Development of pulsed light heating thermoreflectance methods under configurations of rear heating/front detection and front heating/front detection

Thermoreflectance technique holds promise where radiation thermometry faces potential limitations: in temperature measurements around room temperature and of objects requiring sub-micron resolution. Time response of 500 kHz has been experimentally demonstrated. In order to measure thermal diffusivity of thin films, National Metrology Institute of Japan (NMIJ) has developed rear face heating / front face detection picosecond pulsed light heating thermoreflectance methods. This configuration is essentially equivalent to the laser flash method which is the standard method to measure thermal diffusivity of bulk materials. Thermal diffusivity values of thin films can be determined reliably from the thickness of a thin film and the heat diffusion time across a thin film. For application to variety of thin films synthesized on non-transparent substrates, systems have been developed for measuring thermal diffusivity and / or thermal effusivity of thin films under front face heating / front face detection configuration. Area of a diameter of several 10 μm on thin film front face is heated by pulsed light and the same position is irradiated by the probe beam. Then, the history of front face temperature is observed by the thermoreflectance method. In this method, the thermal diffusivity can be calculated from the cooling rate after the pulse heating. In this paper, pulsed light heating thermoreflectance methods under configurations of "rear heating / front detection" and "front heating/front detection" are compared in order to discuss their complementary role, advantages, limitations, and calibration method.