Characterization of thin-film adhesion and phonon lifetimes in Al/Si membranes by picosecond ultrasonics

We quantitatively study interfacial adhesion in a two-layer membrane system consisting of Al and Si with femtosecond time-resolved laser spectroscopy. High-frequency acoustic pulses in the sub-THz regime are utilized to characterize the membrane system. In order to explain the distinct features of the measured data, a spring model for the Al/Si interface is employed. We show that acoustic dissipation in this system needs to be included for accurate modeling of the interface adhesion over a broad frequency range. This modeling approach yields a spring constant of , an acoustic phonon lifetime of ps at 240 GHz in polycrystalline Al and a frequency dependence of the lifetime in Si in the frequency range from 50-800 GHz.