Ultra-long focal depth annular lithography for fabricating micro ring-shaped metasurface unit cells on highly curved substrates

The research team proposed a method called aberration-converged annular lithography (ACAL) system, as schematic diagram shown in Figure 1. The Gaussian beam is converted into a collimated annular beam after passing through two axicons. The annular beam then incident the convex lens group. The aberration in the second convex lens is essential for reducing the annular beam width in the radial direction. By adjusting the spacing between the axicons and the convex lenses, the diameter of the annular spot can be modified. The team constructed an experimental setup and conducted corresponding performance tests‌.

 

The captured spot of the ACAL lithography system at the image plane is shown in Figure 2a, where both the diameter and structure of the annular spot are adjustable to meet diverse application requirements. Figure 2b demonstrates that by switching the convex lens groups, the ACAL system can modify the adjustable range of the annular diameter. Compared with recent typical annular lithography experimental results, the minimum annular structure width (feature size) has been improved by more than 10 times, achieving an average line width of 1.79 µm for the smallest annular structures. Additionally, the adjustable range ratio of the annular diameter far exceeds comparative results.

 

Unlike conventional direct laser writing lithography (DLWL), the ACAL system exhibits exceptional robustness against defocus. Figure 3 illustrates the experiments and results of the equivalent focal depth range experiment, confirming that the ACAL method can produce micro-annular structures with comparable line widths and edge quality even when the defocus distance reaches approximately 10 times that of DLWL.

 

The ACAL method also demonstrated excellent performance on curved substrates coated with photoresist. Using the substrate shown in Figure 4a for validation, only lateral movement of the curved substrate was performed during exposure without adjusting the vertical ACAL optical path. The exposure results in Figure 4b show that despite Ring 1 having a significant substrate-tilt angle and large defocus, its outline remains clearly defined. Ring 3 near the vertex of the curved substrate (Figure 4c) exhibited the best exposure quality, with line width and edge quality comparable to planar exposure results. These findings confirm that due to the ACAL method's advantage of extended depth of focus, defocus and positional errors have minimal impact on actual exposure outcomes.

 

This study proposes a highly feasible lithography method (ACAL) for manufacturing micro-annular structures, addressing key challenges in curved substrate lithography, such as difficult positioning adjustments, spatial conflicts, and low manufacturing efficiency. This method leverages the aberration characteristics of spherical lenses to achieve advantages like extended depth of focus, long working distance, and single-exposure generation of complete rings, effectively overcoming obstacles of curved substrate lithography. The simplicity and efficiency of this method make it particularly suitable for the lithographic fabrication of curved metasurface unit cells.  Furthermore, as a ring-beam convergence technique, it can be integrated with other methods such as two-photon polymerization lithography, additive manufacturing, laser drilling, and laser etching. We believe this creative technology is promising for applications in the future.

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