From trend charts to control charts: setup tests for making the leap

Statistical tests are developed to provide an operational definition of the term `under control´ for application in deciding whether trend charts in semiconductor operations should be implemented as control charts, which often require considerable investment in responding to special cause flags. The decision rules derive from Monte Carlo simulations of traditional special cause flags modified for sample size to provide constant and low probabilities of inappropriate process rejection, to help ensure easy tracing of underlying problems and to help ensure large improvements in process capability from investment in fixes. The tests maintain the high efficiency of time series analysis intrinsic to traditional special cause flags. That uncompensated effects of sample size in control chart setup invite wildly uncontrolled risk of rejecting good processes is shown. A table of decision rules is provided to facilitate application of the setup tests at a constant and low risk of rejecting good processes