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

Excimer laser ablation of gold films deposited on a quartz crystal microbalance is investigated. The ablation rate is directly obtained from the frequency shift of the microbalance. The measured single-shot ablation rate is found to be at least two orders of magnitude higher than the numerical predictions based on a surface vaporization model. Surface morphologystudies indicate that hydrodynamicablation plays a leading role in excimer laser ablation of thin gold films. In situ reflectivity and scattering measurements 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 nanosecond melting duration in the case of a bulk target. This longer duration of melting may promote liquid motion, which leads to hydrodynamic ablation at a much higher rate compared with that of atomic vaporization from the surface. Experiments show that the ablation rate is also a strong function of the background gas pressure, which may be the result of the interactions between the gold vapor evaporated from the surface and the hydrodynamic motion in the molten gold. PACS: 81.60; 85.40; 78.65; 44.10 Laser micromachining of thin-film materials has drawn great attention from many researchers in microelectronics and micromechanics for applications such as optical recording, circuit patterning, and mask generation [1–4]. The short pulse width and the strong intensity mean that the excimer laser can induce heating, melting, and vaporization of metals on a time scale of nanoseconds to microseconds. Localized ablation leads to precise micromachining of metallic thin films on dielectric substrates. Compared with visible and infrared pulsed lasers, an excimer laser offers unique advantages in micromachining of thin metallic films deposited on dielectric substrates. The coupling of excimer laser energy into the thin films can be greatly enhanced because the reflectivity of metallic thin films in the UV range is generally lower than in longer wavelength ranges. Recent progress in excimer laser manufacturing has provided higher beam output, larger beam dimensions, and better homogeneity of the beam intensity distribution. These advances hold great promise even for broader applications in the fields of microelectronics and micromechanics. In order to seek the optimal process conditions in excimer laser ablation of thin metallic films, better understanding of the physical process is needed for process design and control in an industrial environment. It has been shown that the surface topography growth is not significant during the first few pulses for the ablation of bulk metals because of the fast solidification of the molten layer due to the good heat transfer into the bulk solid [5, 6]. Compared with laser ablation of bulk materials, which for example is used in pulsed laser deposition of thin films, thin film ablation with an excimer laser is far more complicated. The distinct difference lies in the fact that due to the poor thermal conductivity of the underlying dielectric substrate, the laser energy is more confined within the metallic film, causing a substantially longer melting duration. The long period of the molten state in the thin film promotes both vaporization and hydrodynamic development. Excimer laser ablation of metallic films with various laser wavelengths, fluences, and dielectric substrate combinations have been investigated, and several physical models have been proposed to explain the experimental observations [7–9]. The vaporization mechanism consists of atomic removal of particles from the surface at elevated temperatures during and after the excimer laser irradiation of thin metal films [10]. The explosion mechanism implies that instantaneous gasification occurs at the film–substrate interface, due to the high temperature reached across the film [11]. This model is essentially based on the mechanism of heterogeneous vapor nucleation at the interface. Thus the vapor explosion and the subsequent liquid expulsion contribute to ablation. The third model suggests that the ablation process is a combination of vaporization from the surface and hydrodynamic development of the molten surface during the prolonged melting [12, 13]. Recent high-speed photography in excimer laser irradiation and melting of copper thin films deposited on PMMA has indicated that the hydrodynamic motion, evolving over several microseconds, induces the removal of the thin film from the substrate [1]. On the other hand, the single-shot abla-