Comparison of ablation mechanisms at low fluence for ultrashort and short-pulse laser exposure of very thin molybdenum films on glass.

Abstract

Complete removal of a loosely adhered very thin molybdenum film from a glass substrate is investigated for both femtosecond and nanosecond lasers at different wavelengths. Atomic force microscopy and scanning electron microscopy confirm that ablation of the molybdenum film by femtosecond pulses occurs close to the damage threshold fluence, creating minimal damage to the substrate. This is in contrast to nanosecond laser processing where significant substrate damage at the equivalent damage threshold fluence is observed. Simulations predict a two-stage mechanical buckling mechanism in the femtosecond case. Out-of-plane thermal expansion first results in a tensile expansion of molybdenum film from the glass substrate; this locally delaminated film is then buckled by a subsequent compressive stress, leading to thin film spallation. Ablation by nanosecond laser pulses behaves differently. The appreciable heat diffusion length (∼700 nm) in molybdenum, observed for the nanosecond case, results in an increased thermal expansion of the glass. The thermally induced stress generated by the molten glass creates a delaminated area, which "pushes" the compressed film away from the substrate. These findings are relevant to future selective laser patterning of very thin molybdenum layers.