Observation of Heat Diffusion across Submicrometer Metal Thin Films Using a Picosecond Thermoreflectance Technique

Abstract

We have observed heat diffusion across submicrometer metal thin films for the first time using a picosecond thermoreflectance method. The boundary between a film and a transparent substrate is heated by a picosecond laser pulse. Heat generated by the pump laser pulse diffuses towards the front surface of the thin film. The temperature change on the front surface opposite to the heated area is probed by the reflectivity of another picosecond laser pulse. Thermoreflectance signals of a molybdenum thin film and an aluminum thin film with nominal thickness of 100 nm deposited on Pyrex 7740 glass substrates were observed at room temperature. Thermal energy transfer inside the molybdenum and aluminum thin films under picosecond heating can be explained by the classical heat diffusion equation. Thermal diffusivity values are close to those of bulk molybdenum and bulk aluminum, respectively.