Chemistry Reference
In-Depth Information
control the etch rate, etch selectivity, solution stability, and quality of the etched surface.
A major difference between these two systems is that the etch rate of silicon HF solu-
tions is similar among the various crystalline orientations, i.e., is isotropic, whereas in
alkaline solutions it strongly depends on the crystalline orientation, i.e., is anisotropic.
Another difference is that silicon oxide, which may be present on the silicon surface
prior to or during an etching process, etches fast in HF solutions whereas it etches very
slowly in alkaline solutions relative to the etch rate of silicon.
Examples of the specific etch rates in various solutions reported in the literature
are given in Table 7.1. Several points may be made regarding the data in Table 7.1. (1)
Silicon can be etched at a wide range of rates, as much as nine orders of magnitude;
(2) the highest etch rates are observed in HF solutions; (3) the etch rates of the three
major crystal planes vary only marginally in HF solutions but vary greatly in alkaline
solutions; (4) the etch rate is specific to a given set of etching conditions. In addition
to material and solution conditions, the etch rate of silicon in a given system depends
on many operational parameters such as size and geometry of the sample, volume of
the solution, stirring condition, ambient (light and air) control, and etching time. Thus,
for a given silicon material and solution composition, the etch rate may vary signifi-
cantly when it is measured under different operating conditions. This is responsible for
the sometimes large difference in etch rates that can be found in identical systems
studied by different investigators.
There are three major etching solution systems with respect to the uniformity of
the etched surface, that is, relative etching rates on silicon surfaces of different crys-
tallographic characters: (1) isotropic etching system represented by (2)
anisotropic etching system represented by alkaline solutions, and (3) defect etching
system represented by solutions (Fig. 7.1).
Table 7.2 shows the activation energies determined for the various etching
systems. In general, the apparent activation energy, as determined from the dependence
of etch rate on temperature, is 3-6 kcal/mol or 0.13-0.26eV for diffusion-limited reac-
tions, whereas it is 10-20 kcal/mol or 0.44-0.87 eV for surface-controlled reactions.
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Using these criteria to evaluate the values of activation energy in Table 7.2, it appears
