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Thermal diffusivity of polycrystalline tin-doped indium oxide (ITO) films with a thickness of 200 nm has been characterized quantitatively by subnanosecond laser pulse irradiation and thermoreflectance measurement. ITO films sandwiched by molybdenum (Mo) films were prepared on a fused silica substrate by dc magnetron sputtering using an oxide ceramic ITO target (90 wt % In2O3 and 10 wt % SnO2). The resistivity and carrier density of the ITO films ranged from 2.9×10−4 to 3.2×10−3 Ω cm and from 1.9×1020 to 1.2×1021 cm−3, respectively. The thermal diffusivity of the ITO films was (1.5–2.2)×10−6 m2/s, depending on the electrical conductivity. The thermal conductivity carried by free electrons was estimated using the Wiedemann–Franz law. The phonon contribution to the heat transfer in ITO films with various resistivities was found to be almost constant (λph=3.95 W/m K), which was about twice that for amorphous indium zinc oxide films.

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.

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.

We have developed an apparatus to measure thermal effusivity distribution in solid materials with a high spatial resolution better than 10μm by the thermoreflectance technique and the periodic heating method. A metal film sputtered on the surface of a sample is periodically heated by a modulated laser beam. The temperature response is measured by using another thin laser beam as a thermoreflectance signal. The thermal effusivity of the sample is derived from the phase lag of the temperature response from the periodic heating. Measurements of a functionally graded material and a fiber composite material are presented as application examples of this thermal effusivity distribution measurement technique.