Research On Lateral Shearing Interference Absolute Measurement Method Of Wavefront Aberration Of Lithography Lens

Lithography technology,as a precise micro-machining technology in the IC manufacturing,determines the development of the semiconductor industry in a country.Photolithography lens,the core component of mask aligner,is used to transfer the pattern from mask to silicon slice,and its imaging quality will directly affect the characteristic line-width of mask aligner.Optical transfer function can be used to evaluate the imaging quality of lens.But a photolithography lens is the minimal aberration system,and the wavefront aberration calibration currently is the only way to evaluate the imaging quality of the lithography lens.Wavefront aberration calibration can guide the assembly of the lithography lens.The famous manufacturers of lithography lens all have their own teams to develop the methods to test the wavefront aberration of the lithography lens.These methods can be summarized as three categories: Hartman-Shark method,point diffraction interferometry,and grating lateral shear interferometry.These three methods measure the wavefront slope or curvature,and there's no need to consider the problem about the limitation of the accuracy of the reference surface.However,for the lithography lens,whose work wavelength is 193 nm,the H-S method can hardly achieve the sub-nanometer accuracy.According to the principle of the point diffraction interferometry,the diameter of the spatial pinhole filter decreases with the decrease of wavelength,so the ideal spherical wavefront cannot be obtained.The theory of the grating lateral shearing interferometry is simple,and it is easy to complete the construction of its equipment in the laboratory.It may achieve the sub-nanometer measurement accuracy after completing the difficult wavefront reconstruction with computer softwares.Therefore,on the basis of the urgent domestic needs,this thesis develops the research of absolute calibration methods to satisfy the wavefront aberration measurement in grating lateral shearing interferometry.The major research contents are as follows:(1)The methods,which apply to grating lateral shearing interferometry,have been researched,and then the 3-step average algorithm and the weighted 3-step average algorithm have been presented on the basis of the relation between the system errors variation and the rotation angles.The situations with constant system error and variable system error are simulated to verify the accuracy of methods.The experimental results show that the accuracy of the RMS of the traditional algorithm can reach about 0.91 nm,and the accuracy can respectively reaches about 0.71 nm and 0.54 nm by the 3-step average algorithm and the weighted 3-step average algorithm.(2)A 2-step method is proposed to restrain the influence of decentration errors.This method ensures the rotation angles with the minimum decentration and chooses the optimal angles to decrease the influence of decentration errors.The situations with constant system error and variable system error are simulated to verify the validity of the 2-step method.The experimental results show that the RMS of the residuals of wavefront aberrations between original position and other three positions(90°,135°,and 180°)by error-immune algorithm are respectively 2.01 nm,4.89 nm,and 7.78 nm,and the RMS of the residual of wavefront aberrations between original position and other two positions(315° and 90°)by the 2-step algorithm are respectively 1.46 nm and 4.16 nm.We estimate the uncertainty matrix of the wavefront aberrations at all angular positions by 2-step algorithm,whose standard deviation is 2 nm.(3)A fitting method based on the decentration in rotation measurement is proposed.This method uses the rotation angles with the minimum decentration to protocol the real wavefront aberration of the lens,and uses least square method to compensate the wavefront aberrations of the lens at all positions.The simulation and the experiment verify the fitting method.We estimate the uncertainty matrix of the wavefront aberrations at all angular positions by the fitting algorithm,whose standard deviation is 0.097 nm.It shows that the fitting method can restrain the influence of decentration more effectively.