Process-parameter variability in the manufacture of m.o.s. integrated circuits

The integrated circuits produced on a manufacturer´s process line incorporate a standardised m.o.s. transistor providing a set of seven process-parameter values per die at the wafer-probe stage. A statistical analysis of the parameters measured on batches of wafers withdrawn in consecutive weeks from the line shows that the batches received different process treatments. A similar analytical procedure applied to the individual wafers of the batches yields the more surprising result that the wafers were also dissimilarly processed.