Optical Measurement of Thermal Conductivity Using Fiber Aligned Frequency Domain Thermoreflectance

Abstract

Fiber aligned frequency domain thermoreflectance (FAFDTR) is a simple noncontact optical technique for accurately measuring the thermal conductivity of thin films and bulk samples for a wide range of materials, including electrically conducting samples. FAFDTR is a single-sided measurement that requires minimal sample preparation and no microfabrication. Like existing thermoreflectance techniques, a modulated pump laser heats the sample surface, and a probe laser monitors the resultant thermal wave via the temperature dependent reflectance of the surface. Via the use of inexpensive fiber coupled diode lasers and common mode rejection, FAFDTR addresses three challenges of existing optical methods: complexity in setup, uncertainty in pump-probe alignment, and noise in the probe laser. FAFDTR was validated for thermal conductivities spanning three orders of magnitude (0.1–100 W/m K), and thin film thermal conductances greater than 10 W/m2 K. Uncertainties of 10–15% were typical, and were dominated by uncertainties in the laser spot size. A parametric study of sensitivity for thin film samples shows that high thermal conductivity contrast between film and substrate is essential for making accurate measurements.

Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays

“…S4b for a bare silicon substrate and a silicon substrate with a 50 nm PbS NCA film demonstrate the high sensitivity of the FDTR technique to the presence of the NCA film. The resulting thermal conductivity has an uncertainty of 15-20% due to goodness of fit of the model and input parameter uncertainties (NCA film thickness, NCA heat capacity, Si substrate thermal conductivity, Au film thermal conductivity, Au film thickness, laser spot diameter) 12 . To minimize this uncertainty, we separately measured the thermal conductivities of the Si substrate (using FDTR) and gold film (using either FDTR or the Wiedemann-Franz law through sheet resistance measurements made on a dielectric sample that was coated with Au at the same time as the NCAs).…”

Experimental estimates of in-plane thermal conductivity in FePt-C granular thin film heat assisted magnetic recording media using a model layered system

“…The thermal conductivities of the film stacks in the direction normal to the layers were characterized using the FrequencyDomain Thermoreflectance (FDTR) technique, [16][17][18][19] as illustrated in Figure 1(c), and we take those to be representative of the in-plane thermal conductivity of granular films when the lateral grain size is similar to the FePt thickness in the model samples. To estimate the effect of media grain size on the inplane lateral thermal conduction, the thickness of FePt layer was varied from 14.5 nm to 29 nm.…”

“…( In the FDTR method, 16,18,19 schematically illustrated in Figure 2(a), two continuous wave lasers periodically heat the sample and measure its thermal response to determine the unknown thermal transport properties of the FePt/C stack. The 1/e 2 effective radius of the Gaussian laser spots on the sample surface was measured by knife-edge to be 2.6 6 0.2 lm.…”

“…These values were averaged over 3 measurements at 3 locations on the samples. The reported uncertainty was estimated following the uncertainty analysis described elsewhere 16 assuming 62% uncertainties in the value of q, C p , and thickness of the Au layer, [FePt/C] N /FePt stack and Si substrate; 65% uncertainty in the k th of the Au layer; and 610% in the k th of the Si substrate and laser spot size. The resulting thermal conductivities have uncertainties of 4%-12%.…”

Parametric study of the frequency-domain thermoreflectance technique

“…11 To remove the complexity of lens alignments, fiber-aligned thermoreflectance is also under study. 14 Precise modeling of the heat transport process in the layered-sample system is significant because the reflected signal only has an implicit relationship with the thermal properties of the system. Material properties or other parameters are extracted using an inverse method.…”