Ultraviolet metasurface-enabled flat-top beam shaping with size preservation uniformity and broadband robustness

Ultraviolet lithography based on Gaussian beam sources plays a critical role in integrated circuit fabrication, yet the intrinsic strong central intensity and rapid edge decay of Gaussian beams often lead to blurred feature edges, tapered sidewalls, and pattern distortion during exposure. Converting Gaussian beams into flat-top beams while preserving the original beam size is therefore highly desirable but remains challenging for compact optical systems. In this work, we investigate metasurface-based Gaussian-to-flat-top beam shaping using two independent phase-design strategies: the geometrical mapping technique (GMT) and the iterative Fourier transform algorithm (IFTA). The GMT provides a deterministic reference solution based on energy redistribution, while the IFTA offers a flexible iterative optimization framework for controlling the output intensity distribution. Based on these design approaches, a hafnium dioxide nanopillar metasurface operating at a wavelength of 300 nm is proposed to generate flat-top beams with excellent beam-size preservation. Numerical simulations show that the generated flat-top beam exhibits an intensity uniformity of approximately 0.965 and a beam-shaping efficiency of about 79.5%, while the relative deviation between the flat-top beam width and the incident Gaussian beam width is maintained at only 0.24%. The metasurface also demonstrates stable beam-shaping performance across multiple target planes, over a broadband ultraviolet range from 240 to 360 nm, and under incident angles up to 10°. The proposed metasurface beam-shaping strategy provides a compact and versatile approach for generating uniform flat-top beams with strict beam-size control, offering promising potential for applications in ultraviolet lithography and laser micromachining.