2.2 Oxidation

Formation of silicon dioxide layer is one of the most frequently used steps in integrated circuits manufacture. Silicon dioxide is an electrical insulator and is usually created in situ by thermal oxidation. The SiO2 layer is used as a mask during dopant diffusion, as a junction passivation, as an insulating field oxide and as a gate dielectric.

Dry oxidation is performed by exposing silicon to oxygen at a bout 1000 C. Oxidation rate is more rapid if moisture is mixed in with oxygen. This process is called wet oxidation. Dry oxidation produces high quality thin oxide layers of 0.05 µm while wet oxidation produces thick oxide layers of 0.5 µm.

 



Fig 6. A stack of wafers being loaded into furnace for oxidation.

 

Oxidation takes place at silicon-silicon dioxide interface. Oxygen must diffuse through SiO2 layer to this interface to react. Because SiO2 is a barrier to oxygen diffusion, growth rate of SiO2 decreases with time. For initial processing steps a thickness of 500 A or 0.05 µm is adequate and is usually grown within 30 minutes at 900 C in oxygen atmosphere.