[Zurück]

J. Hell, M. Chirtoc, C. Eisenmenger-Sittner, P. Kijamnajsuk, M. Kitzmantel, E. Neubauer, K. Zellhofer:
"Interface modifications in the Cu-Diamond system";
Vortrag: 15th International Conference on Thin Films (ICTF 15), Kyoto; 08.11.2011 - 12.11.2011.

Introduction and motivation
Modern technical and industrial processes tend towards operation in extreme environmental conditions. High performance systems constantly increase the requirements for the used materials. One possibility to meet these developments is the application of new composite materials, as metal matrix composites (MMCs) which can be implemented as advanced heat sink materials in various processes. The impact of high power densities as in modern CPUs, particle accelerator beam lines or nuclear fusion applications indicate the demand for effective cooling devices.
The combination of copper and diamond makes the resulting material a promising candidate for effective heat sinks as both constituents exhibit excellent thermal conductivity (Cu: ~400 W/m2K, Dia: ~2000 W/m2K [1]). The embedding of diamond particles as reinforcements into a Cu matrix reduces the high coefficient of thermal expansion (CTE) of Cu (CTECu: ~16 ppm/K, CTEDia: ~1,5 ppm/K). Moreover, to avoid thermal failure, the CTE of the device to be cooled can be matched by varying the volume fractions of Cu and diamond. Unfortunately the thermodynamic immiscibility of Cu and C results in low mechanical adhesion and a high thermal contact resistance (TCR) at the Cu-diamond interface. To overcome this issue thin niobium or boron interlayers may be a solution.
Experimental Procedures
To model the surface of the reinforcing particles, synthetic single crystal diamonds of plain geometry were used as substrate material. Thin boron or niobium films were deposited by RF- or DC magnetron sputter deposition, respectively. These interlayers were covered by a copper top layer deposited by DC magnetron sputtering. A selection of the samples were subjected to thermal treatment for 30 min. at 800° C under high vacuum (HV) conditions. The thermal interface of these samples was characterized by modulated infrared (IR) radiometry, where thermal waves in the sample are excited by a modulated laser beam. Variations in the amplitude and the phase shift of the IR radiation emitted by the sample can be associated with values for the thermal contact resistance (TCR) by fitting the data with theoretical models [2].
Results and Discussion
For all samples the TCR was found to be lower for the thermally treated samples than for the untreated ones. While the sample without interlayer had a TCR of 5∙10-8 m2K/W after heat treatment the samples with the 5 nm B and 5 nm Nb interlayer both exhibited values of 2∙10-8 m2K/W. In addition, the TCR of the thermally treated sample with 5 nm Nb interlayer was reduced to 5% of the TCR of the untreated sample. These results show that even very thin interlayers are able to influence the mechanical and thermal interface of the copper-diamond system. Interdiffusion of B and Nb into the Cu coating and the diamond substrate results in a manipulation of the wetting behavior of Cu at the Cu-diamond interface. Moreover, the heat conduction by phonons in diamond may be matched by Nb as it is a carbide forming metal which additionally strengthens the mechanical interface. In the case of Boron, apart from carbide formation, the semiconducting properties of boron may gradually shift the heat transport mechanism from the electronic one of the highly conductive Cu to the phononic one of electrically insulating diamond.
Acknowledgement
This work was supported by the Austrian Science Fund (FWF), grant no.: P-19379
References
[1] R. Zehringer, A. Stuck, T. Lang, Solid State Electronics, Vol. 42, No. 12 (1998), 2139-2151
[2] D. Schäfer et al., Surface and Coatings Technology, 205 (2011), 3729-3735