Understanding Thermal Transport in Graded, Layered and Hybrid Materials

Characterization of Cu/diamond interface thermal conductance (hc) and an improved understanding of factors affecting it are important, as Cu-diamond composites are increasingly being considered for electronic packaging applications. In this study, ~90 nm thick Cu layer was deposited on synthetic as well as natural diamond substrates. In several specimens, a Ti-interface layer of thickness ≤ 3.5 nm was sputtered between the diamond substrate and the Cu top layer. The hc across Cu/diamond interfaces for the specimens with and without a Ti-interface layer was determined with time-domain thermoreflectance. The hc is ~ 2× higher for specimens with the synthetic diamond substrate than with natural diamond. The roughness of synthetic diamond substrate is ~ 2 × higher than natural diamond. The surface nitrogen concentration of synthetic diamond substrate is an order of magnitude lower than natural diamond and bulk nitrogen concentration is four orders of magnitude lower in synthetic diamond. These differences in roughness and nitrogen concentration can potentially explain the variations in hc. Furthermore, the hc was observed to increase with an increase of Ti-interface layer thickness. This is explained by invoking Fuchs-Sondheimer (size-effect) theory, which suggests that with an increase of Ti-interface layer thickness, the effective mean free path of electrons in Ti-layer increases with a concomitant increase in its thermal conductivity. 7 Approved for public release; distribution unlimited.