The Development Of Laser Synthetic Wavelength Interferometer For Nanometer Displacement Measurement

Nanometer displacement measurement technology and instruments with large range and high accuracy is a key technology to support the improvement of advanced manufacturing equipments.In recent years,with the rapid development in some key equipment manufacturing technologies,such as in the ultra precision fabrication,micro electronics manufacturing technologies,the machining accuracy of the parts is continually improved while the processing scope is more and more large,the line width of IC is continually narrowing while the diameter of silicon wafer is more and more large.Therefore,an ambitious challenge for displacement measurement capable of millimeter range with nanometer accuracy has been put forward in these high-tech fields,such as the moving detection and calibration of ultra precision CNC machine worktable,the motion controlling and positioning of the scanning table in IC manufacturing,and the calibration of SPM,CMM,laser interferometer,grating and other displacement sensors in the field of precision metrology technology,etc.In this dissertation,a laser synthetic wavelength interferometer for nanometer displacement measurement is proposed and developed based on the synthetic wavelength interferometry fringe subdivision principle.The nanometer displacement of the measured object is achieved by determining a millimeter or micrometer displacement of a reference mirror.And a single wavelength interference signal acts as a marker reference signal.Therefore,the displacement measurement with millimeter range and nanometer accuracy is realized at the same time in the laser synthetic wavelength interferometer,and the measurement result has the traceability to the definition of the meter.From the actual implementation method of displacement measurement in laser synthetic wavelength interferometer,nonlinear error analysis method is proposed and modeled by detecting the simultaneous zero-crossing position errors of the two interference signals.The relationships between the initial position and step displacement of the measurement mirror and the nonlinear error caused by the elliptically polarized laser beams,polarization leakage and misalignment of polarized beam splitter are analyzed and simulated.Moreover,the influences of the air refractive index error and the material expansion error are analyzed theoretically,and corresponding compensation methods are discussed.As a result,the related theory of the laser synthetic wavelength interferometry for displacement measurement with large range and high accuracy is established.After analyzing and comparing different interferometer optical configurations from three aspects of nonlinear error,common path anti-interference and deadpath error,this dissertation proposes a novel laser synthetic wavelength interferometer optical configuration which has the featres of separating wavelengths before forming the interference signals and common path.So it can effectively reduce the nonlinear error,reduce the deadpath error and improve the anti-interference ability of the interferometer.Consequently it ensures the implementation of high precision nanometer displacement measurement.According to the characteristic of the laser synthetic wavelength interference signals,the fractional displacement measurement is obtained by detecting the simultaneous zero-crossing positions of interference signals.And a linear regression fitting algorithm is used to improve the simultaneous zero-crossing detecting accuracy.Furthermore,a novel fringe counting method which corrects the software integral counting number based on the fractional displacement is proposed,thus the correct combination of the integer and fraction fringe counting is solved.And the combining of hardware counting and software counting is adopted to realize the integer fringe counting.It eliminates the integer fringe counting error caused by the stage oscillation when the measured stage starts and stops moving or when the measured displacement is around integral multiple of ?2/2.The optical configuration,mechanical structure,interference signal processing circuits,interference signal detecting and controlling software of the laser synthetic wavelength interferometer have been designed,and the development of the interferometer has been accomplished.The stability experiments,displacement measurement experiments,comparison experiments and application experiments have been carried out to verify the feasibility,effectiveness and practicability of the developed interferometer.Experimental results show that the displacement measurement resolution is 0.02nm.When using the displacement of precision stages as a reference,the displacement measurement accuracies are lnm in the range of micrometer with nanometer step displacement,7.1nm-21.4nm in the range of millimeter with micrometer step displacement and 37nm-51nm in the range of millimeter with millimeter step displacement,respectviely.These measurement results are in accordance with the technical data of stages.When using the displacement measurement results of similar foreign equipments as a reference,in the range of micrometer and millimeter,the standard deviation of the results is about 20nm.In the application of the laser synthetic wavelength interferometry principle,a novel laser wavelength measurement method based on the measurement of synthetic wavelength is proposed.The unknown wavelength is obtained by measuring the value of the synthetic wavelength produced by the unknown and reference wavelengths.Secondly,a novel method for measuring air refractive index fluctuation based on the laser synthetic wavelength interferometry is proposed.The change of air refractive index is regarded as an equivalent measured displacement in the measurement arm,which can be realized by tracking a large compensative displacement of the reference mirror in the reference arm of the laser synthetic wavelength interferometer.Theoretical analysis and experiments verified the effectiveness and feasibility of these methods.Therefore,it provides a new way for the detection or calibration of laser wavelength,measurement of air refractive index with high.accuracy.This dissertation systematically studies the measurement methods of three important parameters of displacement,laser wavelength and refractive index of air in the precision length metrology.