Etching of Oxides - Electrochemistry of Silicon and Its Oxide - page 154

On the other hand, according to Wirth and Gieskes, 897 the rate-determining reaction
during etching of silica is determined by the surface charge, and any ionic species,
which affects the surface charge, would affect the etch rate.
In alkaline solutions, according to Hooley, 527 the dissolution of silica follows two
consecutive reactions. First, an adsorption of water occurs followed by reaction with
hydroxyl ions to produce soluble products:
It is argued that the surface of silica consists largely of oxygen ions because the small,
more highly charged silicon ions are well shielded by the highly polarized oxygen ions
so that the hydroxyl ions do not react with them. The absorption of water by hydrogen
bonding to the oxygen ions on the surface results in transfer of charge toward the water
molecule. This decreases the screening so that silicon ions can combine with hydroxyl
ions to form soluble complex. This mechanism explains the lack of dissolution of
silicon oxides in NaOH containing methanol. It also explains the maximum rate with
respect to NaOH concentration (Fig. 4.15). At low molarity and therefore high water
content, reaction 4.5 is the rate-determining step so that the etch rate will increase with
increasing molarity. On the other hand, at high molarity and thus low water concen-
tration, reaction 4.4 becomes the rate-limiting step and thus the rate decreases with