5-6. Diffusion & Ion Implantation

o Gas-source systems supply the dopant species directly to the furnace tube in the
gaseous state.
- The common gas sources are extremely toxic, and additional input purging and
trapping systems are required to ensure that all the source gas is removed from
the system before wafer entry or removal.
- In addition, most diffusion processes either do not use all of the source gas or
produce undesirable reaction by-products.
- Therefore, the output of diffusion systems must be processed by burning or by
chemical or water scrubbing before being exhausted into the atmosphere.
o Boron is the only commonly used p-type dopant.
- The diffusion coefficients of aluminum and gallium are quite high in Si dioxide,
and there elements cannot be masked effectively by SiO₂.
- Indium is not used, because it is a relatively deep-level acceptor.
o Antimony, phosphorus, and arsenic can all be masked by Si dioxide and are all
routinely used as n-type dopants in Si processing.
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