Method to measurement of the thin film thickness based on digital Moiré technique

Abstract

Optical interferometry is the most accurate known method to measurement of the thin film thickness. With phase shifting interferometry (PSI) can achieve fast automatic measurement of the thin film thickness, however, this method requires high-precision phase shifter, such as PZT phase shifter, to the introduction of phase shift. This paper will use Digital Moire technique to obtain the information of the thin film thickness by processing and analysis a static interferogram. The static interferogram with a certain spatial frequency introduced by the interferometer, then a virtual sinusoidal grating with this spatial frequency is generated in the computer. By changing the initial phase of the sinusoidal grating, π/2 phase shift between adjacent sinusoidal gratings will be made accurately. 4 sinusoidal gratings are obtained in a grating period, then overlapped these 4 sinusoidal gratings on the interferogram to get 4 moire patterns. Processing and analysis the moire patterns then the thin film thickness can be obtained by 4-bucket algorithm. In this method, the static interferogram is analyzed with PSI algorithm but there is no need to have phase shifter. The phase shift is introduced mathematically to avoid the associated error.