Photoacoustic characterization of the mechanical properties of thin films

Abstract

Narrow band photoacoustics (laser ultrasonics) are used to characterize the properties of free-standing nanometer-sized thin films. Photoacoustic generation is achieved by use of a microchip laser which depositspulsed laser energy in the form of a spatially periodic source on the structure. The resulting narrow band ultrasonic modes are monitored using a Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of acoustic wave numbers is probed. Results are presented for two-layer thin film aluminum/silicon-nitride ( Al/Si 3 N 4 ) membranes. For such thin films, only the two lowest order guided modes are generated and these in turn can be related to sheet and flexural modes in plates. The mechanical properties and residual stress in the thin films are evaluated from measured acoustic dispersion curves for these two lowest order modes.