1. A method for in-line monitoring process performance during wafer fabrication using a fabrication tool, comprising the steps of: (a) collecting first signals generated by the fabrication tool when a model wafer is processed; (b) filtering the first signals to exclude abnormal signals; (c) regulating and normalizing the filtered first signals to generate model wafer data; (d) measuring the model wafer to generate a measured value representing the process quality thereof; (e) building a correlation model of the model wafer data and the measured value of the model wafer; (f) collecting second signals generated by the fabrication tool when a run wafer is processed; (g) filtering the second signals to exclude abnormal signals; (h) regulating and normalizing the filtered second signals to generate run wafer data; and (i) generating a predicted value representing the process quality of the run wafer by inputting the run wafer data into the correlation model.