Design And Fabrication Of High Accuracy Crossed Planar Phase Grating For Metrology

Modern measurement systems,whether they are coordinate measuring machines or precision CNC machining centers,require accurate measurement and positioning of the crossed direction of the system worktable.For most applied crossed planar micro-displacementmeasurement systems,comparedwith theone-dimensional measurement device installed in crossed directions,crossed gratings used for measurement not only can measure the crossed plane micro-displacement at the same time,but also has the advantages of high precision,compact structure,less abbe error and so on.In the crossed grating measurement system,the crossed grating is the core optical element of the entire measurement system,so designing and manufacturing a high-precision crossed planar grating is of great significance.In this paper,combining the application of crossed gratings and related diffraction theory of gratings,the crossed gratings are analyzed and studied.The main work contents are as follows:(1)The working principle of the crossed grating in the displacement measurement process is introduced,and the correspondence between the output signal of the measurement system and the technical parameters characterizing the grating has been established.(2)Analyze the diffraction process of crossed gratings,establish a crossed grating incidence model.The design of crossed planar reflecting phase grating used under the Littrow condition with circularly polarized light at 780 nm wavelength has been carried out.The aim of the design is to find out the range of structure parameters of crossed-grating that has higher-1~stt order diffraction efficiency and good efficiency equilibrium for both of TE-and TM-polarized incident lights.By adoption of the Fourier modal method(FMM),the microstructure parameters of the 1200line/mm crossed grating with the duty cycle range of 10%to 50%and the profile depth of 150 nm to 350 nm have been searched exactly.The calculation results show that:When the duty cycle range of gold-plated the grating is42%to 44%and profile depth is 210 nm to 220 nm,the-1~stt diffraction efficiencies of TE-and TM-polarized lights are both above 60%and the efficiency equilibrium is better than80%.(3)The study on the fabrication of crossed gold-plated gratings with holographic method has been carried out.The crossed gold-plated gratings under different technological conditions are made by holographic interferometry,plasma etching and vacuum coating technology.The diffraction efficiency is tested,the measured values of diffraction efficiency are compared with the theoretical values,and the causes of the error are analyzed.Combined with the actual AFM measurement results of the crossed grating micro structure,the model of the crossed grating is improved.The design of the crossed grating has been optimized on the basis of the terraces model which is more consistent with the actual production process,and the process range that satisfies the design index under the terraces model is given.