Fabrication of High Aspect Ratio Taper‐Free Microtrenches in Copper by Line‐Shaped Flat‐Top Femtosecond Laser Machining Under Flowing Water

High‐precision taper‐free microtrenches in copper are of interest for microelectronic and microscale heat‐exchanger applications. High‐aspect‐ratio trenches can increase the effective interfacial area, improve fluid transport, and enhance heat‐transfer performance. However, copper micromachining is challenging because its high ductility promotes burr formation, whereas laser ablation can generate recast and redeposited material that degrades the trench geometry. In this study, a line‐shaped flat‐top beam was introduced to provide a uniform energy distribution within the axial defocus range. Femtosecond laser ablation combined with flowing water circulation was employed to facilitate debris evacuation and suppress thermal damage. To mitigate the depth‐saturation effect, the target depth was divided into multiple processing layers using a dynamic Z‐stepping strategy. Taper‐free microtrenches with a width of 30 μm and a depth of 90 μm, corresponding to an aspect ratio of 3, were fabricated by optimizing the laser power and polarization orientation. Laser‐induced periodic surface structures (LIPSS) on the sidewalls were further analyzed, revealing a strong polarization dependence of the sidewall morphology. This study demonstrated an effective approach for fabricating high‐aspect‐ratio, taper‐free microtrenches in copper for microfluidic and heat‐sink applications.