Characterization of ultrathin films by laser-induced sub-picosecond photoacoustics with coherent extreme ultraviolet detection

Photoacoustic spectroscopy is a powerful tool for characterizing thin films. In this paper we demonstrate a new photoacoustic technique that allows us to precisely characterize the mechanical properties of ultrathin films. We focus an ultrafast laser onto a nano-patterned thin film sample, launching both surface acoustic waves (SAWs) and longitudinal acoustic waves (LAWs). Coherent extreme ultraviolet pulses are then used to probe the propagation dynamics of both the SAWs and LAWs. The resulting photoacoustic signal on both short (picosecond) and long (nanosecond) time scales yields important information. In the first 100ps, a fast oscillation followed by an echo signal corresponds to LAWs traveling inside the nanostructures and the thin film, from which the LAW velocities in the two materials can be extracted. On longer time-scales, SAW oscillations are observed. By combining the measured SAW frequency with the wavelength (determined by the nanostructure period) the SAW velocity can be accurately determined, even for very short wavelength surface acoustic waves with very small penetration depths. Using this technique, the elastic properties, including the Young's modulus and Poisson ratio for the thin film, can be obtained in a single measurement, this technique can be extended to sub-10nm thin films.

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