Diffusion Systems

The diffusion process is carried out in systems called diffusion furnaces which may be either sealed or open tube type. The silicon wafer and the dopants are placed into the quartz tube before sealing and subsequently after the high temperature process; the tube is broken to remove the diffused wafers. Thus the sealed tubes are for one time use and can be maintained contamination free. The open tube method is preferred because multiple diffusion runs can be carried out. Separate tubes for each type of dopants are maintained because tubes are contaminated with the respective impurity. The insertion of wafers and solid dopant sources are done from one end of the tube and the other end is used for the flow of gases or impurities in vapour form. The diffusion furnaces are usually operated at temperatures 600C-1200°C and constant temperature zones are maintained with a ± 0.5°C tolerance.

Three main sources of dopants such as gaseous, liquid and solids source of dopant atoms are used in semiconductor doping processes. The gaseous sources are most widely used since they are reliable and convenient. The typical examples are BF₃ (diborane), PH₃ (phosphine) and AsH₃ . Source gas reacts with oxygen at the wafer surface to form a dopant oxide, the dopant then diffuses from the oxide into silicon resulting in a uniform dopant concentration across the surface.