Etching of Oxides - Electrochemistry of Silicon and Its Oxide - page 149

etch rates of anodic oxides, as shown in Fig. 4.2, are much higher than for other oxide
types.

The etch rate of anodic oxide can be determined by methods similar to those for
thermal oxide or deposited oxides. It may also be estimated from the anodic current of
the oxidized electrode. The anodic i-V curve of a silicon electrode typically shows a
passivation-like peak above which the dissolution occurs through a two-step process:
the formation of oxide film followed by the chemical dissolution of the oxide. The
steady-state anodic current measured at an anodic potential above the peak potential
indicates the dissolution rate of the anodic oxide. Thus, the passivation current,
in Table 5.5 can be used for estimation of the etch rate of the oxide film formed at the
anodic potentials. (A current density of

, listed

corresponds to a silicon etch rate of
3.1Å/s or to a silicon oxide etch rate of about 7 Å/s.) For example, in 1% HF solution,

is and thus the etch rate of the oxide film formed at a potential anodic of

the first current peak is about 35 Å/s. In 2M KOH solution at room temperature,

equivalent to an etch rate of about 0.014 Å/s. These numbers appear to
be in general agreement with the data in Table 4.1.
As can be expected, the etch rate of anodic oxides depends on the formation poten-
tial. Figure 4.26 shows that the dissolution rate of the oxide films formed in 0.05% HF
changes with potential, indicating that the oxides formed at different potentials are dif-
ferent. 286 For the oxide that is formed in the current oscillation region (see Section 5.10),
the etch rate may greatly vary as a function of the distance from the silicon/oxide inter-
face to the oxide/electrolyte interface as shown in Fig. 4.27. 950 Such drastic changes are
partly due to the change of field related to the oscillation of thickness at a given poten-
tial and partly due to the change of the oxide structure during its growing process. In
fluoride-containing solutions, current oscillation can occur in a certain anodic potential
region, which means that the properties and the etch rate of the oxide film oscillate under
that condition. 122,855 The mode of anodization, whether the final potential is imposed in
one step or gradually reached, is also found to influence the etch rate. 98
The high etch rates of anodic oxides are indicative of the loose structure of the
oxides. As shown in Table 3.2, anodic oxide films have low densities and are not