Laser ablation of Si under a water surface has been investigated. The laser used is a KrF excimer laser, which has a wavelength of 248 nm and a pulse duration of 23 ns. It is found that the laser ablation rate of Si varies with the thickness of the water layer above the Si substrates. The laser ablation rate is the most highly enhanced with a water layer of 1.1 mm. It is assumed that the plasma generated in the water confinement regime with an optimal water layer thickness induces the strongest pressure. This high-pressure, high-temperature plasma results in the highest ablation rate. A wide-band microphone is used to detect the audible acoustic wave generated during the laser ablation. The amplitude of the acoustic wave is closely related to the ablation rate. It is found that the first peak-to-peak amplitude of the acoustic wave is the strongest when the water layer thickness is 1.1 mm above the substrate. Fast Fourier transform analysis of the wave forms shows that there are several frequency components included in the acoustic waves. The dominant frequency component decreases from 10.6 to 3.5 kHz as the water layer thickness varies from 1 to 2.2 mm. Diagnostics of the acoustic wave emission can be used to find the optimal water layer thickness to enhance the laser ablation rate. With proper calibration, acoustic-wave detection can be used as a real-time monitoring of the laser ablation.

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