Thermal Boundary Conductance between metals and dielectrics

This work consists in an experimental study of the main parameters governing the transport of heat across an interface between two solids, when it is dominated by phonons. The main experimental tool used is Time-Domain ThermoReflectance (TDTR), which is an optical technique able to measure the Thermal Boundary Conductance (TBC) of single interfaces between metallic layers about 100 nm thick and mm-sized, flat dielectric substrates. Emphasis is put on diamond as a dielectric, because the diamond/metal TBC is among the important parameters governing the thermal conductivity of diamond-based Metal Matrix Composites (MMC) with potential applications as heat sink materials. In a first part, TDTR as a technique is reviewed and its main advantages are identified. Experimental parameters that influence it the most strongly are the size of the laser spots used and the thickness of the metallic layer investigated. An experimental procedure is presented, that provides a reliable way of measuring these quantities. In a second part, the effect on the TBC of surface treatments of the dielectric substrate prior to the metallic layer deposition is investigated. On an AlN substrate, it is found that the formation of a native oxide on its surface decreases the thermal transport across its interface with an Al layer. Etching away this oxide improves the measured TBC from 43 to 241 MWm−2K−1. On a diamond substrate, the results are more nuanced. The as-received surface is found to be hydrogen-terminated and has traces of organic contaminants. Ridding the surface of these contaminants and terminating it with hydrogen using an Ar:H plasma treatment improves TBC in a way that depends on the diamond surface orientation, from 23 to 32 ([111] orientation) or 23 to 54 ([100] orientation) MWm−2K−1. Doing so using a mixture of H2SO4 and HNO3 (1:1) at 200°C partly terminates the surface with oxygen and yields a TBC of 125 MWm−2K−1. Forcing a graphitic sp2 termination of the surface by sputtering Ar ions on it further improves TBC, up to 150 MWm−2K−1. Finally, forcing oxygen termination using an Ar:O plasma yields a high TBC of 180 MWm−2K−1. Still on the oxygenated surface, the deposition of the Al layer by DC sputtering instead of evaporation results in the highest TBC value measured between Al and diamond, of 230 MWm−2K−1, an order of magnitude higher than the TBC of 23 MWm−2K−1 between Al and as-received diamond, and thrice as large as the highest value of 90 MWm−2K−1 reported in the literature. All the treatments applied except for the Ar:H plasma are found to yield TBCs independent of the diamond surface orientation. In a third part, some attempts are made to explain the origin of the observed increase in TBC. By comparing the temperature dependence of the TBC between Al on oxygenated and sp2-terminated diamond, we find that the latter case leads to a distinctly different temperature-dependent behavior. This suggests that the presence of oxygen atoms, presumably forming an