Research On Stray Light In Diffractive Optical System

Lithography is the core equipment for the production of ultra large scaleintegrated circuit. Illumination system, as one of the most important parts oflithography, includes beam expander system, beam shaping system, uniformillumination system and illumination lens system. It enables to realize off-axisillumination modes with diffractive optical element and supply high uniformityintensity distribution on the mask with fly’s eye condenser.In this paper, the stray light of micro optical elements (diffractive optical elementand fly’s eye condenser) in the ArF lithography illumination system is studied. Due tothe stray light will seriously affect the quality of the final exposure, this research isone of the key technologies to ensure the quality of exposure. The purpose of thispaper is that many effective measures can be adopted to suppress the stray light ofmicro optical elements. The main research work in this paper includes the followingparts:(1) Algorithms for the Design of Diffractive Optical Element for Beam Shaping.Through the investigation and comparison of many design algorithms of thediffractive optical elements, a hybrid algorithm is proposed to design diffractiveoptical elements. This advanced algorithm can reduce stray light of diffractive opticalelements effectively.(2) Influences of design parameters of diffractive optical element on stray light.Scalar diffraction theory is used to analysis the reasons of high order diffraction light.Then, a new parameter Trpis defined. It is proportional to the wavelength andinversely proportional to the far-field max diffraction angle and the size of diffractiveoptical element cells. Through the simulation analysis, the stray light of highdiffractive orders can be suppressed effectively by regulating this new parameter.(3) Influences of fabrication errors of diffractive optical element on stray light. Fabrication methods of diffractive optical elements are introduced and the influencesof fabrication errors are analyzed, including step quantization error, alignment error,random etching depth error, as well as spherical etching depth error. Finally,improvements are suggested.(4) An experiment is used to test the performance of diffractive optical elements,including some tests about the surface of diffractive optical elements and tests aboutbeam shaping. The experimental results are compared with the simulation results.Finally, some methods of suppression diffractive stray light are discussed.(5) The method of ray tracing in ASAP is used to simulate the uniform processwith fly’s eye condenser. Fabrication errors of fly’s eye and influences are analyzed.Finally, Fabrication improvements are suggested.(6) Based on the structure of lithography system, energy efficiency of thelithography system is caculatied and energy of stray light from diffractive opticalelement is analyzed. In addition, influences of stray light of diffractive opticalelements on the performance of lithography are discussed with Prolith software.These researches have advantages of reduction of stray light in lithographysystem and the results provide valuable references for the development of deepultraviolet lithography illumination system.