Evolution of hole depth and shape in ultrashort pulse deep drilling in silicon
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We report on the temporal evolution of the percussion drilling process in deep laser drilling. Ultrashort laser pulses at 1030 nm and a duration of 8 ps were used to machine silicon while simultaneously imaging the silhouette of the hole using an illumination wavelength above the band edge. We investigate the influence of the processing parameters fluence and pulse energy on the depth and shape of the hole demonstrating different phases of the drilling process. In the first phase, a tapered hole is formed with highly reproducible shape and depth. In the following, the evolution of the hole shape is irregular and imperfections like bulges, changes of the drilling direction and the formation of multiple hole ends occur. In the final phase, the maximum depth stays constant while the volume still increases due to enlargement of the hole diameter and the possible formation of multiple hole ends. Deviations from the ideal hole shape occur primarily in the lower part of the hole. Their extent can be reduced by increasing the amount of applied pulse energy. Moreover, the pulse energy is chiefly determining the maximum achievable hole depth, which is largely independent of the focusing conditions and corresponding fluence.