Ultrafast thermoreflectance techniques for measuring thermal conductivity and interface thermal conductance of thin films

The thermal conductivity of thin films and interface thermal conductance of dissimilar materials play a critical role in the functionality and the reliability of micro/nanomaterials and devices. The ultrafast laser-based thermoreflectance techniques, including the time-domain thermoreflectance (TDTR) and the frequency-domain thermoreflectance (FDTR) techniques are excellent approaches for the challenging measurements of interface thermal conductance of dissimilar materials. Both TDTR and FDTR signals on a trilayer structure which consists of a thin film metal transducer, a target thin film, and a substrate are studied by a thermal conduction model. The sensitivity of TDTR signals to the thermal conductivity of thin films is analyzed to show that the modulation frequency needs to be selected carefully for a high precision TDTR measurement. However, such a frequency selection, which is closely related to the unknown thermal properties and consequently hard to make before TDTR measurement, can be avoided in FDTR measurement. We also found out that in FDTR method, the heat transport in a trilayer structure could be divided into three regimes, and the thermal conductivity of thin films and interface thermal conductance can be obtained subsequently by fitting the data in different frequency range of one FDTR measurement, based on the regime map. Both TDTR and FDTR measurements are then conducted along with the analysis to obtain the thermal conductivity of SiO 2 thin films and interface thermal conductance between SiO 2 and Si. FDTR measurement results agree well with the TDTR measurements, but promises to be a much easier implementation than TDTR measurements.