Efficiency of silicon micromachining by femtosecond laser pulses in ambient air

Femtosecond lasers have proven to be effective tools for precise micromachining. Taking advantage of the reduced heat diffusion and the sharp ablation threshold at comparatively low energy densities, subdiffraction limit sized craters have been machined on silicon wafers by single near infrared Ti:sapphire laser pulses using a high numerical aperture objective lens. Two different ablation regimes have been identified by varying the laser fluence. While two-photon absorption dominates in the low fluence regime, electronic diffusion is a major energy transport mechanism at higher laser fluences. Time-resolved pump-and-probe side-view imaging has been performed to investigate the energy coupling to the target specimen over a wide range of fluences (up to around 1000 J cm2) at lateral beam dimensions of the order of micrometers. The decrease of the ablation efficiency in the high fluence regime (>10 J cm2) is attributed to the strong interaction of the laser pulse with the laser-induced plasma.