Estimating thin-film thermal conductivity by optical pump thermoreflectance imaging and finite element analysis

We introduce a noncontact experiment method to estimate thermal conductivity of nanoscale thin films by fitting high spatial resolution thermoreflectance images of surface spot heating to a finite element simulated temperature distribution. The thin-film top surface is heated by a 1μm diameter focused, 825 nm wavelength laser spot. The surface temperature distribution in the excited sample is imaged by thermoreflectance microscopy with submicrometer spatial resolution and up to 10 mK temperature resolution. Thin-film thermal conductivity is extracted by fitting a measured surface temperature distribution to a 3D finite element temperature model. The method is demonstrated by estimating thermal conductivity for an isotropic thin-film metal (nickel, 60–260 nm) on a glass substrate. The fitted Ni thermal conductivity was 50 ± 5 W/m K, which is in good agreement with the literature. Also, we present a detailed finite element analysis for an anisotropic thin-film semiconductor sample to show how the method could be extended to estimate thermal conductivity of anisotropic thin films. Advantages of the new method are easy sample preparation (no top surface transducer film or integrated heater required), rapid in situ measurement, and application to a broad range of thin-film materials.