Abstract

Femtosecond (fs) laser ablation is generally considered defect free because it is thought occur through a direct solid-vapor transition [1]. In long-pulsed or CW ablation, the directly irradiated material reaches thermal equilibrium during the application of the laser beam and the beam directly heats the substrate, whereas for fs lasers, equilibrium is reached after the application of the beam. The pulse width of the laser beam is sufficiently short compared to the time necessary for hydrodynamic motion, that it is not necessary to consider the fluid dynamics of the interaction [2]. Recently, a number of groups have speculated that there is a melted layer even in the femtosecond regime. However, we believe that no one has ever been able to provide direct evidence to support this argument. A direct observation of an amorphous layer in the final state of the material would be very conclusive. We have performed a cross-sectional TEM study of holes ablated in Si using a femtosecond laser, and have observed the structure directly.

© 2004 Optical Society of America