Naoyuki Taketoshi

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.

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

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.

Thermal transport properties of polycrystalline tin-doped indium oxide films

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.

Analysis on thermal properties of tin doped indium oxide films by picosecond thermoreflectance measurement

An attempt of observation of thermal diffusion across tin doped indium oxide (ITO) thin films perpendicular to the film surface has been carried out using a picosecond thermoreflectance measurement. ITO films sandwiched by molybdenum (Mo) films were prepared on fused silica substrate by rf magnetron sputtering using ITO and Mo multitargets. Such Mo/ITO/Mo layered structure was fabricated without exposure to the atmosphere between each deposition. The Mo films with a thickness of 70 nm are necessary because the wavelength of pulse laser used in this study is 780 nm at which wavelength ITO is transparent. The ITO films with a different thickness of 27, 46, and 62 nm were prepared as the intermediate layer in order to estimate thermal resistance at the interface between Mo/ITO. The resistivity, carrier density and Hall mobility of the ITO films ranged from 5.2 to 7.6×10−4Ωcm, 3.5 to 3.9×1020cm−3, and 23 to 35cm2∕Vs, respectively. The interface between the Mo films and the fused silica substrate was irradiate...

Thermoreflectance technique to measure thermal effusivity distribution with high spatial resolution

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.

Measurements of temperature dependence of optical and thermal properties of optical disk materials

We have experimentally determined the temperature dependences of the refractive index and thermal conductivity for several optical disk materials from room temperature to 200–300°C. A novel system for obtaining thermal conductivity in the films is also introduced. As a demonstration, temperature simulation inside a simple-structured disk with and without considerating the temperature dependence of the refractive index at a 405 nm wavelength was carried out, and the difference was approximately 15°C at maximum temperature.

Homodyne detection technique using spontaneously generated reference signal in picosecond thermoreflectance measurements

A new detection technique for picosecond thermoreflectance measurements has been developed. Conventional picosecond thermoreflectance measurements detect the signal amplitude of a reflected probe laser beam synchronized with the modulation frequency of the pump laser beam using a lock-in amplifier. Our new detection technique uses the signal phase of the lock-in output instead of the signal amplitude. Thermoreflectance signals for molybdenum thin films deposited on glass substrates were observed at room temperature. The signal-to-noise ratio of the phase detection was about four times better than that of the amplitude detection. The signal phase detected by this model is insensitive to fluctuation of both pump and probe beams. According to this technique, picosecond thermoreflectance measurements can apply not only to single layer metal thin films of around 100 nm thickness, but also to thicker metal films, metal–nonmetal multilayer thin films, and many other thin films.

Temperature Dependence of the Thermal Properties of Optical Memory Materials

We have measured the temperature dependence of the thermal conductivities of Ge2Sb2Te5 (GST) and ZnS–SiO2 using a nano second thermoreflectance measurement system from room temperature to 500–600 °C. The specific heat capacities of these materials also have been determined from -130 to 500 °C for GST and from room temperature to 600 °C for ZnS–SiO2. The Debye temperature was obtained from specific heat capacity measurement. Using the obtained temperature dependence of the thermal conductivities, a temperature simulation inside a simple structured optical disk with and without considering its temperature dependence was carried out, and the difference in maximum temperature was approximate 80 °C.

Electrical delay technique in the picosecond thermoreflectance method for thermophysical property measurements of thin films

A new picosecond thermoreflectance measurement system using an electrical delay control has been developed in order to extend observation time of temperature response of thin films after pulse heating. The new system uses two picosecond titanium sapphire lasers, one for heating a specimen and the other for probing temperature change on the film surface. Temperature history of the film surface is observed by changing the delay time of the probe pulse from the pump pulse electrically instead of changing the difference of path length between the pump beam and the probe beam. It is not difficult to observe temperature history over longer time than the repetition period of the picosecond laser pulses using the electrical delay technique. The electrical delay technique can get rid of the fluctuation of focused position of the beam on the specimen surface which is hard to eliminate after traveling through the optical delay line of variable path length. Temperature history curves of sputtered tungsten thin films ...

Effect of electrical properties on thermal diffusivity of amorphous indium zinc oxide films

Thermal diffusivity of amorphous transparent conductive films, indium zinc oxide (IZO) films, with a thickness of 200nm has been analyzed quantitatively using a newly developed nanosecond thermoreflectance system. IZO films sandwiched by molybdenum (Mo) films were prepared on fused silica substrate by dc magnetron sputtering using an oxide ceramic IZO target (89.3wt% In2O3 and 10.7wt% ZnO). The resistivity, carrier density, and Hall mobility of the IZO films ranged from 4.2×10−4to22.7Ωcm, from 2.6×1016to4.2×1020cm−3, and from 10to51cm2∕Vs, respectively. The thermoreflectance signals were analyzed based on an analytical solution of the one dimensional heat flow across the three-layered film (Mo/IZO/Mo) system. The thermal diffusivity of the IZO films was (0.6–1.3)×10−6m2∕s, depending on the electrical resistivity. The thermal conductivity carried by free electrons was estimated using the Wiedemann-Franz law. The phonon contribution to the heat transfer in IZO films with various resistivities was found to b...