Closed-Loop Two-Photon Lithography for Improved Precision and Repeatability

We present a closed-loop two-photon lithography (TPL) system based on optical diffraction tomography (ODT) and digital micromirror device (DMD)-based multi-focus ultrafast 3D scanning. Traditionally, micro- nanostructures fabricated by TPL can only be observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) with special sample treatments, such as metal coating. On the other hand, focus-ion beam is typically the option to investigate the internal structure quality of a printed structure, which is destructive. Overall, these complex and expensive approaches are post-fabrication observation methods and cannot provide immediate feedback during the printing process, leading to low reproducibility and loose dimensional accuracy when writing sub-diffraction limit features (i.e., &lt; 400 nm).<br/><br/>To address these challenges, we have developed a closed-loop TPL system, where an ODT module is integrated with a parallel TPL system based on a DMD. A 1-kHz regenerative femtosecond laser amplifier is the TPL light source to provide sufficient power to command up to 2,000 individually programmable laser foci for parallel nanofabrication. The ODT module uses a DMD to scan a 532-nm metrology laser at 22.7 kHz via 10 – 50 different angles to construct 3D tomograms of the work volume at a rate of 400 – 1000 Hz. The tomogram gives a 3D refractive index (RI) map of the work volume. As RI is directly related to the laser doses, fast RI monitoring enables closed-loop TPL for the first time.<br/><br/>To characterize the closed-loop TPL system, we designed and fabricated different 3D micro-structures. The results, for the first time, show that in conventional TPL processes, polymerized structures can have an ± 0.01 RI variation due to varying laser doses; in other words, fine structures, e.g., nanowires or bridges, written by thresholding the laser energy will each have different RIs. Microlenses written by different scanning strategies (e.g., fine-scanning on the surface and fast exposure for the interior) will have non-uniform RI distribution. Based on closed-loop TPL, we show micro-structures can be printed with uniform RI distribution. In the experiments, we further demonstrated that closed-loop TPL presents improved dimensional accuracy, i.e., 200 nm ± 30 nm vs. 200 nm ± 100 nm (conventional open-loop TPL printing) and reproducibility.<br/><br/>In summary, we will present the design and characterization of a closed-loop TPL system based on ODT, which addresses the long-standing low reproducibility challenge of TPL, making it now suitable for industrial applications (i.e., high-resolution, high-speed, low-cost, and high yield). As the ODT technology rapidly monitors RI distribution, the impact can be extended beyond TPL; for example, UV stereolithography or label-free imaging.