Thermal Diffusivity Measurement of Thin Films by Fourier Transform Thermo-Reflectance Method under Front Heat – Front Detect Configuration

Thermo-reflectance method is one of the few methods which can measure thermal transport properties in the cross-plane direction of thin films. Thermo-reflectance method under front heat – front detect (FF) configuration, so-called time domain thermo-reflectance (TDTR), is commonly used because it can be applied to a wide variety of samples including non-metallic thin films. In FF configuration, both pump beam and probe beam are focused at the same spot on the surface of a sample and temperature response from the surface is observed. In this case, the temperature response is significantly affected by the penetration of pump beam, which depends on the absorption coefficient of the surface. In TDTR method, a metallic layer, sometimes called transducer, is generally deposited on a sample to safely observe thermo-reflectance signals. In most cases, heat diffusion time in the transducer is assumed to be infinitesimal and the layer can be regarded as heat bath with known heat capacity. To determine thermal effusivity of the sample below the transducer, the rate of temperature decrease is analyzed. This means the penetration of pump beam, which affects the initial distribution of temperature, is not considered. This approach has some ambiguity because thermal diffusion in the transducer can significantly affect the determination of the thermal effusivity of the sample. To examine this issue, we propose an analytical approach which considers the effect of penetration of pump beam. Our approach attributes to recent developments in thermo-reflectance method. The electrical delay technique has enabled the technique to observe temperature response in longer timescale than pulse interval of pump laser, which demonstrates the periodic nature of the temperature response. In our recent study, we developed an analytical approach based on Fourier analysis, which can consider the periodicity and improve the robustness and reliability of determination of thermal properties by thermo-reflectance method. Thanks to this new analytical approach, we could also develop a mathematical model which can consider the contribution from the penetration of pump beam, whereas our conventional model assumes that the energy of pump beam is entirely absorbed at the surface without penetration. We could examine how the penetration affects the temperature response and improve the robustness and reliability of determination of thermal properties by thermo-reflectance method under FF configuration. We measured some actual thin films by picosecond thermo-reflectance method under FF configuration. We are going to introduce notable developments in thermo-reflectance method and actual results obtained from these samples, and discuss the robustness and reliability of the determination.