Chemistry Reference
In-Depth Information
vation energies are involved in the etching process.
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An examination of the data in
Table 4.2 also suggests that for a given solution the activation energy tends to be higher
for denser oxides such as quartz than for less dense ones such as CVD oxides.
4.3. THERMAL OXIDE
Figure 4.3 shows the etch rate of thermal oxide as a function of HF concentra-
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tion and temperature. A nearly seven order of magnitude variation of etching rate,
from less than to near can be obtained by varying the HF concentra-
tion from about 0.0001% to about 10% in the temperature range of 25-90 °C. The
etching reaction has a reaction order larger than 1 in the whole concentration and tem-
perature ranges. Etch rate could not be meaningfully determined in HF solution below
0.0001 M HF by measurement of loss in thickness or weight of etched samples.
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Temperature has a strong influence at high HF concentrations and relatively less
effect at very low concentrations as shown in Fig. 4.3.
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At very low concentrations,
