2. Physics of Thermal Oxidation

THERMAL OXIDATION is a chemical process, where silicon dioxide (SiO$_2$) is grown in an ambient with elevated temperatures. A simple form of thermal oxidation even takes place at room temperature, if silicon is exposed to an oxygen or air ambient. There, a thin native oxide layer with 0.5-1nm will form on the surface rapidly. After that, the growth slows down and effectively stops after a few hours with a final thickness in the order of 1-2nm, because the oxygen atoms have too small energy at room temperature to diffuse through the already formed oxide layer.

SiO$_2$ is used to isolate one device from another, to act as gate oxide in MOS structures, and to serve as a structured mask against implant of dopant atoms. In the beginning of this chapter is described, why thermal grown SiO$_2$ is the most suitable material for such requirements.

This chapter will focus on thermal oxidation, but it should be mentioned that SiO$_2$ layers can also be produced by deposition techniques, like chemical vapor deposition. Deposition normally involves a much smaller thermal budget than thermal oxidation and so it is the only option when wafers have already metal on them. Usually deposited oxides are not used for thin layers under 10nm because the control of the deposition process is not so good as the thermal oxidation process. Another disadvantage is the interface between a deposited oxide and the underlying silicon, which is electrically not so good as thermal oxide. Furthermore, deposited oxide does not have the same high density as thermal grown oxide.

Thermal oxidation is a complex process where a diffusion of oxidants, a chemical reaction, and a volume increase occur simultaneously to convert the silicon substrate into SiO$_2$. This process is strongly influenced by the used oxidant species, the oxidation ambient with temperature and pressure, and also the crystal orientation of the substrate. With these parameters the quality and the growth of the oxide during the manufacturing process can be controlled.

The small dimensions and high performance of modern MOS devices require ultrathin SiO$_2$ layers for gate dielectrics. Apart from the exact thickness control, pure SiO$_2$ has some difficulties to fulfill all requirements at such thin thicknesses. Especially the dopant penetration and direct tunneling for ultrathin oxides can not be handled. It was found that silicon oxynitrides are more suitable materials for such applications. Oxynitrides can be produced by different methods which depend on the desired nitrogen profile and, therefore, on the application.