Area/Planar Defects

Two area defects are:

· Twins

Twinning represents a change in the crystal orientation across a twin plane, such that certain symmetry (like a mirror image) exists across the plane.

· Grain boundaries

A grain boundary represents a transition between crystals having no particular orientation relationship to one another, that is the crystal structure on either side of a grain boundary is different. [2]

Area defects represent a large area discontinuity in the lattice. The defects appear during crystal growth, but crystals having such defects are not considered usable for IC manufacture and are discarded.

Volume defects

· Precipitates of impurity or dopant atoms constitute volume defects. Most impurities have a retrograde solubility, which is defined as a solubility that decreases with decreasing temperature.

· Thus if an impurity is introduced (at a temperature T1) at the maximum concentration allowed by its solubility and the crystal is then cooled to a lower temperature T2, a supersaturated condition is said to exist. The crystal achieves an equilibrium state by precipitating the impurity atoms in excess of the solubility level as a second phase, that is, as a material of different composition and structure.

· Precipitates are generally undesirable because they act as sites for dislocation generation. [2]