Mechanical Properties of Interconnect Metallic Thin Films

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The ULSI technology has brought about a great demand for understanding and controlling of the mechanical properties as well as electromigration (EM) in thin films. Silver may be a candidate for interconnect materials because of the low electric resistivity and an atomically sharp Ag/Si interface. Young's modulus EィイD2fィエD2 of silver films was measured as a function of film thickness, d, in the thickness range between 6 and 150 nm. EィイD2fィエD2 of silver films shows good agreement with the bulk value for films thicker than 50 nm and a decrease with decreasing film thickness below 50 nm, suggesting that the effective thickness of a grain boundary, tィイD2CBィエD2, is about 1.5 nm in a silver mono-metal film. In contrast, we also found that Ag/Pd multilayer films show the supermodulus effects at around the modulation wave length λ=1.9, 2.8 and 3.7 nm where the activation enthalpy for interdiffusion is anomalously high, suggesting that such Ag/Pd multilayer films may be potential materials for h … More eavy duty wiring. We also pursued the effect of alloy elements on the mechanical properties of Al-Si(Cu). Pure-Al and Al-Si(Cu) films prepared by the vacuum deposition show a decrease in Young's modulus for the film thickness below 10 nm and 20 nm, respectively, suggesting that tィイD2CBィエD2 is about 0.5 nm in pure-Al films and about 1 nm in Al-Si(Cu) films. In other words, the addition of Cu alloy element does not suppress the grain boundary diffusion in Al but work as an inhibitor to the self-diffusion of Al. It is also found that Al-Si(Cu) films prepared by RF-sputtering show the supermodulus effects for d below 20 nm where the interatomic spacing shows a decrease with decreasing d, suggesting that the dense grain boundary is mechanically strong. Matter of the fact, the study on the mechanical properties of nanocrystalline Au indicated that Young's modulus of the grain boundary in the dense nano-Au specimens is similar to the bulk value. On the other hand, the study on the collective motion in amorphous alloys suggests that in a nm-scale wiring, the collective motion of fine crystallites would be excited by the high-frequency EM-force. Less