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High Bond Reliability of Newly Developed Silver Alloy Bonding Wire
Keywords: silver bonding wire, reliability, wire bonding
Wire bonding is a key interconnect technology for the electrical connection between semiconductor chip and outer electrode. In recent years, new wire materials have been rapidly replacing the traditional gold (Au) wire in many applications. Today, copper (Cu) wire, particularly palladium (Pd) coated Cu wire is widely used in LSI packages owing to its high performance and low cost. On the other hand, Cu wire is difficult to use in memory device and other applications with weak bond pad due to higher hardness of Cu compared to Au. In these areas, silver (Ag) wire, with its similar mechanical properties to Au, is growing as a better alternative to Au wire. One of the most important factors in selecting wire material is the long term bond reliability. In Ag wire market, Pd doped Ag wire has been conventionally used. The bond reliability of the Pd doped Ag wire depends on the amount of Pd added. The higher the Pd amount is, the better the bond reliability is. On the other hand, electrical resistivity, which is another important wire property, particularly for advanced memory device, increases in proportion to the Pd amount for the Pd doped Ag wire. Thus, in the conventional Pd-doped Ag wire, bond reliability and electrical resistivity are trade-off relation, and it is difficult to achieve high bond reliability and low electrical resistivity at the same time, which is crucial for some applications. In this paper, a new type of Ag alloy wire is introduced and its bond reliability performance is demonstrated. The new Ag wire is doped with an additive element X, and its electrical resistivity is much lower (2.6u*cm) than the conventional Pd-doped Ag wire (3.8u*cm). Highly accelerated storage tests (HAST) were performed using three different Ag wires of 20um diameter: the new Ag wire, conventional Pd-doped Ag wire, and pure 4N (99.99wt%) Ag wire. The results of HAST show that the lifetime of the new Ag wire is more than twice longer than the Pd doped Ag wire. Additional HAST was also performed using the new Ag wire with even lower electrical resistivity (2.0u*cm), and the new wire still shows a better reliability in comparison to Pd-doped Ag wire of similar resistivity. Cross-sectional analyses of bond interface were also done to clarify the improving mechanism of the new wire, using scanning electron microscope and transmission electron microscope. The high resolution analysis shows that the additive element X is enriched at the bond interface and the growth of a corrosive intermetallic compound (IMC), Ag3Al, which is typically observed at failed bond interface, is efficiently suppressed. The experimental findings indicate that the additive element acts as a diffusion barrier to retard the formation of corrosive IMC. The results of this study demonstrate the superior reliability performance of the new Ag wire while maintaining much lower electrical resistivity compared to conventional Pd-doped Ag wire. The new Ag wire realizes high bond reliability and low electrical resistivity simultaneously. The study also shows the mechanism of corrosion inhibition by the additive element, which is crucial in selecting correct wire material. The new Ag wire is a suitable alternative to Au wire for many applications including advanced memory device.
Noritoshi Araki,
Nippon Micrometal Corporation
Iruma-city, Saitama
Japan


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