5-6. Diffusion & Ion Implantation

4.11 Gettering
o Gettering is often used to improve the quality of the Si wafer as one of the first
steps in the fabrication process.
- Getting is used to remove unwanted impurities, typically heavy metals such as
copper, gold, iron, and nickel, from the surface, by providing locations away from
the active device regions where the undesired impurities can precipitate and be
immobilized.
- These unwanted impurities can reduce both lifetime and mobility in Si.
- The heavy metals tend to be fast diffusers in Si, and have high solubility in heavily
doped n-type Si.
o Oxygen, at levels as high as 10 18 ⁄cm3, is incorporated into Si wafers during the
crystal growth process.
- This oxygen can combine with other undesired impurities to form precipitates,
and this technique is commonly referred to as intrinsic gettering.
- To be effective, the oxygen level must be controlled reasonably well during
crystal growth, and a specific heat treatment cycle must be utilized.
- Defects in the original crystal structure or those introduced by epitaxial growth
can also provide gettering sites.
- An Excellent introduction to gettering can be found in reference [31].
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