Anamorphic objective design for extreme ultraviolet lithography at the 5∼1 nm technology node

Extreme ultraviolet lithography (EUVL) has been applied in integrated circuit manufacture at the 9-7 nm technology node, in which the numerical aperture (NA) of the objective is 0.33, and the reduction of the objective is 4 along both the x and y direction. The high NA (≥ 0.55) objective with anamorphic magnification of 8 and 4 reduction ratios in the scanning direction along the y axis (My8) and x axis (Mx4), respectively, was proposed at 5 ∼ 1 nm technology node. In optical theory, a high NA objective corresponds to high resolution of a lithography image. However, high NA objective, large exposure field of view (26 mm × 1.5 ∼ 2 mm), and strict lithography performance are severely restricted or conflict with each other, which makes the optical design too difficult to realize. A new, to the best of our knowledge, design of an anamorphic objective with a free-form surface is implemented in this paper. The design method with manufacturability potential and a high optimization degree of freedom is established. By this design method, the number of terms of x− y free-form surface shape coefficients can be controlled well, the aberration compensation can be improved significantly, and the optimization degree of freedom and manufacturability is optimized. The objective lens design meets the requirements of imaging performance for NA 0.55 EUV lithography.