10.1.8 Oxides

Oxide layers are grown from oxygen or H$_2$O molecules from the gas phase above the wafer. On the ubiquitous silicon wafers oxidation results in a layer of SiO$_2$. The growth rate of the SiO$_2$ layer is influenced by temperature, the doping concentration, and the partial pressure of the oxidizing gas. Their purpose is to act as an insulator between the substrate and conductors on top of them. The oxidation process naturally consumes a certain amount of silicon from the substrate.

Hence for processes where a SiO$_2$ layer has to cover anything but silicon a different process must be used. An alternative to oxidation is the growth of SiO$_2$ layers by CVD. Its advantage is that it can be performed at lower temperatures, which reduces undesirable diffusion of previously introduced doping. On the other hand SiO$_2$ layers generated by CVD are generally less uniform than those generated by oxidation.