Observation of thermal transfer across a Pt thin film at a low temperature using a femtosecond light pulse thermoreflectance method

Abstract

We report the first systematic temperature responses of thermal energy transfer across a 90 nm Pt thin film at low temperature using a rear-heating front-detection-type (RF-type) femtosecond thermoreflectance measurement system. Thermal energy transfer across the thin film became faster as the temperature decreased from 273 K to 15 K. The obtained thermoreflectance curves showed two thermal energy transfer processes: a fast thermal energy transfer within the initial few picoseconds and a slow thermal energy transfer after a few picoseconds. The classical heat diffusion curves were fitted well to the slow temperature increase after a few picoseconds by introducing a ‘characteristic length’ that characterizes the thermal energy penetration depth at the ‘initial’ incident time instead of the given optical penetration depth. The obtained thermal diffusivities from 100 K to 273 K were consistent with the electrical resistivity of the film measured with the van der Pauw method. The characteristic lengths calculated from this analysis increased with the decrease of the temperature, and were larger than the optical penetration depth. The thermoreflectance signal over the initial several picoseconds is considered to be a non-diffusive process. The new RF-type system can not only give the classical thermal diffusivity of thin films at low temperature, but also new information related to the non-thermal energy transfer process.