Brazing of aluminum nitride substrates
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Aluminum nitride (AIN) is currently under investigation as a potential candidate for replacing aluminum oxide (A12O3) as a substrate material for electronic circuit packaging. The requirements for such a material are that it can be metallized and joined to produce hermetic enclosures for semiconductor devices. A technique for brazing AIN using a nonactive metal braze has been investigated. The process involves the in situ decomposition of an active metal hydride. This process improves the wetting of the AIN and led to the development of strong bonding between braze and ceramic. The ceramic-braze interface was studied using scanning electron microscopy (SEM). The nature of the interfacial reactions and the reaction products have been identified using x-ray diffraction (XRD). The progress of the reaction has been followed using differential thermal analysis (DTA). The experimental results have been correlated with thermodynamic predictions of the reaction processes. In addition to joining ceramic to ceramic, braze joints of AIN to copper and to a low expansion iron-nickel lead frame alloy were made. Residual stress resulting from a mismatch of thermal expansion coefficients between AIN and copper caused cracking in the ceramic upon cooldown from the brazing temperature. No cracking occurred in the ceramic when joined to the iron-nickel alloy. The results obtained are important for the realization of AIN as a ceramic packaging material for semiconductor devices.
I. INTRODUCTION
The high thermal conductivity of aluminum nitride (AIN), coupled with its expansion matching to silicon, makes it ideal as a substrate material for electronic packaging applications. Before the widespread commercial potential of AIN can be realized, suitable metallization and joining techniques are required. These are necessary for a number of applications: hermetic package sealing, attachment of the integrated circuit, and subsequent interconnection with other circuit components. A number of studies have investigated the brazing of ceramics, and relevant reviews are available.1'2 The majority of these studies have investigated alumina ceramics. Alloy brazes containing an active metal, one capable of changing the chemistry of the ceramic surface, have been found to enhance bonding. The most commonly used active additive is titanium. Titanium forms a series of oxides, of which TiO is the most stable when measured in terms of free energy of formation per mole of oxygen.3 This oxide is better wetted by copper than other less titanium-rich oxides.4 The second most common process is TiH2 activation, in which the hydride is applied as a powder before applying the nonactive brazing alloy. The TiH2 dissociates at 350-550 °C, to form a wettable titanium coating.1 No attempts using "'Current address: Cornell University, Department of Materials Science and Engineering, Bard Hall, Ithaca, New York 14853. 2172
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 10, Oct 1990
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the hydride activation technique on AIN ceramics
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