Measurement of Thin Film Isotropic and Anisotropic Thermal Conductivity Using 3ω and Thermoreflectance Imaging

The 3ω method is a well established technique for experimentally extracting thermal conductivity of thin films and substrates. The 3ω method calculates thermal conductivity by measuring thin film temperature response to a metal strip heater deposited on the material’s surface. The electrical resistance of the metal strip is used as both heat source and temperature sensor. An important factor in the accuracy of 3ω measurements is the fact that current should be confined to the thin film metal and any current leakage to the substrate will invalidate the results. This is because the heat source would not be localized on the surface anymore and also because any Schottky behavior at metal/semiconductor interface will create nonlinearities that affect the 3ω signal substantially. These problems are especially important at high temperatures where thermionic emission of electrons through oxide insulation layer becomes important. In this paper we propose thermoreflectance imaging as an additional method to determine thermal conductivity of thin film materials. Because thermoreflectance measures temperatures optically, the method is not as sensitive to the electrical properties of the metal heater. In addition, the temperature profile near the heat source can be used to make sure that there is no defect in the thin film metal heater. Theory is presented demonstrating thermoreflectance can also be used to measure anisotropic in-plane and crossplane thermal conductivity in thin films. Preliminary thermoreflectance measurements were analyzed at various locations on the surface of isotropic, InGaAs thin film 3ω test samples. Experimental results are in agreement with simulated temperature distributions.