Laser-induced bubble dynamics optimization for high-aspect-ratio 3D micro-fluidic channels fabrication

In this study, we demonstrate a high-aspect-ratio microhole fabrication method that uses a femtosecond laser to drill from the rear side of transparent borosilicate glass in a specific solution environment with an assisted ultrasonic field. The experimental results show that an ablation enhancement effect could be achieved by mixing ethanol with distilled water with a volume fraction of approximately 20 %. The craters produced in the mixed solution were wider than those produced in water. An assisted ultrasonic field was introduced to further improve the drilling depth and avoid the termination of the drilling process caused by bubble aggregation and debris blockage. The bubble dynamics induced by the femtosecond-laser-induced breakdown effect were observed using shadowgraphs. The shadowgraphs of the high-speed camera revealed that the cavitation bubbles in the mixed solution had more than twice the maximum diameter and a significantly higher number of bubble expansions than those in water, which may cause a stronger impact pressure. The coupling effect of laser ablation enhancement and bubble bursting impact helped achieve a high material removal rate and significantly improve drilling depth. This indicates that an ultrahigh-aspect-ratio straight (∼273) microhole with a ∼2 µm diameter can be fabricated. By designing machining paths, we can machine 3D complex curved microchannels such as spirals. This method shows considerable potential in biomedical and microelectronics applications, for example to fabricate calibrated microholes for container closure integrity testing (CCIT).