology since they cause serious problems
in further processing and eventual performance of devices. However, metal deposition on
silicon, as pointed out in a recent review by Oskam
et al
.,
848
despite its potential impor-
tance in metal/semiconductor contact technology, has not been well investigated. The
recent interest in replacing aluminum metallization by copper has resulted a renewal of
research activities in electrochemical deposition of metals onto silicon.
6.3.1
.
Kinetics
The driving force for metal deposition on a semiconductor electrode is deter-
mined by the difference between the Fermi level of the electrode and the reversible
potential of the metal species in the solution. For the noble metals this difference at the
OCP is generally negative indicating that the deposition process is spontaneous, which
is the basis for electroless deposition. As shown in Fig. 6.8, the redox potentials of the
