Femtosecond laser has been expected as a new tool for the industrial usage in particular to material micromachining. We are developing the dicing technique with femtosecond laser ablation for the ultra thin semiconductor substrates, which are 50 pm thick or less. In this research, we performed drilling for 50 1um thick silicon substrate with femtosecond laser (r=120 fs, 2=800 nm, F =1 kHz) as the basic experiment for dicing, focusing on the influence of a double-pulse irradiation on the processing characteristics.The double-pulse irradiation for 1 8 shots of 10 0/pulse at the pulse separation time from 10 to 20 PS showed the remarkable reduction ofthe height ofthe molten layer around the drilled hole (<0.5 pin). At the same time, however, the ablation depth was the minimum (<2.3 pin). The surface inside the hole got smooth as the pulse separation time of more than 3 ps. We supposed that the second pulse in a double-pulse should generate the another ablation on the surface and its high pressure should prevent the ablated materials by the first pulse from flying out of the hole.
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