Optical-structural-thermal Analysis And Cooling Optimization Of Extreme Ultraviolet Lithography Collector

The collector for extreme ultraviolet(EUV) source is one of the most important parts in EUV lithography system. The mirror will be subjected to serious deformation resulting from the high thermal load absorbed by the collector when the EUV source power is significantly enhanced, which causes larger focal shift and spot size, further makes the energy distribution of the bright spot formed by convergent rays at intermediate focus nonuniform badly. all above disadvantages bring many difficulties in design of the next systems and,eventually, degrade the quality and throughout of lithography process.In order to minimize the thermal distortion of EUV lithography source collector caused by nonuniform temperature in mirror body, thus improve the illumination quality of focus spot, in this paper, with the Wolter type collector as studying object, the arrangement optimization of cooling ducts on mirror backside is presented. The radiation model between radioactive sources an mirror are analyzed, the nonuniform heat flux distribution on mirror surface are calculated in detail, and the overall arrangement scheme of cooling ducts is designed; then a calculation model of heat conduction in mirror is established, an empirical model of thermal convection of cooling water is adopted, both of which are written into the optimizer through the law of energy conversation, eventually complete the calculation of ducts’ optimal axial spaces with arc-length spaces as loop variable and mirror’s temperature difference as optimization goal. The results show that, using discussed optimization method, the temperature difference of outer collector mirror can be controlled within 1 ℃, which is under40 kw source power and with only four single cooling ducts on mirror backside.The analysis process and optimization method in this paper can provide a certain reference to active thermal control of other reflective optical mirrors with nonuniform heat flux on mirror surface.This paper adopts Zernike annular polynomials to fit mirror deformation data, which is solved by ANSYS finite element analysis program. Firstly getting the coefficients of every polynomial, then a user-defined INT file that adds the residual deformation on original mirror surface is written and imported into the CODE V software, eventually evaluate the influence of mirror deformation on optical spot quality at intermediate focus. This method makes the integrated optomechanical analysis possible.