Audible acoustic wave emission in excimer laser interaction with materials

Abstract

Audible acoustic wave generation during excimer laser interaction with materials has been investigated. It is found that the amplitudes of acoustic waves depend on laser fluence, pulse number, and substrate material characteristics and can be used to determine the nature of laser–material interactions. When laser fluence is below the ablation threshold of the materials, the amplitudes are reduced to zero at large pulse number due to the cleaning of contaminants on the substrate surface. As laser fluence becomes higher than the ablation threshold, the amplitudes of acoustic waves also reduce with increasing pulse number but to a constant level instead of zero due to laser ablation of substrate materials. Since the surface contamination can be completely removed by a few pulses at high laser fluence, the constant level is attributed to the material ablation. It is also found that the constant level increases with laser fluence. By establishing a relationship between the amplitudes and laser parameters, real-time monitoring of laser–solid interaction can be achieved. Fast Fourier transform analysis of the wave forms shows that there are several frequency components included in the acoustic waves with a peak around 10.9 kHz as the dominant one, which is related to laser material ablation. The monitoring of the acoustic wave emission can, therefore, be used to find the nature of laser–substrate interaction (i.e., surface cleaning or ablation), and to find the ablation threshold.