Investigation of acoustic monitoring in the laser cleaning of copper

A Q-switched Nd:YAG laser operating at 1064nm and 532nm has been used to investigate acoustic monitoring during the removal of copper oxide from a copper substrate. It was found that the acoustic wave emitted from the surface under laser irradiation can be related to the surface condition - i.e. surface cleanliness and substrate damage. The acoustic waveforms also depend on the laser fluence. The frequency power spectrum produced by FFT(Fast Fourier Transform) provided a clear indication of different levels of surface cleanliness. Removal of copper oxide resulted in a decrease while substrate damage (arising from the overexposure to the laser pulse) resulted in an increase of the spectrum intensity at characteristic frequencies. As a result, surface cleanliness and substrate damage can be monitored in real-time by sensing the acoustic emission during the laser cleaning process. Optimum processing conditions for copper oxide removal could be found from the frequency spectrum and surface analysis. Differences in the mechanism of cleaning at the two laser wavelengths were also observed in the surface morphology. At 532nm the laser pulse produced a slightly sputtered wide area around the laser crater.