Lateral Heat Spreading in Layered Samples
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1172-T04-01
Lateral heat spreading in layered samples Danièle Fournier1, Christian Frétigny2, Mikaël Busson1, Elika Saïdi1, Lionel Aigouy1, Jean Paul Roger1, Nicolas Bergeal1 and Jérôme Lesueur1 1 Laboratoire LPEM, UPR A0005 CNRS / UPMC, ESPCI, 10 rue Vauquelin 75231 Paris Cedex 5, France 2 Laboratoire PPMD, UMR 7615 CNRS / UPMC ESPCI, 10 rue Vauquelin 75231 Paris Cedex 5, France ABSTRACT We describe two techniques dedicated to observe and study the heating of structured materials like micro and nanowires and multilayered compounds. The techniques are thermally modulated fluorescence and thermoreflectance. Thermally modulated fluorescence allows mapping the heating of devices with a sub-wavelength lateral resolution. Thermoreflectance allows deeper physical investigations and can be directly used to determine the thermal conductivity and diffusivity of layered structures. In particular, we will show that by thermally modulating a surface by a point-like source, we are able to determine such quantities for several geometries, taking into account the nature of the substrate (conductive or not) as well as the interface quality between the layers. The experimental results, measured on aluminum thin films of variable thickness and on vanadium dioxide layers are corroborated by an analytical model that analyzes both the amplitude and the phase of the lateral heat diffusion in the structure INTRODUCTION Thermal waves turned out to be a very interesting tool to study various samples without much preparation. In this paper we describe unique setups we have done in our laboratory devoted to thermal imaging and applied them to investigate layered structures. When a thin layer is deposited on a substrate, it is now well established that its physical properties can exhibit large discrepancies with those of the corresponding bulk material. Moreover the interface presents defects which most of the time complicate the investigation of the layer. Optical techniques are well suited to come up to this study. Their advantages are to be non contact, to present a resolution of the order of the wavelength of the light. The investigation of the sample can then be made with spots as small as about one hundred of nanometers. The setups that we have made in the laboratory use an intensity modulated pump beam which creates the thermal waves and a probe beam which is reflected at the surface of the sample. The variation of the reflectivity versus the temperature is a measurement of the local temperature. The scanning of the temperature around the heat source allows determining various thermal parameters after modelling the heat diffusion in the different parts of the sample. We show that our approach in the case of good conductor layer deposited on thermal insulator leads to a quantitative thermal characterisation of the layer: the thermal conductivity can be determined independently of the thermal diffusivity. Moreover if it exists, the thermal resistance appears unambiguously. In the inverse case, poor thermal conductor layer on good conduct
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