Picosecond ultrasound spectroscopy for studying elastic modulus of thin films: a review

This article introduces an advanced acoustic method for measuring the out-of-plane longitudinal-wave modulus of thin films using picosecond ultrasound. The ultrafast light pulse is focused on the film surface to excite the coherent acoustic pulse, which propagates in the thickness direction, and then, the time delayed probing light pulse irradiates the specimen for detecting the acoustic waves. For opaque thin films, the pulse echoes within the film or the thickness resonance frequency of the film is measured to determine the modulus. For transparent films, Brillouin oscillations from the film are observed, and their frequencies yield the modulus. The film thickness and refractive index are measured by X-ray reflectivity and ellipsometry, respectively. This method was applied to various thin films, providing important knowledge about the elasticity of thin films. Most thin films are softer than corresponding bulk materials, but some thin films are significantly stiffer.