The influence of the interface in the copper-diamond system

The material system copper-diamond is of technological relevance since such material would combine a high thermal conductivity with a reduced coefficient of thermal expansion. The interface is one of the main critical areas in this system as a result of the different heat conduction mechanism in copper and diamond. Electrons are responsible for the conduction of heat in copper while phonons are responsible in diamond. This is the limiting fact and therefore the theoretical predicted thermal properties cannot be achieved by a simple mixture of copper and diamond. This work will focus in a comprehensive study on the influence of various intermediate layers on the thermal transport across a copper-diamond interface. In a fist step a plane diamond substrate will be used and coated with copper. By introduction of intermediate layers with different thickness and by applying different heat treatments an improvement of the thermal transfer is expected. For the characterization of the thermal behavior of the interface a photothermal method will be used and give information which type of intermediate layer and which type of subsequent heat treatment is necessary to improve the thermal transport properties across the interface. The results achieved from the studies on the plane substrates will be transferred in a second step to the preparation of a copper-diamond composite material with improved thermal properties. A sputter deposition process will be used which allows to deposit different bond layers directly onto diamond particles of various size. The coated diamond particles will be used and compacted to a dense body. The thermal properties will be characterized and allow an indirect assessment of the thermal interface in the composite material. As a result of this research work a clear statement how the thermal transfer across the copper-diamond interface can be modified should be available.

Heat generation in high power electronic components, machining equipment or large scale integrated circuits has become a major issue because of the tremendous increase of power density which was achieved during the last decades. This poses high requirements to new materials in respect to the combination of thermal conductivity and thermal expansion behavior. Heat removal is essential for preventing thermal overload. A low or tunable coefficient of thermal expansion is paramount to prevent mechanical stresses caused by cyclic heating and cooling during operation, which, in the long run, can lead to mechanical failure. Promising material combinations in this respect are copper based composites reinforced with diamond particles. Thermal conductivities of 600 W/mK have been achieved, but the issue of the modification of the thermal transport across the copper/diamond interface has not been studied systematically. Within the present project intermediate layers which optimize the thermal transport between copper and diamond were studied. A wide group of carbide forming materials (B, various borides, Cr, Mo, W and Nb) was intensively investigated on planar diamond or vitreous carbon substrates. Heat transfer across the thermal interface between copper and carbon was studied by infrared radiometry. The connection between the quality of the thermal interface and the morphological evolution of copper under heat treatment was investigated by various experimental techniques. The chemical composition of the interface bearing the interlayers was characterized with high chemical sensitivity and spatial resolution by secondary ion mass spectroscopy and electron energy loss spectroscopy, respectively. Cross correlating the experimental results allowed for the identification of B and Nb as optimum interlayer materials. It could be shown that only very thin interlayers are needed to significantly improve the thermal transfer and that, to a certain degree, results obtained from vitreous carbon samples can be transferred to diamond. Finally, the optimized interlayers were also applied to diamond granulate and copper diamond composites were manufactured. Here pure boron yielded the best results, if it is located at the copper diamond Interface. Therefore, the detailed scientific investigation of possible interlayer materials led to a reliable method to optimize the thermal conductivity in copper diamond composites.