Unusually there was only in-line monitor to check the post maintenance quality in semiconductor fab, especial in frond-end process. As these are 1∼ 2 months time form frond-end tool to WAT/CP test. If the post maintenance monitor is OK, but something wrong at WAT/CP test, there will be huge loss. There are many methods to mention the post maintenance quality control by FDC metrology. But most of them are too complicated to real time control for Fab operation. We want to introduce a simple method to real-time monitor/control post maintenance quality and can predict key WAT/CP parameter trend.