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15 April 2015 Conditions for random and periodic hole formations during surface ablation of silicon with ultrashort pulsed lasers
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The formation of randomly distributed or periodic holes with a depth of several μm can be observed during the ablation of silicon with ultrafast lasers. In order to investigate the conditions for these hole formations, Si wafers with a thickness of 525 μm were ablated using a 380-fs laser at a wavelength of 520 nm. The surface was scanned parallel and perpendicular to the polarization of the laser beam with different fluences and numbers of scans. The ablated surfaces were analyzed by scanning electron microscopy. Their roughness was measured using confocal microscopy. At a fluences of 1.4 J/cm2, laser-induced periodic surface structures (LIPSS) orientate parallel to the polarization and periodic holes appear in the furrows independently of the polarization. At 4.3 J/cm2, LIPPS can be suppressed and the hole formation is reduced, if the polarization is perpendicular to the scan direction. The surface roughness reduces by a factor of 2. At 14.1 J/cm2, the formation of LIPPS is suppressed independently of the polarization, but the surface roughness increases again due to the increasing amount and size of melt ejections. The analysis of the surface suggests that random stacking of melt ejections creates bumps and holes. The depth of the holes increases with the number of scans. In this way, first random and then—with increasing number of scans—periodic holes appear. This effect can be enhanced by creating LIPPS.
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Matthias Domke "Conditions for random and periodic hole formations during surface ablation of silicon with ultrashort pulsed lasers", Proc. SPIE 9355, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XV, 935511 (15 April 2015);

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