The Frequency Domain Thermoreflectance Technique for Thermal Property Measurements

The frequency domain thermoreflectance (FDTR) technique is a nondestructive method for thermal characterization and imaging with microscale spatial resolution. The technique relies on a pump laser to generate a modulated temperature change in a sample and a probe laser to monitor the sample's local thermal response. The sample's thermal properties are determined by fitting a thermal model to the sample's thermoreflectance response. This article presents detailed protocols for implementing the technique and conducting local measurements of a substrate's local thermal conductivity. Special attention is devoted to discussing how the measurement is influenced by the laser source parameters, error sources in an FDTR measurement, and quantifying the measurement uncertainty. Finally, we discuss a thermal conductivity imaging experiment conducted near a single vertical interface between two surface-activated bonded single-crystal silicon substrates. The thermal conductivity at the interface is suppressed by 3% compared to the bulk value in the adjoining single crystals. The ability to probe interface thermal properties with the FDTR technique may facilitate local studies of heat flow around individual grain boundaries, particularly in thermoelectric materials, where they can be tuned to optimize the performance of thermoelectric devices.