Characterization of nanoimprinting polymer films using picosecond ultrasonics

Abstract

A laser acoustic setup is used to determine mechanical properties of polymer thin films which are used for nanoimprinting. Until now mechanical properties like the Young's Modulus or the Poisson Ratio are not well known for these polymers in such small dimensions (100 - 600 nm thickness). The polymer films are spincoated on a silicon wafer and covered with a thin aluminum layer for a better energy absorption of the laser pulses. The measurements are performed on a femtosecond laser pump- probe setup with a collinear beam guidance. This measurement method is contact-free and non-destructive. Mechanical waves are excited and detected thermoelastically using infrared laser pulses of approximately 80 fs duration. The entire experimental setup is simulated numerically: The heat distribution and wave excitation in the thin films caused by the laser pulse, the wave propagation, and the photoacoustic detection. Results of the simulation are shown and a short overview of the simulation procedure is given. With the simulation it is possible to interpret and assign the various measured wave pulses. The laser acoustic measurements are compared with profilometry measurements performed on the same thin film structures in order to quantify the mechanical properties of the polymer films.