High Frequency Measurements of Thermo-Physical Properties of Thin Films Using a Modified Broad-Band Frequency Domain Thermo-Reflectance Approach

We present a new method to perform high-frequency thermoreflectance measurements on thin films, Differential Broad-Band Frequency Domain Thermo-Reflectance (DBB-FDTR). Our method follows BB-FDTR developed previously [Rev. Sci. Instrum. 84, 064901 (2013)], but it does not require the use of expensive Electro-Optical Modulators. In DBB-FDTR, the thermal phase of interest is recovered by performing two measurements: either at different laser focal positions, or at different pump/probe laser offsets. The unwanted contributions to the phase signal are removed by subtracting the measured phases to calculate the Differential Thermal Phase (DTP). The DTP is then fitted to a solution of thermal diffusion equation to obtain the thermo-physical properties of interest. We also show how to mitigate coherent noise. Moreover, by reducing the laser spot size, the SNR can be increased by both increasing the thermal signal magnitude and reducing the coherent noise.
DBB-FDTR allows us to perform measurement up to ~150 MHz, in order to investigate heat transport in thin structures, and study non-diffusive and non-equilibrium dynamics of the heat carriers.