Research On A New Type Of Optical Thin-film Thickness Monitoring Based On Laser Technology

Spectral properties of the optical thin-film is closely related to the optical thickness of each layer in the thin-film system. In order to make spectral properties meet the performance index, optical thickness of each layer must be real-time monitored. With the rapid development of science and technology, higher requirements for the performance index of optical instruments puts forward. The performance index of optical thin-film is also increase. The requirements on optical thin-film thickness monitoring accuracy for each layer is getting higher and higher.Quartz crystal method is a major approach to monitor the non-normalized coating system, by which the quality of thin-film was monitored, show very high levels of monitoring accuracy of each layer. Compared to the quartz crystal method, the optical monitoring of optical thin-film thickness has automatic compensation function,which demand lower monitoring accuracy. The main technical difficulties are the low light intensity on a single wavelength, the stray light in the vacuum coating machine, various circuit noise cancelling, and the non-normalized coating system monitoring.The research carried out as follows: First of all, a new algorithm for optical thin-film thickness monitoring is presented. Though the mathe-matical calculation, the relationship between optical thickness and transmittance is linear, and the bad effects of optical source fluctuation and conveying noise is eliminated.At last a algorithm simulation through the software of matlab is given out, curves about optical thin-film thickness monitoring is drawn, and degree of linearity is calculated. Secondly, the optical system of thin-film thickness monitoring instrument is designed, the appropriate optical parts is selected, light path is designed, and the optical system is assembled and adjusted. Thirdly, the major noise cancelling method is discussed such as modulation, demodulation, smoothing filtering and phase-locked amplifier. Through the Simulink, a simulation about the process of film thickness monitoring is given out. Using NI PCI-6281data acquisition card, the film thickness signal is acquired and is sent into the compute.Though the digital signal processing, the non-normalized coating system is real-time monitored. The accuracy of this new methord is about2percent.