Comparison between two processes using oxygen in the Cu/AlN bonding

Abstract

The modern electronics industry uses, at the present time, increasingly high intensities of current which produce large quantities of heat which can be dissipated by using a ceramic substrate, having a high thermal conductivity associated with a high electrical resistivity. Aluminium nitride with a good electrical resistivity and thermal conductivity (between 170 and 220 W m −1 K −1) closed to copper, can fulfil this role on condition that its interface with the metal, generally copper, allows an acceptable thermal conduction. On ordinary conditions, it is not possible to get a direct good junction between Cu and AlN. An eutectic phase is necessary to ensure both thermal continuity and good mechanical resistance. This eutectic phase can be seen on the thermodynamic copper-oxygen phase diagram, and is easily formed at 1065 °C. In all cases, oxygen is brought to the interface by copper oxide Cu 2O and by alumina systematically formed by oxidation at the surface of AlN. Two processes have been investigated: - the layer of Cu 2O is formed by oxidation of Cu by oxygen before the contact; - the copper oxide is formed in situ at the interface during contact. These processes require the control of the contact parameters of copper on aluminium nitride: thickness of Cu 2O formed, partial pressure of oxygen, temperature, contact time, are important parameters of the both processes. Good connections between copper and ceramic have been obtained by these methods. The analysis of the interfaces by electron microscope, highlight in all cases the role of residual oxygen. Ceramic metal adherence, has been evaluated by mechanical tests and the heat transfer through the interface, has been measured by the flash-laser technique. The results lead to values of thermal resistance of contact (1.7 × 10 −6 m 2 W −1 K −1) largely improved compared to commercial assemblies by direct bonding copper (7 × 10 −6 m 2 W −1 K −1).