Thermal Transport on the Nanometer Scale and the Effect of Microstructure and Interface Resistance

Abstract

The aim of this research is to provide a better understanding of the physics of phonons involved in thermal transport on nanometer scale and to address the need for systematic information about the thermal properties of ultra-thin films. The present work includes data on thermal transport in dense and porous hydrogen silsesquioxane thin films and thin SiO2 films, across TiN/MgO(001), TiN/MgO(111) and TiN/Al2O3(0001), and in W/Al2O3, Re/Al2O3 and W/B multilayers. The thermal conductivities of low-k dielectric thin films were measured with the 3o method between 80 and 400 K. The strong temperature dependence is not reflected by the minimum thermal conductivity model for homogeneous materials. The differential effective medium model predicts a 1.5 power scaling of thermal conductivity with atomic density, in good agreement with experimental data. The thermal conductances G of TiN/MgO(001), TiN/MgO(111) and TiN/Al2O3(0001), interfaces, measured at temperatures between 79.4 and 294 K using time-domain thermoreflectance, are essentially identical and in good agreement with the predictions of lattice dynamics models and the diffuse mismatch model. Near room temperature G ≈ 700 MW m-2 K-1, ≈5 times larger than the highest values reported previously for any individual interface. For W/Al2O3, multilayers deposited by atomic layer deposition with layers only a few nanometers thick, the high interface density produced a strong impediment to heat transfer, giving a thermal conductivity of ∼0.6 W m-1 K-1. The thermal conductivities of magnetron sputtered multilayers of W/Al2O3, Re/Al 2O3 and W/B decrease with increasing number of layers and the dependence on temperature is similar to that predicted by the diffuse mismatch model. The conductivities of W/Al2O3, and Re/Al 2O3 multilayers were found to be similar, and as low as ∼0.6 W m-1 K-1---suitable for ultra-low conductivity thermal barriers.