Measurement of thermal properties of bulk materials and thin films by modulated thermoreflectance (MTR)

Abstract

Thermal conductivity and thermal diffusivity of materials must be known at high accuracy for thermal engineering applications, in order to understand energy dissipation in devices and engines. Thermal conductivity libraries can be unreliable since their reported values may not reflect the complexities of the samples under investigation, and new materials may not yet be listed. Over the past three decades, modulated thermoreflectance (MTR) has emerged and evolved as a reliable, non-contact, and noninvasive technique to measure the thermal properties of a wide range of bulk and thin film materials and their interfaces. This Tutorial discusses the basis of, and recent advances in, the MTR technique, and its applications in the thermal characterization of a variety of materials. MTR experiments use two micrometer-wide laser beams. Experimental data (amplitude and phase vs the offset between the pump and the probe) give information about heat diffusion around the heat source along several tens of micrometers. Heat diffusion equations are used to fit the experimental data and extract the required thermal properties. Importantly, best fit procedures are not always needed because some analytical approximations provide evidence of the required thermal properties. We present many examples (bulk materials, layered sample, etc.) which illustrate this.