Monday, October 16, 2006

Diffusion Process in Semiconductor Manufacturing

Diffusion of impurity atoms in silicon during processing is important for the electrical characteristics of silicon devices. Various ways of introducing dopants into silicon by diffusion are used and have been studied with the goals of controlling dopant distribution, total dopant concentration, uniformity, and reproducibility.
Diffusion is used to form base, emitter, and collector regions in bipolar device processing, to form source, drain and channel regions, and to dope polysilicon in MOS processing. Dopant atoms that span a wide range of concentrations can be introduced into silicon in many ways. The most commonly used methods are
Diffusion from a chemical source in a vapor form at high temperatures,
Diffusion from a doped-oxide source, and
Diffusion and annealing from an ion-implanted layer (see ion implantation).
The electrical activation of ion-implanted species is carried out by annealing. This causes a redistribution of the impurity atoms which should be as low as possible. In order to optimize the electrical behavior of the device, it is important to know how the impurities redistribute during the anneal. The development of appropriate models and simulation programs to predict the diffusion is one major topic in semiconductor technology research.


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