A photoacoustic method for characterising thin films
A photoacoustic method is presented for characterising thin films. The method uses acoustic surface waves with frequencies up to 200 MHz induced by short laser pulses and detected with a piezoelectric transducer. The surface wave signals are processed by cross-correlation and Fourier transformation to determine a dispersion spectrum (phase velocity depending on frequency) with optimal signal-to-noise ratio. The theoretical curve is fitted to the measured dispersion spectrum to derive film parameters as Young's modulus, density and/or film thickness. The conditions are discussed which enable one, two, or three of these parameters to be obtained. The dispersion spectrum may show normal and anomalous dispersion, which depends on the combination of film and substrate material. Diamond-like carbon and polyamide films on (100) silicon are used to demonstrate this phenomenon. The effects of film thickness, film and substrate material, and the error of the input parameters on the results are d iscussed. The peculiarity of surface wave propagation in cubic single crystals is described. The photoacoustic method is particularly suitable to determine the Young's modulus of the film. This material parameter is sensitively related to important microstructural properties and bonding conditions. The large modulus variation of some important covalent and ionic film materials recommends to use Young's modulus for quality control.