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.