Comparison Study to Evaluate the Relationship Between Equipment Uptime Variability Metrics and Cycle Time

This paper evaluates several measures of equipment uptime variability to determine which most closely track with the cycle time in a semiconductor wafer fab. The analysis is focused on a fleet of similar process tools performing the same function in parallel and the resulting cycle time of lots running through those tools. Discrete-event simulation is used to study the relationship between cycle time and several uptime variability metrics, including coefficient of variation (CV), A80, and A20-A80 for different combinations of fleet size (numbers of tools) and levels of uptime variability based on a range of tool mean time to fail (MTTF) and mean time to repair (MTTR) combinations. For the scenarios analyzed, the CV of shift uptime is found to correlate with cycle time as well or better than the other studied equipment uptime variability metrics.