AUDIBLE ACOUSTIC-WAVE EMISSION IN EXCIMER-LASER INTERACTION WITH MATERIALS

Audible acoustic wave generation during excimer laser interaction with materials has been investigated. It is found that the amplitudes of a coustic waves depend on laser fluence, pulse number, and substrate mat erial characteristics and can be used to determine the nature of laser -material interactions. When laser fluence is below the ablation thres hold of the materials, the amplitudes are reduced to zero at large pul se number due to the cleaning of contaminants on the substrate surface . As laser fluence becomes higher than the ablation threshold, the amp litudes of acoustic waves also reduce with increasing pulse number but to a constant level instead of zero due to laser ablation of substrat e materials. Since the surface contamination can be completely removed by a few pulses at high laser fluence, the constant level is attribut ed to the material ablation. It is also found that the constant level increases with laser fluence. By establishing a relationship between t he amplitudes and laser parameters, real-time monitoring of laser-soli d 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 ac oustic wave emission can, therefore, be used to find the nature of las er-substrate interaction (i.e., surface cleaning or ablation), and to find the ablation threshold. (C) 1996 American Institute of Physics.