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 a coustic waves depend on laser fluence, pulse number and surface condit ion of the substrate and can be used to determine the nature of laser- material interactions such as pulsed-laser-induced cleaning, surface r oughening and ablation. When laser fluence is below the ablation thres hold 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 t he ablation threshold, the amplitudes of acoustic waves also reduce wi th increasing pulse number but to a level larger than zero due to lase r ablation and then increase again due to surface morphology change. B y establishing a relationship between the amplitudes and laser-substra te parameters, i.e., bulk response, surface response, pulse dependence and morphology response, a numerical formula is proposed for real-tim e 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. (C) 1997 Elsevier Sci ence B.V.