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K. Zellhofer, J. Hell, D. Schäfer, C. Eisenmenger-Sittner, E. Neubauer, S. Puchner, H. Hutter:
"PULL-OFF TEST-, SEM-, AFM-, SIMS- AND HEAT-STAGE ESEM INVESTIGATION OF SPUTTER DEPOSITED BORON- AND METAL DOPED BORON INTERLAYER IN THE Cu-C SYSTEM";
Vortrag: 13th Joint Vacuum Conference, Strbske Pleso/Slovakia; 20.06.2010 - 24.06.2010.

In the rapidly developing sector of high-performance-electronics heat generation and conduction plays a decisive role. Due to that fact novel heat sink materials gain more and more in importance. One option to realize such a material are Metal Matrix Composites (MMCs) consisting of a highly thermal conductive matrix, e. g. copper with included reinforcements (e. g. carbon fibers, nanotubes or diamond particles). These inclusions can reduce the high Coefficient of Thermal Expansion (CTE) of copper (CTE copper: 16 ppm/K, various C modifications: approx. 2 ppm/K) to values typical for electronic materials (CTE Si: 2,6 ppm/K). The thermodynamic immiscibility of Cu and C leads to a high thermal contact resistance (TCR) between matrix and inclusion. The insufficient thermal transport across the Cu/C interface may be improved by wetting promoting interlayers. Promising candidate materials for those interlayers are Boron and metal doped Boron.
Substrates of vitreous carbon ("Sigradur G") and diamond were coated by magnetron sputter deposition with B and Cr-, Ti- or Mo-doped boron from a composite target. The amount of included metal was determined by Auger Electron Spectreoscopy and was chosen to be in the range of 3 at% for all materials. Onto these interlayers a Cu film (300 nm) was deposited by magnetron sputtering as well. A sub group of these Cu coated samples was subjected to a heat treatment of 800 °C for 30 min under HV conditions.
The heat treated samples were investigated by pull-off tests and Scanning Electron Microscopy (SEM). In the latter case it was possible to quantify the de-wetting process by determining the areal density of holes formed within the Cu coating after thermal treatment. Further investigations by Scanning Force Microscopy (AFM) affirmed the hole formation process. Different intesities of de-wetting of the Cu coatings could be observed. The comparison of heat treated and non heat treated samples by SIMS analysis gave an idea of the heat activated diffusion process of the interlayer material into the copper coating. A further investigation of non heat treated samples by Environmental Scanning Electron Microscopy combined with an in-situ heating stage (Heat-stage ESEM) showed the progression of the hole formation during the heating process.
The financial support of the Austrian "Fonds zur Förderung der Wissenschaftlichen Forschung" (FWF), Grant No. P-19379-N16 is gratefully acknowledged.