Fabrication of high aspect ratio and low taper angle micro-holes utilizing complex water-assisted femtosecond laser drilling

Abstract

Water-assisted laser drilling has proven to be an effective technique for high-quality micro-holes fabrication. This study introduces an innovative hybrid approach, designated as spray water film and backside flowing water-assisted (SWF-BFWA) femtosecond laser drilling, which integrates the complementary advantages of spray water film-assisted (SWFA) and backside flowing water-assisted (BFWA) technologies. The proposed methodology combines the high processing efficiency and superior aspect ratio capabilities of SWFA with the excellent geometric precision and minimal taper characteristics inherent to BFWA. A sequential processing strategy was developed, incorporating SWF-BFWA penetration followed by BFWA modification, to achieve micro-holes with both high aspect ratios and low taper angles. To maximize processing efficiency, a single-circular modification trajectory was implemented during the BFWA stage, replacing conventional concentric circle patterns. The influence of laser parameters on modification outcomes was systematically investigated, with optimization of modification factors including the number of modifications, the single-circle diameter of the modification, and the amount of modification defocusing. Experimental results demonstrate that the optimized SWF-BFWA process enables the fabrication of high-quality micro-holes with aspect ratios exceeding 12:1 and taper angles below 0.1°. This investigation provides a comprehensive solution for laser-based fabrication of micro-holes with exceptional dimensional characteristics, offering significant potential for applications requiring high aspect ratios and minimal taper angles.