Phonon attenuation in the GHz regime: Measurements and simulations with a visco-elastic material model

Abstract

Aluminum and PMMA thin film samples are investigated regarding their mechanical properties like speed of sound and attenuation. Aluminum is often used as a transducer layer for pump probe laser measurements and different PMMA types have a large importance in the nanoimprinting technique. The measurements are performed on a short pulse laser pump probe setup, where bulk wave packets in the GHz regime are excited and detected using near infrared laser pulses of less than 100 fs duration. This contact-free and non-destructive measurement method is explained. In order to extract the attenuation precisely from the measurements, the entire experimental setup is simulated numerically: The heat distribution and the thermo-elastic wave excitation caused by the laser pulse, the mechanical wave propagation, and the photo-acoustic detection. By means of the visco-elastic modeling of the wave propagation, the simulations are fitted to the measurements by tuning the attenuation parameters in the numerical model. In this way it is possible to extract the attenuation from the measurements. First, two different types of Aluminum on a sapphire substrate are analyzed: Electron beam evaporated Aluminum and sputtered Aluminum, respectively. The thicknesses of the Aluminum films are in the range of 300 nm. It turns out that the attenuation is much higher in the sputtered Aluminum film. Afterwards, PMMA thin films used for nanoimprinting with thicknesses between 300 and 600 nm are analyzed. The PMMA thin films are spincoated onto a Silicon wafer and covered with an Aluminum transducer layer. The very good agreement between the measurements and simulations of the stacked samples allows a reliable determination of the attenuation in the PMMA films in the GHz regime.