Influence of substrate cooling on femtosecond laser machined hole depth and diameter

Substrate temperature was observed to affect the machined hole depth and diameter during Ti:sapphire femtosecond laser machining of a copper substrate. We studied the blind holes drilled on copper specimens by multiple femtosecond laser (fs) pulses under two temperature conditions, namely the liquid nitrogen temperature (similar to 77 K) and the atmospheric temperature (similar to 298 K). Compared to the atmospheric temperature condition, we found that the diameters of the holes are smaller and the depths of the holes are deeper under the liquid nitrogen temperature condition. We attribute the reduction in diameter to the faster heat dissipation of the cooler substrate. We calculated multiple beam reflections in a channel and attribute the increased depth of the cooler substrate to the enhanced multiple laser beam reflections inside the laser drilled hole.