On-line diagnostic monitoring of photoresist ashing

On-line statistical process control (SPC) has been implemented on a single-wafer remote microwave plasma photoresist asher. SPC for ashing is made more difficult because the prior processes, e.g., ion implantation, affect the properties of the resist material, and consequently the ashing behavior. The system presented comprehends the variety of incoming wafer states from a complex process flow. On-line SPC charts track photoresist clear time on a wafer-to-wafer basis using optical emission spectroscopy. The data is corrected for the “first-wafer” effect, whereby the clear time for a wafer decreases as the delay time between ashing wafers increases. The data is standardized using an expected time and variance for each process flow level to allow all results to be presented in a single set of individuals and moving-standard deviation Shewhart charts. Standard SPC rules are applied automatically within each process flow level to test for unnatural variation in the data. Observed abnormal behavior is due mainly to changes in the incoming material for a specific process flow level, not deviations in the ashing process. When a shift in incoming wafer state is detected, the expected response for that process level is automatically updated to reflect the change. The usefulness of on-line monitoring as a means for identifying misprocessing at prior process steps has been demonstrated, Early diagnosis can save money by avoiding expensive downstream processing on previously misprocessed wafers, In our demonstration laboratory, the equipment has processed wafers from a dozen process flow levels