Three-dimensional (3D) printing, particularly multi-photon lithography (MPL), offers sub-micrometer resolution for creating complex micro/nanostructures. However, the traditional point-scanning approach of MPL limits throughput, hindering large-scale manufacturing. While methods like increasing laser foci or employing holography have improved speed, they are often limited to periodic structures or struggle with confining polymerization to a single plane in multi-photon processes. This paper addresses this challenge by combining projection multi-photon lithography with spatiotemporal focusing. Spatiotemporal focusing, achieved by controlling the angular dispersion of femtosecond laser pulses, confines the high-intensity focus to a thin plane, preventing undesired polymerization above and below the target layer. Previous attempts used gratings or DMDs with limited laser repetition rates and a layer-by-layer approach with pauses between layers. This research overcomes these limitations by utilizing a 5 kHz laser and a DMD for dynamic patterning and angular dispersion, enabling a continuous, layer-by-layer fabrication process.

The introduction extensively reviews existing literature on accelerating multi-photon lithography. It highlights the limitations of previous approaches such as point-scanning, multi-foci methods (limited to periodic structures), holographic approaches, and early attempts at spatiotemporal focusing with limited repetition rates and layer-by-layer approaches. The review establishes the need for a continuous, high-throughput method capable of creating complex, non-periodic 3D structures with sub-micron features.

The researchers developed a continuous, layer-by-layer projection two-photon lithography system. A digital micromirror device (DMD) dynamically creates 2D patterns at up to 4 kHz, simultaneously providing the necessary angular dispersion for temporal focusing. An 800 nm, 5 kHz repetition rate laser, along with appropriate lenses and a microscope objective, projects and focuses the patterned light onto a liquid photoresist. A numerical model, incorporating the spatiotemporal focusing and imaging processes, was created and validated against experimental optical imaging and printing results. This model was used to estimate the minimum layer thickness achievable for large area 2D patterns. The system’s performance was assessed by fabricating various structures, including single layers of varying exposure times and intensities, thin suspended lines, and a complex millimeter-scale metamaterial-like structure. Scanning electron microscopy (SEM) was used to characterize the fabricated structures.

The numerical model accurately predicted the spatiotemporal focusing profile, showing a sharp intensity peak confined to a sub-micron range around the focal plane. Experimental results confirmed the ability to print thin layers (thicknesses of just over 2 µm were achieved, with thinner layers possible with improved resist materials). The system demonstrated a continuous fabrication process without pauses between layers, utilizing the 5 kHz laser repetition rate. Sub-micron features were achieved, with line widths less than 200 nm being fabricated. The printing rate exceeded 10⁻³ mm³ s⁻¹, demonstrated by the fabrication of a millimeter-scale complex structure. The analysis showed that while the printed width was smaller than expected due to shrinkage, which could be minimized through using different photoresist materials, sub-micron features were obtained. The print thickness was found to depend on exposure time and laser intensity, with a minimum thickness of slightly above 2 µm. Axial resolution, the minimum distance between layers, was slightly larger than feature size. The researchers also observed a trade-off between printed line width and height, likely due to oxygen inhibition effects.

The developed system successfully addressed the limitations of previous multi-photon lithography methods. The continuous fabrication process, combined with spatiotemporal focusing for precise layer control, significantly increased the printing speed while maintaining sub-micron resolution. The ability to fabricate complex 3D structures with smooth surfaces and features at a high speed opens up new possibilities for applications in micro/nanofabrication. The observed shrinkage effects could be addressed by using optimized photoresist materials, further improving the accuracy and precision of the printing process. This method significantly advances the potential of multi-photon lithography for high-throughput manufacturing of micro and nanoscale devices and structures.

This work demonstrates a novel continuous projection multi-photon 3D printing system achieving significantly improved speed and throughput compared to traditional methods. The combination of spatiotemporal focusing and high-repetition-rate laser processing allows for the creation of complex 3D micro/nanostructures with sub-micron features at speeds exceeding 10⁻³ mm³ s⁻¹. Further research could focus on optimizing the photoresist to reduce shrinkage and improve resolution, as well as exploring the application of this technique to diverse materials and complex geometries for large-scale manufacturing.

The observed shrinkage of the printed structures, although partially addressed through analysis, remains a limitation and could affect the overall accuracy of the final product. The study primarily focuses on a specific photoresist; testing with alternative materials may reveal further improvements in resolution and speed. While the system demonstrates high throughput, scaling up to even larger structures may require further optimization of the optical setup and processing parameters.