LASER-ABLATION OF SILICON AND ALUMINUM - A PHOTOACOUSTIC AND TIME-RESOLVED REFLECTIVITY STUDY

The interaction of KrF (248 nm) excimer laser pulses with silicon has been studied by photoacoustic and time-resolved-reflectivity techniques. Laser-induced acoustic pulses in aluminum and silicon were detected with a piezoelectric transducer. A sharp rise in amplitude of the longitudinal wave normalized to the incident fluence indicated the onset of ablation, thus permitting a fast and sensitive determination of ablation thresholds in solids. Threshold values of 0.50 and 0.95 J cm(-2) were determined for aluminum and silicon, respectively. The same value is inferred for silicon by time-resolved reflectivity of the KrF laser beam that effects the ablation (self-reflectivity). Truncation of the reflected pulse starting at an incident fluence between 0.9 and 1 J cm(-2) indicates the onset of ablation. Surface melting between similar to 0.2 and 0.9 J cm(-2) was also observed in these experiments. Numerical simulation of the temperature evolution of the silicon surface gave values of the fluence required to raise the surface temperature to the melting and vaporization points in line with the observed values. The mechanism of pulse truncation is discussed in terms of time-dependent absorption and scattering from particles emerging from the surface during ablation. In agreement with the conclusion reached in another study, approximate calculations suggest that large particles are ejected initially that are continuously reduced in size during the laser pulse.