Study Of Partially Coherent Illumination In The Maskless Lithography System

Maskless lithography is one of the key manufacturing equipment for electronic instruments and optical devices.This technology is flexible and effective.This technology can be applied to printed circuit board manufacturing,diffractive optical element manufacturing,detector manufacturing,thin film resist manufacturing,biological scaffold manufacturing and so on.The gaps between two micromirrors are one of the causes of imaging defects of products produced by the maskless lithography.This thesis proposes a method to suppress the generation of such kind of defect by manipulating the light source.The thesis takes the step-and-scanning imaging method from maskless lithography as a sample to explain the influence of partially coherent factor on the imaging crack introduced by the gap between micromirrors in the digital micromirror device.Simulations verify that the partially coherent illumination can restrain such kind of imaging crack.The thesis also predicts the proposed method can be applied to three-dimensional helical-structure device manufacturing.Chapter one of this thesis introduces the classification and the manufacturing processes of lithography.Chapter two of this thesis introduces four imaging methods of maskless lithography,including the point-array-scanning imaging method,the step-and-scanning imaging method,the three-dimensional structure imaging method and the virtual-grid imaging method.Chapter three of this thesis systematically introduces four theoretical expressions of partially coherent imaging.The thesis analyzes the advantage of Hopkins’ and Abbe’s method,two theory branches of partially coherent imaging,resulting that the direct discrete expression of Hopkins’ method and the TCC(transmission cross-coefficient matrix)expression of Abbe’s method are suitable for the situation taking the image as the variable and that the model expansion expression of Hopkins’ method and the ICC(illumination cross-coefficient matrix)expression of Abbe’s method are suitable for the situation taking the light source as the variable.Normalized line width is introduced to describe the minimum line width of the target image.The thesis derives the expression,which takes the partially coherent factor as an independent variable and takes the target image shape,minimum line width of the target pattern,numerical aperture of projecting lens and the weight on the different spatial frequencies of the light source as parameter variables.Simulations in chapter four of this thesis shows that imaging crack will be suppressed when the value of sigma is at a particular value,which depends on the normalized line width,while the image remains good imaging quality.This specific value of sigma is called as critical sigma.Afterward,this paper extracts values of critical sigma for several normalized line widths.In the end,this chapter explains why the suppression is independent of the shape of the target image and the rationality of suppressing the imaging crack by adjusting numerical aperture,which was put forward by the former researchers,and compares the suppressing method by adjusting numerical aperture and by utilizing partially coherent imaging.The thesis concludes that the suppressing method by utilizing partially coherent image has one more degree of freedom.