AbstractReal-time monitoring of surface cleanness in laser dry cleaning was investigated by monitoring the acoustic waves emitted from the substrate surface under pulsed laser irradiation. It is found that the acoustic wave from the surface under laser irradiation can reflect the surface condition, i.e., the surface cleanness. Besides the surface condition, the acoustic waveforms also depend on the substrate materials and laser pulse energies (intensities). The frequency spectrum obtained from the Fourier transform can provide clear indication for different surface cleanness. Removing of surface contaminants can generally result in weaker laser-induced acoustic wave and change in the frequency spectrum. The surface cleanness, therefore, can be monitored in real-time by detecting the amplitude and frequency of the acoustic wave during the laser cleaning process. This phenomenon provides the laser cleaning technique a real-time feedback during the cleaning process.