Acoustic wave monitoring of cleaning and ablation during excimer laser interaction with copper surfaces

Acoustic wave generation during excimer laser interaction with copper substrates has been investigated. It is found that the amplitudes of acoustic waves depend on laser fluence, pulse number and surface condition of the substrate and can be used to determine the nature of laser-material interactions such as pulsed-laser-induced cleaning, surface roughening and ablation. When laser fluence is below the ablation threshold of copper, the amplitudes are reduced to zero due to the cleaning of surface contaminants but increase again due to surface roughening if the fluence is large enough. As laser fluence becomes higher than the ablation threshold, the amplitudes of acoustic waves also reduce with increasing pulse number but to a level larger than zero due to laser ablation and then increase again due to surface morphology change. By establishing a relationship between the amplitudes and laser-substrate parameters, i.e., bulk response, surface response, pulse dependence and morphology response, a numerical formula is proposed for real-time monitoring of laser-solid interaction. The bulk response can be used to determine the ablation threshold. The morphology response is found to reach the maximum at the ablation threshold.