Method To Measurement Of Thin Film Thickness Based On Digital Moiré Technique

With the wide application of thin film technique and optoelectronic devices, optical thin film has been widely used in the basic research of the equipment and the development and production of the special devices. With the continuously development and updated of the weapons, equipment and new optoelectronic devices, higher requirements of the precision measurement of the various parameters of the optical film has been put forward. Film thickness is one of the key parameters of the design and manufacturing process of the thin film, especially the rapid development of the nano-scale thin-film technology makes accurate measurement of film thickness to become a hot issue in the research field. Optical interferometry measurement use wavelength as unit, is a recognized precision measurement and testing technology. With the high accuracy and fast speed, Phase-Shifting Interferometry (PSI) technology occupies an important position in the high-precision optical detection. However, this technology requires high-precision phase shifter and environmental noise during the data acquisition should be effectively eliminated.In this paper, the Digital Moiré technique will be introduced. The thin film parameter are obtained from the static interferogram with a certain spatial carrier frequency introduced by tilting the reference mirror of the reference mirror of the interferometer. The value of the carrier frequency can be obtained by the Fourier analysis. A virtual sinusoidal grating with this spatial frequency can be generated in the computer and overlapped on the interferogram to get a moire pattern. Another moire pattern with π2 phase shift can be obtained by moving the grating 1/4 of the grating period. In the same way, two more moire patterns with π/2 phase shift and generated 4 moire pattern in a period of the grating. Filter out the high frequency fringes from the above four moire pattern by low pass filtering, four interferograms with accurate π/2 phase shift between the adjacent ones are remained. Use the 4-bucket algorithm of the Phase Shift Interferometry (PSI) to retrieve the wavefront. The static interferogram is analyzed with PSI algorithm but there is no need to have phase shifter. Besides, the phase shift is introduced mathematically to avoid the associated error.In this paper, the theory about the Digital Moiré technique is introduced and analyzed, the program for the method is designed, and we compared the thin film thickness measurement by Digital Moiré technique and Zygo interferometer, The results of the relative deviation was less than 1%, which shows that Digital Moiré technique are quite feasible, with high accuracy in thin film thickness measurement.