In-plane Thermal Conductivity Determination in Silicon on Insulator (SOI) Structures Through Thermoreflectance Measureme
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In-Plane Thermal Conductivity Determination in Silicon on Insulator (SOI) structures through Thermoreflectance measurements Max S. Aubain and Prabhakar R. Bandaru Mechanical Engineering Department, Materials Science Program, University of California-San Diego, San Diego, CA 92093, U.S.A. ABSTRACT Heat dissipation in Silicon-On-Insulator (SOI) based microdevices is hindered in the silicon device layer by the low thermal conductivity of the neighboring oxide and reduced in-plane thermal conductivity in very thin layers. This work shows that the in-plane thermal conductivity of a 260 nm thick device layers in SOI substrates can be characterized by measuring the temperature distributions induced by AC joule heating through microfabricated heaters by a scanning thermoreflectance technique. These data were fitted to numerical solutions of the heat conduction equation calculated using COMSOL® Multiphysics modeling software, suggesting the in-plane thermal conductivity of the device layer is reduced to 90±10 W/(m.K), which is consistent with phonon boundary scattering theory predictions. INTRODUCTION Silicon-On-Insulator (SOI) wafers are widely utilized in microdevice fabrication for the advantages offered by the electrical insulation of the device layer from the substrate, allowing for smaller parasitic capacitances, higher device density, and reduced power requirements. Heat dissipation is a central issue to the performance of these devices, which is significantly hindered by low thermal conductivity of the buried oxide and possibly reduced in-plane thermal conductivity in very thin single-crystal silicon device layers [1]. Modeling of thermal conductivity of silicon is typically done by solving the Boltzmann Transport Equation (BTE) for acoustic phonons; thermal energy stored by optical phonons is primarily capacitive and does not significantly contribute to heat conduction due to small phonon group velocity [2]. As phononboundary scattering has been identified to be the primary mechanism by which thermal conductivity is reduced in two-dimensional structures, the BTE was modified to include an additional boundary scattering term for thin films [3]. Single-crystal silicon thin films and SOI based substrates have been extensively measured and modeled within this framework [4-9]. In this study, we demonstrate an experimental method for the determination in-plane thermal conductivity of the device layer in SOI substrates using scanning thermoreflectance. Numerical solutions of the Fourier Heat Conduction Equation obtained with COMSOL® are fit to experimental thermal profiles, implying a reduction in the thermal conductivity of the device layer consistent with BTE models. Thermoreflectance thermometry is a differential optical technique which monitors a relative change in reflectance in a conducting material that is proportional to the thermal variation of the sample. In semiconductors, the complex index of refraction varies as a function of temperature which results in a temperature dependent reflectance value.
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