ABSTRACT
There are two types of impurities in silicon wafers. Any nonsilicon atoms are considered impurities. Some of them are added intentionally, whereas others are present despite the best efforts to drive them away from the silicon lattice. As was discussed earlier, the silicon lattice is not very dense, and its lling factor is much less when compared to other close-packed structures, so there is a lot of scope to occupy the interstitial positions in the lattice. This accommodative property of silicon traps many metallic ions, which are relatively small, and it is difcult to drive them from these locations. These ions also get accommodated if vacancy sites are available in the regular silicon lattice sites participating in chemical bonding. The presence of these nonsilicon atoms disturbs the electrical properties of the silicon to a great extent and, ultimately, may affect many electrical and reliability issues of devices fabricated using these wafers. The atoms/ions present in the interstitial positions move freely when the silicon wafer undergoes high-temperature thermal cycles, and this movement agglomerates many moving species, leading to precipitations. This may, in turn, lead to unwanted electrical paths and may contribute to device leakage paths.
