Chemistry Reference
In-Depth Information
HF constitutes a large fraction of the dissolved species, it does not participate in the
reaction since a solution consisting of only HF molecules is not able to attack the
structure at an appreciable rate. They found that the etch rates fit the equation
The relative contribution of and in the etching process changes with total
HF concentration as shown in Fig. 4.38. At very low HF concentration, etching is pri-
marily due to
At high HF concentration, the etching by
dominates. The rel-
ative contribution in the etching by
or
can be controlled by adding
HCl, or
Vondeling
671
found that the etch rate of quartz in
solutions can
be described by
Based on this empirical equation, the author suggested that positive ions and HF are
the important species in the etching reactions on quartz and that, unlike in the etching
does not appear to significantly affect the
etch rate of quartz. It was also suggested that the etching mechanism involves the
adsorption of positive ions, e.g., onto the quartz surface, which changes the elec-
tronic configuration of the surface and facilitates the etching reactions.
Osseo-Asare
123
proposed a model described Eqs. (4.10) to (4.14). According to
this model, the effect of pH on the concentration of surface fluoride species is related
to the effect of pH on (1) the concentration of the SiOH sites, (2) the concentration of
fluoride-containing species, and (3) the competitive adsorption of fluoride-containing
species and hydroxyl ions. The surface site balance can be written as
of thermal oxides, the concentration of
