Effect of Shot Number on Femtosecond Laser Drilling of Silicon

Silicon wafer drilling has been under heavy investigation for some time already. Several different laser types and methods have been introduced to different applications. In the industry usually the nanosecond lasers are used. Femtosecond lasers have been considered as high cost and not suitable for the industrial production. At present femtosecond lasers offer really high pulse energies and good reliability, and also high repetition rates and relatively high pulse energies are possible in the sub picosecond range. Processing accuracy with femtosecond laser is very good and heat input is negligible in the processed surface. In this paper we demonstrate the effect of the number of pulses in silicon wafer drilling with a femtosecond laser in free air. Silicon wafers used were p-type, [100] oriented and double-side polished. From the experiments we can see that the drilled hole depth increases rapidly until a turning point is reached, and the ablation rate becomes much smaller. The transition occurs at around several hundreds to 1000 shots.