Electronic Processes in Laser Ablation of Semiconductors and Insulators

Abstract

Surface ablation by high-power, nanosecond-pulsed lasers is playing an increasingly important role in materials processing technologies [2.1] and it is a matter of significant scientific interest in its own right. Laser ablation in the nanosecond regime has traditionally been viewed as resulting from rapid heating of the surface layer on the grounds that typical thermalization times, even in insulators, are much shorter than the laser-pulse duration. While the interaction of laser light with metal surfaces in ultra-high vacuum has thermal characteristics [2.2], it is increasingly clear that electronic effects play a significant, and possibly dominant, role in laser ablation from nonmetallic solid surfaces. For example, color centers or other electronic defects may be created by the initial laser irradiation, changing the absorption characteristics-and thus the deposition of laser energy — for photons late in a given laser pulse or in subsequent laser pulses. Diffusion of electronic defects to the surface weakens atomic bonds, resulting in preferential ejection or evaporation of certain atomic or ionic species. Surface-conditioning or incubation effects occur even for interpulse spacings which are clearly much longer than characteristic thermal diffusion times. The challenge, in the light of the varied phenomenology of laser ablation in nonmetallic materials, is to identify the common underlying mechanisms.