Laser-induced ultrasound for the detection of buried micro- and nano-structures
In this thesis we show how laser-induced ultrasound can be used to detect micro- and nano-structures, usually gratings buried underneath optically opaque layers, which is one of the biggest challenge for the semiconductor industry in the wafer alignment process. The experimental setups used in this thesis are based on a pump-probe scheme. When an opaque material is illuminated at the surface by a femtosecond pump pulse, an acoustic wave is generated that can travel deeper into the material, also through layers that are opaque to light. When the sound wave reaches the hidden grating, the acoustic wave is reflected back with a wavefront resembling the shape of the hidden grating. When the ''acoustic replica'' of the grating returns to the surface, it spatially and periodically displaces the atoms or changes the optical properties of the material near the surface. These changes can be detected by a second, delayed probe pulse which is diffracted off the acoustic grating. Thus, the measurement of a diffracted signal indicates the presence of a buried grating. This work shows that laser-induced acoustic waves can be used as a non-invasive, non-contact method to detect micro- and nano-structures buried underneath optically opaque materials. We also show that the photoacoustic signal strength, which is usually very small, can be enhanced by either optical scattering by interface roughness, or by employing plasmonic structures.
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