2.3.3 Silicon Dioxide

Table 2.3: Physical properties of silicon dioxide [19]

Atomic number
Silicon	14
Oxygen	8
Atom radius
Silicon	1.18	Å
Oxygen	0.65	Å
Molecular density (thermal)	2.3 1022	cm-3
Density (thermal dry oxidation)	2270	kg/m-3
Density (thermal wet oxidation)	2180	kg/m-3
Thermal conductivity	3.2 10-3	W/cm K

Silicon dioxide (SiO$_2$) is used as an insulator between semiconductor devices or as a high quality dielectric for MOSFET gates. Whenever silicon is exposed to air a small film of silicon dioxide (native oxide) with a thickness of approximately 1 nm is formed on the surface. Two types of processes are used to build thicker SiO$_2$ layers. On the one hand side thermal oxidation of silicon is performed which generates high quality interfaces between silicon and silicon dioxide, and on the other hand side oxide films can be deposited by a chemical vapor reaction. This technique is preferred to fill trenches, to create a thin insulator between layers, or to build a diffusion source or getterer. For these applications the oxide is doped during deposition. The thermally grown as well as the deposited silicon dioxide is amorphous, the average length of a silicon oxide bond is 0.162 nm and the distance between the oxygen ions is 0.227 nm. Some properties of SiO$_2$ are summarized in Tab. 2.3, where also some differences between thermal grown and deposited SiO$_2$ are emphasized.