Spatially Resolved Thermal Conductivity Measurements Using a Thermoreflectance Microprobe

A transient thermoreflectance technique capable of creating maps of the out-of-plane thermal conductivity of materials with spatial resolution of 5 μm is presented. The applied noncontact optical microprobe uses a thin reflective metal film as temperature transducer and heat reservoir. The parameters of the experimental setup guarantee that the heat flow is one dimensional. Thus, relatively simple mathematical relations can be used to extract the thermal conductivity from the recorded signal shape, which is important for thermal conductivity maps with high pixel count. For reference materials (soda-lime glass and PbTe), the determined thermal conductivities were in good agreement with bulk values, indicating a typical measurement uncertainty of 10%. For low-conductivity substrates, the measuring scheme is relatively insensitive to thermal interface resistance. Thermal conductivity maps of lead-antimony-silver-tellurium surfaces showed distinct features which could be correlated with optical surface micrographs. The local conductivity fluctuations can possibly be attributed to local changes in chemical composition and secondary phase formation.