Design Of Coaxial Extreme Ultraviolet Lithographic Projection Objective

Extreme ultraviolet lithography (EUVL) is a very promising technology for theintegrated circuit (IC) manufacturing industry. Exposure tools are the core components ofthe EUVL system. Design of projection objective and the illumination system is one of themost important issues and the technical forerunner of the EUVL, which plays an importantrule for the development of the IC manufacturing equipment in our country. This thesismainly keeps focus on the design method of EUVL objective initial structure, optimizationmethod of the aspheric structure, and the modeling method for illumination system and itsoptimization. With the design methods of this thesis, the optimized reaches the diffractionlimit of resolution and good image quality.A grouping design method for the initial structure of EUVL projection objective andits interactive design program are presented in this thesis. Aimed at the shortcuts of theslow calculation speed, the incompleteness of the search for structures, the low degree ofautomation, and the ill-conditioned man-machine interaction, the grouping design methodfor all spherical coaxial reflective objective and corresponding interactive design programare introduced. By separating the whole objective into several mirror groups and designingthem individually with their corresponding light path specifications, the reflective elementstructure, and optical characteristics, the non obstruction or obstruction controlled initialdesign is obtained after connecting the mirror groups with the image-object and pupil-stopmatching rules. This method united the artificially design and the interactive searchprogram, and easy to realize and with lower time cost; the process of calculation and searchare visible and controlled for the designer. Many potential designs are obtained in a shorttime, and the analysis for the designer’s needs is available.With the initial spherical structure design method and interactive design program, weget several of results of projection objective design with six mirrors, eight mirrors, or tenmirrors, and corresponding numerical apertures(NA) are between0.3~0.7. The groupingdesign method is applied in the projection objective design in details: the combinedavailable range of parameters of mirror groups are defined, proper mirror groups are pickedup from the combined ranges and connected as whole initial designs which match the needs of optical characteristics of EUVL objective. Meanwhile, an adjustment program withstepper optimization on the first order parameters is introduced to make the initial structuremuch more compact, balanced and reasonable.The optimization method for aspheric surface started from the all spherical initialstructures is introduced, which is a gradually optimization method based the damped leastsquare (DLS) method. The gradual optimization method suggests optimize the objectivefrom small numerical aperture (NA) and lower aspheric coefficients, and then move onwith small increments of the NA with higher aspheric coefficients step by step, while theclassified constraints are imposed due to the optimization situation. The optimizationmethod is convenient to operate with rapid convergence, and easy to learn and control forthe designer. The gradual optimization method was applied on the initial structures whichare obtained by the grouping design method. Some balanced initial structure was optimizedwith the aspheric coefficients, and the image qualities of optimized system were analyzed.The resolution of the objective within the static exposure fields achieves the diffractionlimit with good image quality; the distortion and the critical dimension uniformity are wellcontrolled, and the other imaging specification satisfies the needs of industrial EUVL.A reverse design method for EUVL illumination system is presented, which suggeststart modeling the illumination system from the objective side. Firstly, the parameters of therelay mirrors and fly’s eyes are decided to match the dimension and the emergent angle ofgiven plasma source, and then the para-position of fly’s eyes is settled down to get highillumination quality system. The para-position method of fly’s eye can make furtheroptimization of the illumination uniformity to guarantee the utility for the EUVL system.The reverse design methods is very helpful to provide the matching for the any givenprojection objective with high energy utilization and in good speediness of modeling anddesign.