Single-exposure holographic lithography of ultra-high aspect-ratio microstructures

TL;DR

This work tackles the throughput-resolution bottleneck in 3D microfabrication by introducing single-exposure volumetric lithography guided by inverse-designed holographic phase masks. The method reconstructs a prescribed 3D dose distribution inside a photoresist (SU-8), achieving extended depth of field while preserving ~4 μm lateral resolution, enabling ultrahigh aspect-ratio structures (exceeding 120:1) in volumes up to 800×800×720 μm^3 within ~20 s. It demonstrates versatile 3D geometries (including hexagonal close-packed lattices and Penrose tilings), tiling and overlapping exposures for quasi-3D complexity, and confirms functional performance via capillary flow and in situ nanoindentation (Young’s modulus ~5.7 GPa). The approach provides a scalable, reconfigurable route to high-fidelity volumetric fabrication compatible with existing photolithographic infrastructure, with broad implications for architected materials, microfluidics, MEMS, and micro-optical systems.

Abstract

Volumetric lithography offers a path to scalable fabrication of complex three-dimensional (3D) micro- and nanoscale architectures, yet existing approaches are limited by quasi-two-dimensional exposure physics or slow serial writing. We present a single-exposure volumetric fabrication strategy that enables creation of ultrahigh-aspect-ratio 3D structures with 6 um minimum features. An inverse-designed volumetric (holographic) phase mask generates an extended-depth-of-field intensity distribution inside a photoresist volume while preserving high transverse resolution, enabling uniform polymerization of the full volume in a single exposure. With exposure times of approximately 20 s, we fabricate lattices, Penrose tilings, and micromechanical elements with feature sizes down to 6 um over volumes up to 800 x 800 x 720 um^3, achieving aspect ratios exceeding 120:1. Quantitative analysis of capillary flow in hollow lattices demonstrates controlled fluid transport with an effective capillary transport coefficient of 176.3 um/(ms)^(1/2). In situ nanoindentation-based micro-compression reveals that the printed 3D hexagonal close-packed lattices exhibit a well-defined linear elastic regime with an effective Young's modulus of 5.7 GPa, followed by progressive buckling and densification characteristic of mechanically robust cellular architectures. Overlapping, tilted and multi-mask exposures further enable quasi-3D complex geometries with potential for reconfigurability. This approach establishes a new regime of high-throughput volumetric fabrication.

Figures (6)

• Figure 1: Single-exposure fabrication of $>$120:1 aspect-ratio microstructures. (a) A conventional binary photomask produces only low-aspect-ratio features because diffraction rapidly degrades optical contrast during propagation. (b) An inverse-designed phase mask suppresses diffraction menon2023perspectives, enabling the projection of high-aspect-ratio geometries deep into a photoresist film, and allowing true single-exposure volumetric fabrication. (c) Simulated optical contrast as a function of propagation distance from the mask shows that the binary mask loses contrast rapidly, whereas the inverse-designed phase mask maintains high and nearly uniform contrast over extended distances, a key requirement for high-aspect-ratio lithography. (d) Scanning electron micrograph of a hexagonal close-packed (HCP) lattice with overall dimensions of 800 × 800 × 720 µ m$^3$. (e) Magnified view of the HCP lattice showing a wall width of $\sim$6 µ m, corresponding to an aspect ratio 120:1. (f) Because the entire structure is formed in a single exposure step with a typical duration of 20 s, the resulting lattice walls are smooth, in contrast to the rough surfaces commonly observed in structures fabricated by conventional two-photon polymerization lithography (see Supplementary Note S8).
• Figure 2: Inverse-design and optical characterization of phase mask. (a) Schematic of the inverse-design workflow used to compute the phase mask that reconstructs a prescribed 3D intensity distribution. (b) The inverse-designed phase mask, shown by its phase profile (bottom), illuminated by a collimated laser beam at $\lambda=$ 405 nm to generate an extended hexagonal close-packed (HCP) lattice intensity pattern. (c) Optical micrograph of a representative region of the fabricated phase mask, confirming faithful realization of the designed phase features. (d) Peak signal-to-noise ratio (PSNR) extracted from the measured images as a function of propagation distance, indicating high image fidelity throughout the target volume. (e) Measured intensity images at different axial distances from the phase mask, corresponding to the mask–resist gap and propagation distances of Z = 20, 20.225 and 20.450 mm, demonstrating uniform image formation over an extended axial range. This range corresponds to effective propagation distances of 20, 20.36, and 20.72 mm in SU-8 photoresist (refractive index, n=1.613 at $\lambda=$ 405 nm). Also see Fig. S2 and Supplementary Video 1.
• Figure 3: Single-exposure fabrication of high-aspect-ratio 3D microstructures. (a) Schematic of the optical setup used for single-exposure volumetric lithography.(b) Photograph of the phase mask under illumination, corresponding to the region indicated by red dashed lines in (a). (c–f) Optical micrographs of developed SU-8 photoresist showing representative 3D geometries fabricated using a single exposure, including a hexagonal close-packed (HCP) lattice, a Cartesian lattice, a Penrose tiling, and the aperiodic “hat” monotile monotile. All structures have nominal dimensions of 800 × 800 × 720 µ m$^3$ and were exposed for 20 s. The target slices are indicated in the top-right inset in each panel. (g,h) Scanning electron micrographs of the HCP and Cartesian lattice structures, respectively.(i) Magnified view of the region highlighted by the red square in (h), resolving individual lattice walls. (j) Scanning electron micrograph of the Penrose tiling structure. All SU-8 structures were coated with a thin Ti/Pt layer to mitigate charging during electron microscopy. Also see Supplementary Note S9.
• Figure 4: Enhancing complexity of printed microstructures via stepped exposures. (a) By stepping the SU-8 wafer relative to the mask and doing multiple exposures, one can tile the HCP lattice to create a much larger HCP lattice. The phase mask is kept fixed in this example. (b) By stepping the SU-8 wafer and also using different masks, one can combine multiple patterns to create a larger structure. Additional images including prints from single exposures are provided in Fig. S7.
• Figure 5: Enhancing complexity of printed microstructures via tilted and overlapping exposures. (a) By tilting the illumination relative to the mask, we create tilted 3D intensity distribution inside the SU-8, thereby creating a tilted HCP lattice. The monitoring optics, including the CMOS sensor and relay optics, are tilted to maintain mask–resist alignment. The middle panel illustrates recorded images as function of distance from the mask illustrating the tilt in the 3D pattern. (b) Complex structures can be created by overlapping two exposures with a relative translation between them as illustrated in the left panel. composite structures. In the scanning-electron micrograph, we illustrate double exposure with a HCP lattice, and the magnified views confirm the increased geometric complexity. Additional images including prints from single exposures are provided in Fig. S8.