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Influence of ambient pressure on the hole formation process in ultrashort pulse laser deep drilling

We investigate the influence of the ambient pressure on the hole formation process during percussion drilling of silicon by applying an in-situ imaging technique. In this study the pressure is varied from atmospheric conditions down to medium vacuum of 10 !bar. Drilling was performed using an ultrashort pulse system providing 8 ps pulses with up to 125 μJ at 1030 nm. At this wavelength, the ablation behavior of silicon is comparable to metals. At the beginning of the drilling process, we observe an increased drilling efficiency by 40% already for a moderate pressure decrease to 100 mbar. The formation of an ideally shaped hole lasts for approximately 200 pulses instead of only 100 as for atmospheric conditions and therefore leads to 3 times the depth at this point. The effect can be enhanced by increasing the pulse energy, but not by decreasing pressure further. However, the number of pulses till the end of the drilling process is extended by decreasing the pressure further. For a low ambient pressure of 10 μbar, this is accompanied by an increase of the maximum achievable depth of more than 100%. Simultaneously the hole shape changes from a few ends and bulges at atmospheric conditions to numerous branches over the complete lower part of the hole at low pressure. This drilling behavior can be attributed to a better removal of ablated particles from the hole capillary with decreasing pressure, which leads to lower scattering losses for the pulse propagation inside the hole.