Frequency Domain Thermoreflectance

# Frequency Domain Thermoreflectance

Frequency-domain thermoreflectance (FDTR) is a non-contact optical technique capable of a wide range of thermal conductivity measurements for solid materials. FDTR measures the phase lag between a periodic surface heat flux and surface temperature over a range of heating frequencies (100 kHz-200 MHz). The unknown thermal parameters (most commonly thermal conductivity and thermal boundary resistance) are extracted by fitting experimental data to an analytical model. In order to make a measurement, a thin metal layer (typically gold) is deposited on the sample surface. This metal layer, referred to as the transducer, undergoes a temperature-induced change in reflectance due to the periodic heat flux.

Two lasers, referred to as the pump and probe, are required to extract the phase lag. The pump (λ=488 nm) is absorbed within the transducer to create the periodic heat flux. This heat flux creates a temperature oscillation which is captured using the probe laser (λ=532 nm). The phase data is obtained by measuring the phase lag between the reflected pump and probe using a lock-in amplifier2,3. In addition to measuring bulk samples, FDTR is capable of measuring thin films (including anisotropic films1) with thermal conductivities ranging from 1 W/m-K up to roughly 1000 W/m-K. FDTR can also measure thermal boundary resistance between the transducer and sample so long as the resistance is larger than approximately 5 m2-K/GW. The laser spot sizes (1/e2 diameter) can range from 5-40 microns. FDTR relies on specular laser reflections so only samples with optically smooth surfaces can be measured.

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Simultaneous measurement of in-plane and through-plane thermal conductivity using beam-offset frequency domain thermoreflectance