Chapter 6: MOS Capacitors

The fabrication of the oxide of an MOS structure is one of the critical steps when fabricating MOSFETs. This is in part due to the need for an ideal oxide-semiconductor interface with low surface-state density but also because of the extremely thin oxides that are currently used for sub-micron MOSFETs. Two techniques are commonly used to form silicon dioxide. One involves the oxidation of the silicon yielding a thermal oxide. The other technique relies on the deposition of SiO₂ using a chemical vapor-deposition (CVD) process.

The thermal oxidation of silicon is obtained by heating the wafer in an oxygen or water vapor ambient. Typical temperatures range from 800 to 1200°C. The oxidation of a silicon surface also occurs at room temperature but the resulting 3 nm layer of oxide limits any further oxidation. At high temperatures, oxygen or water molecules can diffuse through the oxide so that further oxidation takes place. The oxidation in oxygen ambient is called a dry oxidation. The one in water vapor is a wet oxidation. The thermal oxidation provides a high quality interface and oxide. It is used less these days because of the high process temperatures.

The deposition of SiO₂ using a CDV process is one where two gases, silane and oxygen, react to form silicon dioxide, which then sublimes onto any solid surface. The wafers are heated to 200 - 400°C yielding high quality oxides. The lower process temperature and the quality of the deposited layers make CVD deposition the preferred method to fabricate MOS oxides.