Diagnostic monitoring of photoresist ashing

Wafer-by-wafer statistical quality control (SQC) has been implemented on a single-wafer remote microwave plasma photoresist asher. SQC 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 SQC charts track photoresist clear time on a run-by-run basis using optical emission spectroscopy. The data is corrected for a modeled ´first-wafer´ effect whereby the clear time for a wafer decreases as the delay time between ashing wafer increases. The data is standardized using an expected time and variance for each process flow level to allow all results to be presented in single individuals and moving-range Shewhart charts. Standard SQC rules are applied automatically within each process flow level to test fo 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 is updated to reflect the change. The equipment has processed wafers in a demonstration laboratory from a dozen process flow levels.