Components of variance for photo StS & WiS analysis

By baseline define & variance test of process variance that can show to engineer whether the process is stable or not. Furthermore the root cause can be found more quickly. Photo can't find out the issue point directly and monitor shot performance efficiently by "LtL, WtW, WiW (Lot to Lot,Wafer to Wafer & Within Wafer)" analysis. Base on WiW to develop "StS WiS (Shot to Shot Within Shot)" analysis, it's useful at checking shot health.