Measuring Method Of Grating Structure Parameters Based On Rigorous Coupled Wave Theory

As an important optical component,diffraction grating has an irreplaceably important role in many fields.Therefore,the measurement of its structural parameters is of great significance.With the continuous development of grating manufacturing technology and application,the existing methods,such as atomic force microscopy,scanning tunneling microscopy,scanning electron microscopy and transmission electron microscopy,have been unable to take into account the fast,low cost,non-destructive and high precision measurement requirements.In recent years,the optical scatterometry method provides a new solution for taking into account the above measurement needs.In this paper,two important problems need to be solved in the actual analysis,which are the positive problem of grating modeling and inverse problem of structural parameter solving.Concerning the above problems,this paper presents a method to measure the parameters of gratings based on Rigorous Coupled Wave Theory.The key problems,which are grating modeling and parameter solving,is analyzed.Combined with experimental verification,the measurement of grating structure parameters is realized.The main contents of the thesis are as follows:(1)Study on grating modeling based on the Rigorous Coupled Wave Theory.As a vector diffraction theory,the Rigorous Coupled Wave Theory can accurately establish the optical characteristic model of the grating and calculate the diffraction efficiency.Aiming at the one-dimensional grating,it is approximated as a mathematical equivalent model,and the optical characteristic model of the grating is established based on the Rigorous Coupled Wave Theory.Through this model,the accurate solution of the grating diffraction efficiency can be realized.Based on the above model simulation,the influence of different incident conditions and different grating parameters on the diffraction efficiency is analyzed,which provides a theoretical basis for the inversion problem of the grating structure parameters.(2)Study on grating diffraction efficiency inversion of grating structure parameters.The grating diffractive model is very complex.The calculation of diffraction efficiency is not analytic function.Based on the inverse problem analysis method,this paper chooses the Back Propagation Neural Network to construct the inversion model to explore the mapping relationship between the grating diffraction efficiency and the structural parameters.Using the typical three-layer structure,six input variables are determined,which are the diffraction efficiency under different incident conditions,and the two output variables are groove depth and duty cycle respectively.The nonlinear mapping between input and output variables is analyzed.And the optimization process of LM algorithm is used to improve the accuracy as well as the convergence rate.The model of diffraction efficiency inversion of grating structure parameters is established.The correlation analysis of the model proves that the model has strong generalization ability and learning ability.(3)Experimental verification of grating structure parameter measurement.In this paper,a grating diffraction efficiency measurement system is established,and the diffraction efficiency of a variety of gratings is measured.The measured results are within ±3% error of the simulation results.Combining with the grating optical characteristic model and the structural parameter inversion model,the structural parameters of the grating are retrieved by measuring the obtained diffraction efficiency.Compared with the AFM calibration of the grating groove depth of 434 nm and the duty ratio of 0.325,the relative error of the groove depth obtained by the measurement method is 0.23%,the relative error of the duty ratio is 0.92%.The experimental results show that the method of inversion of structural parameters based on grating diffraction efficiency can achieve fast,low cost,non-destructive and high precision measurement of grating structure parameters.