We developed a linear tradeoff model by making various assumptions about the capability of flatness measuring tools, the ability of the flatness specifications to predict die failure due to flatness deviations, the characteristic distribution of site flatness values, and the revenue losses associated with wafer flatness testing. Modeling showed that, if the die price exceeds a critical value, it is more cost-effective to 100% inspect and sort, and if the die price is less than this value, it is more cost-effective not to inspect and sort. If the process capability is high and each rejected site causes only one die failure, and if the defective sites are randomly distributed, the critical value is equal to the value of the wafer. Poor process capability, multiple die failures for each defective site, and non-random site failures all decrease the critical value below the wafer value. Both incorrect specification limits and metrology inaccuracies might affect the results obtained by this model, but neither of these factors has yet been fully evaluated.
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