Copper/diamond Composites for Heat Sink Applications

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Fig. 1: SEM image showing formation of chromium carbide at the surface of a diamond particle.
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Fig. 2: HR-SEM image of the fracture surface of hot- pressed CuCr/60% diamond composite using SE signal.
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Fig. 3: TEM image of the interface, exposed by FIB milling in the CuCr/60% diamond composite and the electron diffraction pattern of the interfacial carbide.

Thermal aspects are becoming increasingly important for the reliability of electronic components driven by continuous progress of the electronics industry. Effective thermal management is therefore a key issue in the packaging of high performance semiconductors. The ideal material for a heat sink and heat spreader should have a coefficient of thermal expansion (CTE) of 4-8*10-6/K and high thermal conductivity. Metal matrix composites (MMCs) offer the possibility to tailor the properties of a metal by adding an appropriate reinforcement phase thus meeting the demands for thermal management.

When diamond particles are embedded in a copper matrix, the interface plays a crucial role in determining thermal conductivity, CTE and mechanical properties of the composite. The ideal interface should provide good adhesion and minimum thermal barrier resistance. Pure liquid copper (Cu) does not wet diamond and pure Cu/diamond composites have been shown to feature weak interfacial bonding; debonding occurs upon thermal cycling. It is well known that alloying of copper with a strong carbide-forming element like Ti, Cr, B or Zr promotes wetting and bonding of diamond. Electrons dominate heat conduction in copper, whereas phonons dominate that in diamond. Hence, for heat conductivity of the metal matrix composite (MMC), energy transfer must occur between electrons and phonons. A very thin interface layer of a carbide phase can presumably aid the necessary electron-phonon coupling.

In order to solve the interface problem between copper and diamond, the use of different carbide formers as alloying elements to the copper matrix is being investigated. High thermal conductivities were achieved with diamond reinforced CuCr matrix composites. Fast pressure-assisted sintering of the corresponding powder mixtures with heating/cooling rates of 100-150 K/min and holding times up to a few minutes result in the most promising thermal conductivity values compared to conventional hot pressing with heating/cooling rates of about 10 K/min. Experimental results confirm that alloying the copper matrix with the carbide-forming element chromium delivers better thermal properties in CuCr/60% diamond composites compared to a pure copper matrix. Without alloying, rather low thermal conductivities of the composite (~ 200 W/mK) were measured indicating high thermal boundary resistance. This is because there is no chemical affinity between copper and diamond, and therefore it is difficult to produce a bond of low thermal resistance and high mechanical strength between the matrix and the reinforcement. A high thermal conductivity of about 640 W/mK along with a CTE of about 7*10-6/K was achieved in the CuCr/diamond composites.

A detailed study of the interfaces was done on diamonds released by simple chemical etching with nitric acid from the composites. X-ray analyses reveal the formation of Cr3C2 as the main reaction product in CuCr/diamond composites. Cr3C2 is the thermodynamically stable reaction product that forms between chromium and carbon. The XRD patterns also reveal a shift of the (112) peak of Cr3C2 indicating a larger lattice parameter b compared to the equilibrium orthorhombic carbide. Using the half peak width a crystal size between 20 and 40 nm in the interfacial carbide was estimated.

SEM pictures using the BSE signal of the interfaces in the released diamonds show carbide formation on all diamond faces – {100} and {111} (Fig. 1). This carbide interlayer with a thickness of about 100 nm is clearly visible in the SE mode using HR-SEM (Fig. 2). TEM observations, performed on FIB processed thin foils, revealed the nanocrystalline structure of the interlayer. Electron diffraction analysis revealed the presence of chromium carbide (Cr3C2) at the Cu/diamond interface (Fig. 3).

EDS element mapping of the interface area of a FIB processed sample shows high chromium content in the carbide interlayer (Fig. 4). These analyses confirmed the continuous nature of the thin carbide layer also observed at the fracture surface.

This work demonstrates that high thermal conductivities are possible with diamond reinforced CuCr matrix composites. Controlling the interfacial reaction results in the formation of a thin Cr3C2 layer, and this is necessary to enable the manufacturing of Cu/diamond heat sinks with high thermal conductivities. Obviously, rapid heating (during directly heated hot pressing) can cause a smaller critical radius and a higher number of nuclei for carbide formation resulting in finer and smoother interfacial structures compared to conventional hot pressing. This presumably correlates with the achieved higher thermal conductivity of the composites.

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Fig. 4: EDS element mapping at the interface – FIB milled specimen of a CuCr/60% diamond composite.
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