Synthesis and characterization of the diamond/copper composites produced by the pulse plasma sintering (PPS) method

The rapidly advancing miniaturization of micro-electronic devices leads to a considerable increase of the amount of heat evolved by electronic circuits. This, combined with the inexorable increase in the clock speed of running such devices, results in skyrocketing power densities in modern devices such as microprocessors and other high-performance chips. It is anticipated that, in the current decade, the power density will reach the limiting value possible to dissipate by the materials used at the present. As predicted by Patrick Gelsinger (Intel CTO), during the next few years the semiconductor industry will be "heating to a meltdown", with the trend of power densities in modern microprocessors literally escalating toward levels found within a nuclear reactor. In order to enable the packing density of micro-electronic devices to be further increased, we need new materials of higher thermal conductivity. Another requirement is that these materials should have a thermal expansion coefficient comparable with that of the microelectronic substrate material so as to avoid damage to the heat sink/substrate joint due to the thermal stresses induced by cyclic temperature variation. These requirements can be satisfied by the diamond/metal composites with the metal matrix of high thermal conductivity, such as e.g. Cu. The thermal properties (conductivity, thermal expansion) of the composites can easily be tailored by modifying the metal/diamond proportion. However, within the temperature range of consolidation of these composites, diamond is a metastable phase and may, during the consolidation, be transformed into its stable phase i.e. graphite. This can be avoided by conducting the process under conditions of thermodynamic stability of diamond, i.e. by applying appropriately high consolidation pressure (4-5 GPa), which however increases the production costs. The authors of the present study experimented with producing copper/diamond composites with 50 vol.% of diamond particles under conditions of thermodynamic instability of diamond by consolidating the composite using the pulse plasma sintering (PPS) method. The process temperature was 900 C, the pressure was 80 MPa and the sintering time was 10 min. The phase composition, density and microstructure of the composites thus obtained were examined. The Cu/diamond PPSconsolidated composites had a theoretical density of 96% and the diamond particles were distributed uniformly within the copper matrix. The major challenge in the development of this kind of composites is to obtain a well bonded interface between the copper and the diamond. To increase the interfacial bonding in the Cu/diamond composites the copper was alloyed with chromium to form Cu0.8Cr. The Cu0.8Cr/diamond composite had a theoretical density of 99.8% and was characterized by a strong bond between the diamond and the copper matrix, which was due to formation the interface between diamond and copper matrix. This paper presents " the results of TEM examinations of this interface and describes the method of preparation of thin foils cut through it using a FIB technique. (C) 2012 Elsevier B.V. All rights reserved.