Oxygen diffusion

One of the strengths of amorphous (vitreous) SiO₂ as a gate dielectric material, when thermally grown directly on the active region of a single-crystalline Si semiconductor, is its thermal stability in further fabrication steps, in strict accordance with the phase diagram of the Si-O system. More specifically, this means that during further processing steps, usually performed at temperatures between 500 and 1100°C in different atmospheres, and therefore at different oxygen partial pressures, oxygen is adsorbed at the surface of SiO₂ films on Si and diffuses therein, having the following consequences:

Atomic transport is limited to species coming from the gas phase. Oxygen, in particular, diffuses as molecular oxygen (O₂) in the films without interacting with the SiO₂ network to react in the near-interface region with non-fully oxidized silicon atoms (Si^nfo) or with Si atoms from the substrate. Alternatively, oxygen from the gas phase is exchanged for fixed oxygen from the solid phase in the near-surface region. Si remains essentially immobile. Although Si interstitials are produced as a result of oxygen arrival and reaction at the Si/SiO₂ interface, injected Si atoms are not observed in the bulk of the SiO₂ films after the thermal oxidation processing.

Apart from additional, stoichiometric or sub-stoichiometric SiO₂ formation due to reaction of diffusing oxygen with substrate-Si, there are no other chemical reactions at the film interfaces with singlecrystalline Si (active region of the transistor) or even with polycrystalline Si (gate electrode), which is deposited on the SiO₂/Si structures.

In the bulk of SiO₂ films on Si with thicknesses above 4–5 nm, chemical composition does not change. However, ultrathin silicon oxide films (thicknesses below 3–4 nm) on Si can be appreciably oxygen deficient and 126and, in this case, thermal processing in O₂ completes the oxidation of Si^nfo, leading to a film with average stoichiometry closer to SiO₂.