Cu/Diamond Composites Produced By Pulse Plasma Sintering Technique

The rapidly advancing miniaturization of micro-electronic devices leads to a considerable increase of the amount of heat evolved by electronic circuits. It is anticipated that, in the current decade, it will reach the limiting value possible to dissipate by the materials used at the present. In order to enable the packing density of micro-electronic devices to be further increased, we need new materials of higher thermal conductivity but with a comparable value of the thermal expansion coefficient. 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 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 modified 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 degrees C, the pressure was 60 MPa and the process lasted for 5 to 30 min. The phase composition, density and microstructure of the composites thus obtained were examined. The PPS-consolidated composites had a relative density of 98% and the diamond particles were distributed uniformly within the copper matrix. No graphite was found at the Cu/diamond interface (the composite consolidated at a temperature of 900 degrees C for 30 min). Improvements in properties of the composites were achieved using copper alloy with chromium to increase the interfacial bonding in Cu/diamond composites. The Cu0.8Cr/diamond composite was characterized by a strong bond between the diamond and the copper matrix which was due to the chromium carbide transition layer formed there.