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

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.

💡 Research Summary

The authors introduce a single‑exposure volumetric lithography platform that combines ultra‑high aspect‑ratio capability with micron‑scale lateral resolution and high throughput. By employing an inverse‑designed phase mask, the system generates an extended‑depth‑of‑field (EDOF) intensity distribution inside a thick SU‑8 photoresist layer while preserving a transverse feature size of ~4–6 µm. The phase mask is optimized using a fully differentiable forward model based on the angular spectrum method implemented in TensorFlow; the loss function enforces fidelity across all axial slices of the target 3D intensity pattern. The resulting mask (1000 × 1000 pixels, 2 µm pitch) is fabricated via grayscale direct‑write lithography and verified with confocal profilometry.

Optical characterization shows that the reconstructed intensity maintains >83 % contrast and a peak signal‑to‑noise ratio above 30 dB over a 20 mm propagation distance (equivalent to ~720 µm in SU‑8). This contrasts sharply with conventional binary masks, whose contrast degrades after ~40 µm. The exposure setup uses a 405 nm diode laser, a 20 mm air gap, and a computer‑controlled shutter to deliver exposure times of 12–32 s (typically 20 s). After exposure, a two‑step post‑exposure bake (65 °C 5 min, 110 °C 120 min) ensures uniform cross‑linking throughout the full resist thickness, followed by PGMEA development.

Using this workflow, the authors fabricate a range of 3D geometries—hexagonal close‑packed (HCP) lattices, Cartesian lattices, Penrose tilings, and the aperiodic “hat” monotile—each with a volume of 800 × 800 × 720 µm³ and wall thickness of ~6 µm, corresponding to an aspect ratio exceeding 120:1. Scanning electron microscopy confirms smooth, uniform walls without the stitching artifacts typical of voxel‑by‑voxel two‑photon polymerization.

Mechanical performance is evaluated by in‑situ nanoindentation compression of the HCP lattice. The structure exhibits a well‑defined linear elastic regime with an effective Young’s modulus of 5.7 GPa, followed by progressive buckling and densification, characteristic of robust cellular architectures. Fluid transport is probed in hollow lattices, yielding an effective capillary transport coefficient of 176.3 µm·ms⁻¹⁄², demonstrating controlled capillary flow suitable for microfluidic applications.

Scalability is demonstrated by mounting both the phase mask and the SU‑8 substrate on motorized translation stages. Keeping the mask fixed while translating the substrate enables rapid tiling: three consecutive 20 s exposures produce a 2400 × 800 × 720 µm³ tiled lattice in 68 s, a three‑fold increase in fabricated volume. Multi‑mask and tilted exposures further allow quasi‑3D composite structures, expanding design freedom beyond a single volumetric field.

In summary, the work delivers three key innovations: (1) inverse‑designed phase masks that suppress diffraction and enforce global 3D intensity fidelity, (2) an EDOF optical field that delivers uniform dose over hundreds of microns while retaining micron‑scale lateral resolution, and (3) a rapid (~20 s) single‑exposure process that bridges the gap between high‑resolution serial techniques (e.g., 2PP) and high‑throughput volumetric manufacturing. This approach achieves feature sizes down to 6 µm—far below the 25–100 µm limit of current volumetric additive manufacturing—and aspect ratios >120:1, opening new possibilities for micro‑optics, MEMS, microfluidics, and lightweight high‑strength architected materials.