Thin-film interactions and their relevance to electronic packaging
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I. INTRODUCTION Thin-film structures have been increasingly used in various aspects of electronic industries. Devices, packaging, bonding, protective coatings all employ thin metal films in different structures. Interactions involving such metals, therefore, constitute a challenge to both science and technology. Understanding the interactions and application of such knowledge to technology can be exciting and rewarding. For electronic packaging the yield and performance of the products fabricated can often be limited by an undesired outdiffusion, with subsequent oxidation on the surface, and extensive interdiffusion in the metal films used. Furthermore, the interactions can be profoundly affected by the ambient gases used during the fabrication and subsequent operation of the products. In this paper we will describe several aspects of thin-film interactions: the ambient effects, interactions across a diffusion barrier, and effect on solder spread. The ambient effects on metal-metal and metal-semiconductor interactions are reviewed in order to provide information on reaction mechanisms at interfaces, which is difficult to obtain otherwise. Studies on the reaction across a diffusion barrier help choose such a layer for different applications. The effect of undesired outdiffusion on the solder spread provides an example that connects thin-film interactions with materials joining important to packaging.
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FIG. 1. Auger spectrum of the as-deposited Au/Cu (2000 A/2000 A/SiO 2 ), showing outdiffused Cu on the gold surface.
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The ambient effects on thin-film interactions are illustrated using the metal-metal interactions. Take the Au/Cu/Cr structure, a widely used connection metallurgy in packaging, for example. The Cr layer is used for adhesion, Cu for the electrical conduction, and Au for
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II. AMBIENT EFFECTS ON THIN-FILM INTERACTIONS
J. Mater. Res. 2 (5), Sep/Oct 1987
the protection of Cu from oxidation. Ideally, one would like to have no reaction between these layers, with complete protection of the Cu layer by Au. In reality, however, Cu outdiffuses to the Au surface and becomes oxidized there. The oxide thus formed interferes with the subsequent joining or bonding process and is clearly undesirable. Furthermore, the outdiffusion of Cu through Au can be enhanced by the ambient used, since it is higher in air than in vacuum.l In fact, upon a short storage in air, the as-deposited Au/Cu sample shows outdiffused Cu on the Au surface,2 as shown in Fig. 1. The ambient effect is further manifested by lower interdiffusion between Au and Cu in hydrogen than in forming gas ( N 2 - H 2 9:1 ), 2 as shown in Fig. 2. Similar ambient effects have been observed for other systems: an enhanced outdiffusion by air and oxygen through a Au layer has been observed for Cr,3 Ni,4 Fe, 5 Ti, 6 Pd,6 Si,7 Ge, 8 and GaAs, 9 an
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