DETERMINATION OF ELASTIC-CONSTANTS OF THIN-FILMS FROM PHASE-VELOCITY DISPERSION OF DIFFERENT SURFACE-ACOUSTIC-WAVE MODES

We have carried out a systematic investigation of the achievable accur acy in the determination of the independent elastic constants c(11) an d c(12) and the related constant c(44) of thin isotropic films from th e phase velocity dispersion of surface acoustic waves (SAWs). As a mod el system SiO2 and Au films deposited on Y-cut LiNbO3 were considered. The phase velocity dispersion was calculated for real elastic constan ts in dependence on the him thickness. These input data were used to d etermine the least-squares fits of the input dispersion and the phase velocity obtained by modifying c(11) and c(12). The minimum: of this e rror field describes the solution of the inverse wave propagation prob lem. Error fields of the least-squares fits were calculated with +/-5% variation for c(11) and +/-30% for c(12). Different functional behavi or and various magnitudes of the dispersion were compared. When simula ting a small measuring uncertainty for the phase velocity the solution of the inverse problem becomes unstable which results in an insuffici ent accuracy of the interesting elastic constants. By superposition of two SAW modes the inaccuracy was significantly reduced. For the model system each independent SAW mode offers the ability to determine one elastic constant or one relation between different elastic parameters. (C) 1995 American Institute of Physics.