Two methods to loosen optical manufacture tolerances for ArF projection optics

Small field ArF projection optics for experiment is favorable to the prospective study and key technique development for lithography equipment. An ArF projection optics has been designed with specifications of 0.75 numerical aperture (NA), 100μm image field of view and 40X reduction ratio. The designed composite root-mean-square (RMS) of wave front error of the system can reach 0.03λ (λ=193.29nm). To achieve diffraction limited resolution, the optical manufacture tolerances of this high-NA projection optics are too tight to manufacture, such as radius error within ±0.01%, central thickness within ±2μm and surface figure error less than peak-valley (P-V) of 1/30λ (λ=632.8nm). In this paper, two effective compensatory methods are presented to relax the manufacture tolerances before assembly. One method is re-computation of air spaces including actual measured values of the radii of curvature and center thickness for each element. The second method is rotating lenses to find the best matching manner of measured surface figure. The results show that radius error, central thickness error and surface figure error can be compensated significantly, and the best performance of the projection optics disregarding assembly error has been acquired. The composite wave front error RMS can reach 0.09λ (λ=193.29 nm) after compensation, which is much smaller than 0.25λ before compensation under the same optical manufacture error. Both methods can also be used in development of industrial lithographic projection optics.