CONTRASTING DAMAGE CHARACTERISTICS IN DIRECT INCIDENCE AND SURFACE-PLASMON MEDIATED SINGLE-SHOT LASER-ABLATION OF ALUMINUM FILMS

Thin, oxidised Al films grown an one face of fused silica prisms are e xposed. tinder ambient conditions, to single shots from an excimer las er operating at wavelength 248 nm. Preliminary characterisation of the films using attenuated total reflection yields optical and thickness data for the Al and Al oxide layers; this step facilitates the subsequ ent, accurate tuning of the excimer laser pulse to the: surface plasmo n resonance at the Al/(oxide)/air interface and the calculation of the fluence actually absorbed by the thin film system. Ablation damage is characterised using scanning electron, and atomic force microscopy. W hen the laser pulse is incident, through the prism on the sample at le ss than critical angle, the damage features are molten in nature with small islands of sub-micrometer dimension much in evidence, a mechanis m of film melt-through and subsegment blow-off due to the build up of vapour pressure at the substrate/film interface is appropriate. By con trast, when the optical input is surface plasmon mediated, predominate ly mechanical damage results with the film fragmenting into large flak es of dimensions on the order of 10 mu m. It is suggested that the abi lity of surface plasmons to transport energy leads to enhanced, prefer ential absorption of energy at defect sites causing stress throughout the film which exceeds the ultimate tensile stress for the film: this in turn leads to film break-up before melting can onset. (C) 1998 Else vier Science B.V.