Influence of pulse duration on the hole formation during short and ultrashort pulse laser deep drilling

We investigated the influence of the pulse duration on the laser drilling process in the femtosecond, picosecond and nanosecond regime by in-situ imaging of the hole formation in silicon for pulse energies from 25 μJ to 500 μJ. For percussion drilling, we used a Ti:Sa CPA laser system that provides pulses with a duration of 50 fs up to 10 ns at 800 nm. At this wavelength, silicon shows linear absorption and its ablation behavior is comparable to metals. The temporal evolution of the longitudinal silhouette of the hole was visualized during the drilling progress. Deep holes with a depth larger than 1 mm and aspect ratios up to 30:1 were generated. In terms of maximum achievable depth, ultrashort pulses with a duration below 5 ps show comparable efficiency for pulse energies below 100 μJ, while ns-pulses only lead to shallow depths. The situation changes for pulse energies higher than 100 μJ. The depth of holes drilled with ns-pulses increases linearly with pulse energy, while ultrashort pulses show a saturation of achievable depth, which is most distinctive for the shortest pulse duration of 50 fs. The increase in depth for ns-pulses is accompanied by an increasing number of pulses required to reach this depth, which can be 10 times as much as for ultrashort pulses at the same pulse energy. The drilling process consists of an iterative sequence of forward drilling and increase of hole diameter. The increase in diameter leads to numerous deviations from a cylindrical hole shape in the form of bulges, cavities and finger-like structures. This is less pronounced for ps-pulses. fs-pulses show the best achievable hole geometry at a tapered shape without noticeable deviations.