Picosecond Laser Direct Writing of Square Micro-Holes on 316L Stainless Steel Using Skywriting-Based Synchronized Gating and Orthogonal Cross-Hatch Scanning

Finite-length raster scanning can cause endpoint over-ablation, whereas repeated scanning along a fixed direction can reinforce directional bottom textures. In this study, skywriting-based synchronized gating was combined with four-direction orthogonal cross-hatch scanning to fabricate square micro-holes on 316L stainless steel using a picosecond laser. Scanner acceleration and deceleration occurred outside the target region, and laser output was restricted to the constant-velocity segment. The local height variation at the line start decreased from approximately 8.0 to 1.3 μm. At a nominal processing area of 320 μm × 320 μm and a cumulative count of 140 full-area scans, four-direction scanning produced a bottom-surface Sa of 0.325 μm, 52.3% lower than that obtained by unidirectional raster scanning. It also yielded lower Sa and Sq values than continuous serpentine raster and continuous square-spiral scanning. Increasing the scan count from 84 to 532 increased the machining depth from 22.4 to 135.4 μm, with an approximately linear relationship over the tested range. However, bottom roughness and bottom-area shrinkage increased with depth. These results demonstrate that synchronized gating and directional alternation improve endpoint consistency and average bottom uniformity, while depth extension requires balancing removal depth against bottom quality.