Detection of hidden gratings using light and sound
This thesis demonstrates the application of laser-induced ultrasound to detect buried gratings in the context of wafer alignment. We fabricated metal gratings on top of flat layers of metal/ dielectric layers on a glass substrate and performed pump-probe experiments from the substrate side. Hence, both the 400 nm pump pulse and the 800 nm probe pulse see a nominally flat surface, and the grating is effectively optically hidden . The femtosecond pump pulse generates an acoustic wave in the metal layer that propagates through the metal/dielectric layers and reflects off the buried grating . The acoustic wavefront returning to the metal-glass interface now has a shape resembling that of the buried grating . This gives rise to a ‘ grating -shaped’ acoustic wave at the metal-glass interface. A delayed probe pulse diffracts from this grating , and the diffracted signal is observed. The observation of a diffraction signal indicates that we can detect the presence of a buried grating using the pump-probe technique.
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