ONLINE DIAGNOSTIC MONITORING OF PHOTORESIST ASHING

On-line statistical process control (SPC) has been implemented on a si ngle-wafer remote microwave plasma photoresist asher. SPC for ashing i s made more difficult because the prior processes, e.g., ion implantat ion, affect the properties of the resist material, and consequently th e ashing behavior. The system presented comprehends the variety of inc oming wafer states from a complex process how. On-line SPC charts trac k photoresist clear time on a wafer-to-wafer basis using optical emiss ion spectroscopy. The data is corrected for the ''first-wafer'' effect , whereby the clear time for a wafer decreases as the delay time betwe en 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 devia tion Shewhart charts. Standard SPC rules are applied automatically wit hin each process how 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 ashi ng process. When a shift in incoming wafer state is detected, the expe cted response for that process level is automatically updated to refle ct the change. The usefulness of on-line monitoring as a means for ide ntifying misprocessing at prior process steps has been demonstrated. E arly diagnosis can save money by avoiding expensive downstream process ing on previously misprocessed wafers. In our demonstration laboratory , the equipment has processed wafers from a dozen process flow levels.