Femtosecond laser can minimize the undesirable thermal effected zone, and hence is suitable for the processing of brittle materials. Taking advantage of this method, some glassy materials can be cut without any micro-cracks on the surface. This paper presents the drilling of ultra thin silicon wafers irradiated with femtosecond laser. In order to understand fundamental characteristics of the ablation using femtosecond laser, influence of laser parameters such as pulse energy, pulse separation time, etc. is investigated. As the pulse energy increased, the distribution of the air breakdown grew only to the side of the focusing lens and the ablation depth was saturated by the breakdown plasma which causes scattering of the laser. Therefore the highest processing rate was obtained with the maximum pulse energy not to generate the breakdown plasma. Double pulse method was useful for restraint of the swelling around the ablation crater. The swelling height was within 0.5μm with the most suitable pulse separation time (10ps-20ps).
