KrF excimer laser ablation of thin gold films on a quartz crystal microbalance at various argon background pressures

Excimer laser ablation of gold film (0.5 μm) deposited on quartz crystal microbalance is investigated. Ablation rate is directly obtained from the frequency shift of a quartz crystal microbalance (QCM). A numerical model based on surface thermal vaporization mechanism is developed to predict the surface temperature, melting duration, and ablation rate. The measured single-shot ablation rate is found to be at least two orders of magnitude larger than the numerical predictions. Surface morphology studies indicate that hydrodynamic ablation plays a leading role in excimer laser ablation of thin gold films. In situ reflectivity and scattering measurement of the gold film surface during the transient heating and melting upon excimer laser irradiation show that the melting duration is of microsecond order, which is much longer than the melting nanosecond duration in the bulk case. This longer duration of melting may promote liquid motion which leads to hydrodynamic ablation at much a higher rate compared with atomic vaporization from the surface. Experiments also show that the ablation rate is also a strong function of the background gas pressure, which may be the result from the interactions between the gold vapor from the surface and the hydrodynamic motion in the molten gold. The laser ablation rate with the rectangular spot of a larger aspect ratio (5:1) is higher than that of a smaller aspect ratio (1.75:1). The possibility of argon plasma ignition during the laser ablation process at higher argon background pressures is discussed.