Development of a thermal diffusivity measurement system for metal thin films using a picosecond thermoreflectance technique

A picosecond thermoreflectance measurement system has been developed in the National Metrology Institute of Japan in order to measure thermal diffusivities of metal thin films. A laser beam from a picosecond Ti-sapphire laser is focused onto the surface of a metal thin film with a spot size of 100 µm and absorbed within the skin depth of the order of 10 nm. Then, heat diffuses towards the opposite side of the thin film one-dimensionally, and the temperature of the heated face decreases over the time scale from ten picoseconds to several hundreds of picoseconds. This ultrafast temperature response is observed with a thermoreflectance method using probe picosecond pulses from the same source of the Ti-sapphire laser. Thermoreflectance signals for aluminium thin films with the thickness of 50 nm, 100 nm, and 500 nm sputtered on Pyrex 7740 glass substrates were observed under the front heating front detection (FF) configuration. We also developed a rear heating front detection (RF) type picosecond thermoreflectance measurement system. Thermoreflectance signals of molybdenum thin films and aluminium thin films with nominal thickness of 100 nm deposited on Pyrex 7740 glass substrates were observed at room temperature under RF configuration. Thermal energy transfer inside the molybdenum and aluminium thin films is dominated by the classical Fourier law. In-plane thermal diffusivities of the thin films are close to those of the bulk materials although out-of plane electrical resistivities measured by the four-probe method are larger than the resistivities of the bulk materials.