Chemistry Reference
In-Depth Information
larger than 1 and tends to be different for different oxides. (2) The etch rates
differ by several orders of magnitude for different types of oxides with quartz being
the slowest and anodic oxide the fastest, reflecting the large difference in the density
and structure of these oxides. (3) The etch rate of thermal oxides is much slower com-
pared to CVD and anodic oxides. (4) The consistency of the data obtained from many
different investigations indicates that the etch rate of thermal oxides is not significantly
affected by formation conditions such as temperature, pressure, ambient composition,
and duration. (5) Silicon, as a solid, is extremely stable in HF solutions compared to
its oxide, as its dissolution rate is several orders of magnitude smaller than even that
of quartz.
The activation energy for dissolution of silicon oxides varies over a wide range
depending on the solution composition and the type of oxide. It appears that, based on
the data in Table 4.2, the activation energy is higher in nonfluoride solutions than in
fluoride solutions. In general, low activation energy in the range of 3-6cal/mol or
0.13-0.26 eV is indicative of a diffusion-limited process whereas that in the range of
10-20kal/mol or 0.44-0.87 eV is indicative of a surface reaction-limited process.
According to Harrap,
153
the activation energy of thermal oxide in HF solution increases
with HF concentration,
C
, and follows the empirical relation
with and Using the above equation, for example, the
activation energy is calculated as 0.18eV in 0.1% HF and 0.37 eV in 10% HF. An acti-
vation energy lower than that estimated by this equation is observed in dilute HF solu-
tions, for example, 0.07 eV in 0.01 M HF.
239
Also, the activation energy tends to be
higher at high temperatures.
153
In BHF solutions, the activation energy increases with
the ratio of
to HF, suggesting that several parallel reaction paths of different acti-
