Chemistry Reference
In-Depth Information
such as gold, silver, copper, nickel, cadmium, zinc, cobalt, and chromium are also oxi-
dized by and dissolved by the complexing effectiveness of the ammonium hydrox-
ide; copper, for example, forms the amino-complex.
203,488,630
The second treatment step exposes the rinsed wafer to a solution known as SC2,
which is a hot mixture of
(30%), HC1 (37%), and
in the proportions of
1:1:6 to 2:1:8 by volume. This cleaning solution is designed to remove alkali ions
and that form -insoluble hydroxides in the
alkaline solution. It also cleans metallic contaminants that are not entirely removed in
SC1 solution such as gold and prevents displacement replacing from solution by
forming soluble complexes with the resulting ions.
884
A preliminary cleaning step with
hot mixture (2:1) can be used for grossly contaminated wafers having
visible residues. A short dipping in dilute HF solution may also be used after SC1 clean-
ing to remove the hydrous oxide film formed in SC1 solution.
203
More recently, this
cleaning solution has often been used with a concentration only one-tenth of the orig-
inal solution in order to avoid surface roughness and to reduce production cost and the
effect on the environment.
1013
Strict control of the purity of water and materials used for preparation of the
cleaning solutions is essential in obtaining good results, as recontamination may also
occur in the cleaning solutions. Figure 7.60 shows, as an example, that the surface con-
centration of Fe, Ni, Cu, and Zn increases linearly with the impurity concentration
in SC1 solution.
1017
The deposition process is found to follow the Langmuir adsorp-
tion/desorption theory. Cu, Ag, Au, and Mo at in HF and BHF solutions
can all cause deposition at levels higher than Deposition of metal impuri-
ties at ppm levels in HF solutions is severe for Cu, Au, and Cr and is much lower for
Fe and even less for Zn.
883
The precipitation of metals onto a silicon surface is essentially a corrosion
process, in which the ionic metal species in the solution are reduced and deposited on
the surface simultaneously with oxidation of the silicon atoms. The rate of metal depo-
and cations such as
