COMPLICATIONSOFS.P.C.IN

SEMICONDUCTORPROCESSING

3.

Nestedvariance

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There are at least three levels of variation (lot-lot, wafer-wafer, within-wafer)
Effects on SPC - requirementto control multiple levels of variation; complications in establishing control limits.(Advantage:can provide clues to cause of out-of-control condition; can use to establish response procedures)

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4.

Non-normal distributions

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Several key parameters have non-normal distributions (examples: HFE, contact resistance, sheet resistance)
Effects on SPC - "X-bar" (lot means) may not approach normality (central limit theorem), because lot-lot is often the only level of variation with respect to time

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