Monitor signal processing of optic thin film based on generalized Kalman filters

In optical monitor system(OMS) for thin film deposition,the film thickness is insensitive to the monitor signal when the signal is near the peak.The accuracy of the monitor system is limited by the noise of the signal.A generalized Kalman filter(KF) was applied to process the monitor signal.Based on nonlinearity of OMS,mathematical model and arithmetic of extend KF was constituted.An ideal monitor curve of 4-layer AR film was generated by TFCalc and mixed with a Gaussian noise.As a result,the signal-to-noise improvement of monitor signal after KF reached 16dB with excellent dynamic tracking characteristic and the delay time was less than 1s.By comparing the theoretic peak coordinates and zero coordinates of first derivative after KF,the calculated thickness error was less than 4nm.The study improves the accuracy of the film thickness detection.