Chemistry Reference
In-Depth Information
972
range of to Table 2.9 shows the surface concentration of
various metals after exposure for 1000 s to aqueous solutions containing M of
each metal.
977
The surface concentration appears to strongly depend on pH. Even the
very active elements such as
and
can be deposited on the silicon surface,
to
reaching a surface concentration of
at a low concentration
of
The amount of deposition generally increases with the concentration of
975
Fe, Zn, and Ni in the SC-1 cleaning solution as shown in Fig. 2.15.
At an impurity level of metal concentration on the order of deposition
may occur by physical adsorption, chemical absorption, and displacement
processes.
96,99,976
Physisorption due to van der Waals interactions takes place when the
metallic impurities are in solid form such as iron in neutral or alkaline solutions. Alu-
minate deposition in neutral or alkaline medium is attributed to chemisorption. The
deposition of noble metals such as Cu is generally via displacement reactions. The
deposited metal can be bonded to silicon, attached to surface species such as OH groups,
or embedded in native oxide.
915,972,975
According to Loewenstein
et al.,
972
the deposition of the contaminant level of
metals from cleaning solution onto the silicon or silicon oxide surface occurs by replac-
ing the hydrogen ion on the surface silanol groups:
Metals and compete for the available surface sites, the number of which depends
on the manner of surface preparation. The surface concentration of the metals,
can
be described by the Langmuir model for adsorption:
