Chemistry Reference
In-Depth Information
491
A similar rate equation has been proposed by Proksche
et a
l
.
in
solu-
tions. They plotted the ratio of etch rate to the maximum rate as a function of
concentration in Fig. 4.37, revealing that the etch rates of various oxides as a function
of concentration can be divided into two regions. For low concentrations (<12%)
the etch rates increase to a maximum, with the slope of the normalized etch rate the
same for all oxide types. For high concentrations (>12%) the etch rate strongly depends
on the oxide type. The activation energy is also different in the two regions for differ-
ent oxides, indicating that different mechanisms are involved in the two regions. In the
low-concentration region the etching is mainly due to the reaction with
whereas
in the high-concentration region reaction with HF becomes more important.
In strong acidic solutions, with addition of HCl, the dissolution of silica is first
order with respect to HF concentration.
910
Verhaverbeke
et al
.
238
considered the effect of HF dimer,
in addition to
on the etching of
in HF and HF/HCl solutions. It was reasoned that although
