Silicon cluster formation in the laser ablation of SiO at 308 nm

Neutral silicon cluster formation in the laser (308 nm) ablation of silicon monoxide was investigated through the analysis of composition and dynamics of the ablation plume under different laser fluence conditions. The neutral species were ionized by a second laser (193 nm) and the positionized species detected by TOF-MS (time-of-flight mass spectrometry). At low laser fluences, plume composition is dominated by SiO; above 0.6 J/cm Si, SiO and Si have comparable intensity and Si (n ≤ 7) clusters are observed. Flow velocities and temperatures of the ejected species are nearly mass-independent, indicating that the plume dynamics are close to the strong expansion limit, implying a collisional regime. Through the relation between the estimated values of terminal flow velocity and surface temperature, u ∝ T, it is found that, at low laser fluences, the surface temperature increases linearly with laser fluence, whereas, at the laser fluence at which Si clusters are observed, the increase of temperature is below the linear dependence. The population distribution of the ejected Si provides some indication of a formation mechanism based on condensation. Analogies between the ablation behavior of silicon monoxide and silicon targets are considered. © Springer-Verlag 2004.