The three-level ripples induced by femtosecond laser on a 6H-SiC single crystal and the formation mechanism

In this paper, a line-scanning irradiation of 6H-SiC single crystal by a femtosecond laser focus was implemented for laser fluence of 2.2 J/cm2 at different scanning velocities from 1000 down to 100 μm/s. The morphology of the obtained ablation lines characterized by a scanning electron microscope shows that there progressively appear three-level ripples with average period of about 222, 600 nm and that between them. Possible formation mechanisms for these three-level ripples were analyzed by numerical simulation employing finite-difference time-domain method (FDTD). Analysis shows that as Obara proposed, the interference of incident wave with the wave scattered by scratches possibly left by fine polishing is the most possible reason for inducing 222 nm-period and 600 nm-period ripples. Main discussions were focused on the orientation characteristics of ripples. Both isolated scratches and crossed scratches with different orientation angles were considered for evaluating their role in field redistribution. The detailed simulations indicate that for scratches oriented 90° with respect to laser polarization generate a pair of strongest ripple-like optical field enhancement compared to scratches with other orientation angles. It is these advantages of 90°-oriented scratch in field enhancement that make the final ripple perpendicular to laser polarization most competitive and finally left on the surface. This complements the physical picture of Obara et al. in terms of their negligence in addressing the orientation characteristics of ripples. Further FDTD simulation of the interaction of incident wave with as-formed 600 nm-period ripples shows that the optical field enhancement located between any two adjacent 600 nm-period ripples is the driving force for splitting 600 nm-period ripples into the third ripples with period between 222 and 600 nm.