Diffusional breakdown of nickel protective coatings on copper substrate in silver-copper eutectic melts
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INTRODUCTION
DIFFUSION barriers are often required to prevent or control interdiffusion between two dissimilar materials, and therefore to retard the degradation of mechanical or electrical properties of the materials in service. The thickness of such diffusion barriers can range from nanometers to micrometers, depending on their applications. For example, in metallization processes for integrated circuit technology, most thin film diffusion barriers such as TiN and Ti3Si5 are only tens of nanometers thick. Such thin films are needed to minimize the size as well as the electrical resistance of the device. In contrast, the thickness of diffusion barriers for high-temperature oxidation prevention can reach tens of microns in order that these coatings be able to withstand severe environmental attack. Three types of diffusion barriers have been categorized by Nicolet:[1] (a) stuffed barriers (b) passive compound barriers, and (c) sacrificial barriers. Extensive investigations have been undertaken on these diffusion barriers;[2–5] however, most of them emphasized the electrical or mechanical properties of the diffusion products. Diffusion-induced structural and morphological changes were studied on only a few systems. Among them, the interdiffusion behavior of the Cu/(Sn-Ni)/Au diffusion couple was studied by Pinnel and Bennet.[6] They found that above 250 7C, a 12.5-mm Sn-Ni layer disintegrated into fine particles and the gold and copper interdiffused as if the Sn-Ni layer were never present. Tin-rich stringers were found in the gold layer and also at the gold surface. No mechanism for this behavior W.D. ZHUANG, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is Research Engineer, RSM Sensitron, New York, NY 11729. T.W. EAGAR, Professor of Materials Engineering, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology. Manuscript submitted August 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
was determined. More recently, Lin et al.[7] studied diffusional breakdown of a Ag diffusion barrier in a Cu-Ag-Ni diffusion couple at 760 7C. The Ag barrier was breached by successive processes of Cu interdiffusion, interface instability of the Ag-Ni interface, and growth of Cu-Ni protrusions from the Ag-Ni interface to ultimately bridge the Ag barrier. Based on their experimental observations, a model of barrier breakdown was proposed by assuming Nabarro–Herring creep of Ag to accommodate protrusion growth of Cu-Ni solid solution. High-temperature diffusion barriers usually consist of passive compounds. One typical system has been investigated by Walters and Covino.[8] They studied the effectiveness of diffusion barriers in preventing interdiffusion between a molybdenum alloy (TZM), Mo-0.5Ti-0.07Zr0.01C, and electrodeposited platinum coatings. The diffusion barriers TiC, TiN, TaC, TiB2, TiB2/TiC, and TiB2/TiN were tested at 1200 7C, 1300 7C, and 1400 7C for 10, 5, and 2.5 hours, respective
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