Defects kill yield and drive up manufacturing cost, so defect inspection is vital in the Fab.

Defects on the masks or wafers can cause electrical short circuits between aluminum lines that are not supposed to be connected together. They can also cause open circuits or breaks in aluminum traces. Either of these is fatal to the functionality of the chip. With millions of transistors per chip, one can see how defect control is critical. Defects originate either from the atmosphere in the Fab (hence the ultra-clean room requirement), or from the materials used, or from the tools that are used. For a given defect density in a Fab/process, the smaller the die, the larger the population of die per wafer, and the lower the statistical impact of the defects. Also, the smaller the defects are in size, the less is the chance that they will cause fatal problems.

It is not only important to get yields up as high as possible, but it is also important to get them up to their attainable level quickly. Time is money, so a fast ramp-up of yield will get a Fab to profitability sooner.

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