Thermal processes for nanofabrication

Annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable.

Annealing involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, and then cooling.

In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. As the material cools it recrystallizes. For many materials, the crystal grain size and phase composition, which ultimately determine the bulk material properties, are dependent on the heating and cooling rates.

Thermal oxidation is a technique for producing a thin layer of oxide on the surface of a sample. The technique forces an oxidizing agent to diffuse into the sample at high temperature and to react with the bulk material. The rate of oxide growth is often predicted by the Deal-Grove model.

Thermal oxidation may be applied to a variety of materials, but most commonly involves the 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:

Si + 2 H2O ? SiO2 + 2 H2 (g)

Si + O2 ? SiO2

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

• Generally performs best on whole round wafers
• May not be suitable for samples that cannot tolerate high temperatures