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 technique s. Laser-induced acoustic pulses in aluminum and silicon were detected with a piezoelectric transducer. A sharp rise in amplitude of the lon gitudinal wave normalized to the incident fluence indicated the onset of ablation, thus permitting a fast and sensitive determination of abl ation thresholds in solids. Threshold values of 0.50 and 0.95 J cm(-2) were determined for aluminum and silicon, respectively. The same valu e is inferred for silicon by time-resolved reflectivity of the KrF las er beam that effects the ablation (self-reflectivity). Truncation of t he reflected pulse starting at an incident fluence between 0.9 and 1 J cm(-2) indicates the onset of ablation. Surface melting between simil ar to 0.2 and 0.9 J cm(-2) was also observed in these experiments. Num erical simulation of the temperature evolution of the silicon surface gave values of the fluence required to raise the surface temperature t o the melting and vaporization points in line with the observed values . The mechanism of pulse truncation is discussed in terms of time-depe ndent absorption and scattering from particles emerging from the surfa ce during ablation. In agreement with the conclusion reached in anothe r study, approximate calculations suggest that large particles are eje cted initially that are continuously reduced in size during the laser pulse.