Chemistry Reference
In-Depth Information
In solutions of low HF concentrations the surface was considered to be termi-
nated by hydroxyl groups and the anodic reaction proceeds with the formation of
where is the number of holes involved in the reaction depending on the type of silicon.
Figure 5.61 illustrates the steps involved in reaction (5.8). In this reaction, step (b) is
rate limiting because it requires a certain thermal activation for a group to
swing away from the corresponding Si atom. At potentials positive of the passivation
potential, the reaction further proceeds to form silicon oxide:
is not soluble in water so that its formation in the absence of HF in the solution
leads to the passivation of the surface. In the presence of HF the oxide dissolves to
form fluoride complex:
The overall reaction valence from reactions (5.8) to (5.10) is 4, which accounts for the
reactions occurring in the electropolishing region in HF solutions. To account for the
effective dissolution valence of 2 at potentials below the passivation potential (poten-
tial of the peak current,
it was proposed that silicon reacts directly with HF.
