Thermal oxidation

In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rate of oxide growth is often predicted by the Deal-Grove model. Thermal oxidation may be applied to different materials, but this article will only consider oxidation of silicon substrates to produce silicon dioxide.

Thermal oxidation of silicon is usually performed at a temperature between 800 and 1200 °C, resulting in so called High Temperature Oxide layer (HTO). It may use either water vapor (usually UHP steam) or molecular oxygen as the oxidant; it is consequently called either wet or dry oxidation. The reaction is one of the following:

The oxidizing ambient may also contain several percent of hydrochloric acid (HCl). The chlorine removes metal ions that may occur in the oxide.

Thermal oxide incorporates silicon consumed from the substrate and oxygen supplied from the ambient. Thus, it grows both down into the wafer and up out of it. For every unit thickness of silicon consumed, 2.17 unit thicknesses of oxide will appear. If a bare silicon surface is oxidized, 44% of the oxide thickness will lie below the original surface, and 56% above it.

According to the commonly used Deal-Grove model, the time τ required to grow an oxide of thickness X_o, at a constant temperature, on a bare silicon surface, is:

where the constants A and B encapsulate the properties of the reaction and the oxide layer, respectively. This model has further been adapted to account for self-limiting oxidation processes, as used for the fabrication and morphological design of Si nanowires and other nanostructures.

If a wafer that already contains oxide is placed in an oxidizing ambient, this equation must be modified by adding a corrective term τ, the time that would have been required to grow the pre-existing oxide under current conditions. This term may be found using the equation for t above.

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