Ripple formation at laser ablation of chromium thin film

Abstract

Ablation characteristics of chromium thin film on a glass substrate by nanosecond laser pulses were investigated. The laser beam was tightly focused through the glass substrate to a stripe-like spot using the acylindrical lens. The metal was removed only by the central part of the laser beam, where local laser fluence exceeded the well-defined ablation threshold. Formation of a wide area cleaned by the series of laser pulses caused some side effects. The stripe ablated by a single laser pulse had sharp edges on both sides, while the partially overlapping pulses formed a wide stripe with a complicated structure made of the remaining metal. Regular structures, ripples, were developed when laser fluence was slightly above the threshold and the shift between pulses was less than a half width of the line ablated by a single laser pulse. The ripples were located periodically (~4 &mgr;) and were orientated perpendicularly to the long axis of the beam spot (in parallel to the laser pulse shift direction). Their direction did not depend on the laser beam polarization. Different models of the ripple formation in the thin metal film were considered, and instability of the moving vapor-liquid- solid contact line during evaporation of thin liquid films appear to be the most appropriate process responsible for the observed phenomena. Regular gratings with the unlimited line length can be produced by using the technique.