Optical Path Design For Four-wavelength Surface Topography Interference Measurement System

In recent years, interference microscopy which integrates optical metrology techniques and information processing technology, for the advantages of fast measurement, high precision, non-contact, has been widely used in the surface topography measurement of MEMS devices. This paper thoroughly studies the principle of interference microscope method and the structure of the interference microscope, and combines the advantages of phase-shift interferometry and multiple wavelengths, then proposes a four-wavelength interference measurement system for surface topography. The main research contents of the paper include the following aspects:Firstly, Michelson, Mirau, Linnik based on the principle of interference microscope have been analysed and compared. The present situation of white light phase shift interferometry, wavelength scanning interferometry and dual-wavelength or multiple wavelength interferometry which are using the three microscopic interference structure has been studied.Secondly,based on the principle of phase shift interferometry,dual-wavelength,three-wavelength and four-wavelength interferometry principles are stated and compared. Then the principle of four-wavelength surface morphology interference measurement system is designed.Thirdly,the optical path of the measuring system is designed by Zemax optical design software,including collimation beam system,Linnik interfering optical path. The theoretical analysis of the designed system has been conducted by spot diagram and wavefront map.Fourthly, according to the principle of mechanical design, the mechanical structure of the measuring system is designed by Solidworks and Auto CAD,including the overall architecture, component arrangementFifth,image data processing algorithm of four-wavelength interference is deduced, which based on the basic principle of phase shift interferometry. The algorithm divides the image data processing into five steps:(1) Average gray value sequences of four wavelength interference images are obtained respectively.(2) Initial phases of each pixel are calculated by four-step phase shift algorithm after processing the average gray value sequences based on ellipse fitting method.(3)The calculated initial phases are dealed with zero processing by defining the center of the interference field as a reference point, and the phase differences between two near wavelengths and two far wavelengths is obtained.(4)The formula of absolute height is derived through the interference theory, equivalent wavelength and data correction thought.(5)The corresponding single wavelength interference levels and absolute phases are deduced based on the formula of equivalent wavelength absolute height. And three-dimensional topography of the measured surface is restored through the corrected phase data of single wavelength.Sixth,the measuring system is verified by testing the standard glass specimens which are calibrated by China Metrology Institute. To further verify the designed measurement system, the experimental results are compared with the calibrated data and the measurement data of VR-3200 laser confocal microscopy.