Silicon/Electrolyte Interface - Electrochemistry of Silicon and Its Oxide - page 68

The presence of HF in water limits the thickness of native oxide. Oxide does not
form when the concentration of HF in water is higher than l0ppm. 978 The oxide thick-
ness is found to be near zero in sulfuric acid containing 0.1% HF. 126 Generally, in
aqueous solutions the thickness at the steady state is determined by the oxide growth
rate, which is a function of the oxidant concentration in the solution, and by the oxide
dissolution rate, which depends on the concentration of HF in the solution. The oxide

tends to be relatively thick in solutions containing a high concentration of oxidant such
and a low concentration of HF. On the other hand, the oxide tends to be thin
in solutions with a low oxidant content and a relatively high concentration of HF. When
the concentrations of both oxidant and HF are high, the silicon is rapidly etched through
oxide formation and dissolution processes. In solutions the formation of oxide
film depends on concentration. 1090 At concentration below 10%, the formation
rate increases with concentration. At concentration higher than 10%, oxide does not
form because the dissolution rate is higher than the formation rate.
Dissolution of silicon in water occurs during the growth of native oxide in water
as shown in Fig. 2.21. 579 The number of dissolved Si atoms is over one order of mag-
nitude larger than the number of Si atoms in native oxide. The formation of native oxide
in pure water is thus influenced by the dissolution of silicon in water. On the other hand,
little dissolution of silicon in pure water was observed for the samples with native
oxides formed in air or in solutions of or

as

This indicates

that the oxide film formed in pure water is a relatively poor barrier.