Thin Nano- and Microcrystalline CVD Diamond Films for Micro-channel Cooling: Thermal and Elastic Properties
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Thin Nano- and Microcrystalline CVD Diamond Films for Micro-channel Cooling: Thermal and Elastic Properties Robbe Salenbien1, Jan Sermeus1, Paulius Pobedinskas2, Christ Glorieux1, Ken Haenen2,3 1 Katholieke Universiteit Leuven, Departement Natuurkunde, Laboratorium voor Akoestiek en Thermische Fysica, Leuven, Belgium 2 Hasselt University, Institute for Materials Research (IMO), Diepenbeek, Belgium 3 IMEC vzw, Division IMOMEC, Diepenbeek, Belgium ABSTRACT Thin nano- to microcrystalline diamond (N/MCD) films were deposited on silicon substrates using plasma enhanced microwave chemical vapor deposition. Selected layers were covered with a thin metal layer of Cr to enhance their optical absorption characteristics for photothermal and photoacoustic experiments. A heterodyne diffraction method was used to investigate the thermoelastic signatures of the N/MCD layers. While the dispersion of surface acoustic waves turned out to be difficult to determine due to high optical scattering from the diamond crystallites, it was found that a diamond film of ~ 2 μm thick is enhancing the thermal diffusion along the surface. INTRODUCTION Diamond is a material that possesses an unusual combination of several properties. While being electrically isolating it has the highest thermal conductivity at room temperature of all known materials (~2200 Wm-1K-1 for single crystal IIa diamond). However, due to size limitations, cost effectiveness and difficulties in processing, the use of single crystal diamond for thermal applications excludes the use of single crystal films. Plasma enhanced microwave chemical vapor deposition (PE MW CVD) is a technique that enables the deposition of thin films, coating flat substrates but also more complex three dimensional structures. Although thermal conductivity values for CVD diamond have been reported to reach that of natural diamonds [1], these reports usually deal with flat, polished, freestanding films of several hundred micron thickness [2]. This limits the use to in-plane heat spreaders, sometimes connected to a more conventional heat sink material. The reason to use such thick films can be found in the specific growth mechanism of diamond, leading to larger grains and less grains boundaries when a film gets thicker. The properties of the grains will approach those of single crystal diamond, while the reduced amount of grain boundaries will limit the detrimental effects connected to their presence. Nevertheless, when an application like micro-channel cooling is envisaged, the use of sub-micron films is preferred in order to minimize deposition times while retaining an acceptable surface roughness, as polishing of 3D structures is not an option. A recent trend in diamond research focuses on the deposition of polycrystalline diamond films with a better control of the grain size and form, going down to nanometer sized grains, forming so-called nanocrystalline diamond (NCD) films [3]. Up to now, literature contains few reports on the thermal properties of such material. A paper by Philip et al. reports th
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