Measurement of thin film isotropic and anisotropic thermal conductivity using 3ω and thermoreflectance imaging

The 3 omega method is a well established technique for measuring thermal conductivity of thin films and substrates. The method extracts thermal conductivity by measuring temperature response when current flows through a metal strip heater deposited on the material's surface. The metal strip is used both as heat source and temperature sensor. An important factor in the accuracy of 3 omega measurements is that the current should be confined to the metal strip resistor and any leakage to the substrate will invalidate the results. This is because the heat source would no longer be localized on the surface and also because any Schottky behavior at metal/semiconductor interface will create nonlinearities that affect the 3 omega 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 less dependent on the electrical properties of the metal heater. Additionally, the temperature profile near the heat source can be used to ensure 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 cross-plane thermal conductivity in thin films. Preliminary thermoreflectance measurements were analyzed at various locations on the surface of isotropic, InGaAs and anisotropic ScN/ZrN superlattice thin film 3 omega test samples. Experimental results are in agreement with simulated temperature distributions.