The effect of the contact point asymmetry on the accuracy of thin films thermal conductivity measurement by scanning thermal microscopy using Wollaston probes

Scanning thermal microscopy is a widely recognized technique nowadays for thermal conductivity measurement of bulk and nanostructured materials. Wollaston probes are presently used in contact or noncontact mode for thermal conductivity measurement. They can be batch or laboratory fabricated and offer an appropriate spatial resolution from a few micrometers to hundreds of nanometers. A study is reported herewith on the errors that can affect the average temperature rise and related probe thermal resistance with a direct impact on thermal conductivity measurement, as a consequence of a contact point asymmetry. The new proposed theoretical model and its results can be used or adapted to any kind and size of probe. The study is based on the fin diffusive heat conduction equation applied on three regions of the probe: left, middle, and right, with respect to the contact point. The thermal conductivity measurement for a thin film on a substrate is simulated and the errors that arise from using an asymmetric contact point are inferred for the three values of the asymmetry. They are next compared to simulations obtained using a simplified model of heat transfer inside the probe and from the probe to the sample. The accuracy of the two models is comparatively analyzed in order to select the optimum one. A primary validation of the asymmetric model is performed using the experimental data from the literature. This analysis can serve as a criterion for the experimental accuracy of the method and improvement possibilities.