Plasticity contributions to interface adhesion in thin-film interconnect structures
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Anna Vainchteinb) and Huajian Gao Department of Mechanical Engineering, Stanford University, Stanford, California 94305-3030 (Received 5 June 2000; accepted 6 September 2000)
The effects of plasticity in thin copper layers on the interface fracture resistance in thin-film interconnect structures were explored using experiments and multiscale simulations. Particular attention was given to the relationship between the intrinsic work of adhesion, Go, and the measured macroscopic fracture energy, Gc. Specifically, the TaN/SiO2 interface fracture energy was measured in thin-film Cu/TaN/SiO2 structures in which the Cu layer was varied over a wide range of thickness. A continuum/FEM model with cohesive surface elements was employed to calculate the macroscopic fracture energy of the layered structure. Published yield properties together with a plastic flow model for the metal layers were used to predict the plasticity contribution to interface fracture resistance where the film thickness (0.25–2.5 m) dominated deformation behavior. For thicker metal layers, a transition region was identified in which the plastic deformation and associated plastic energy contributions to Gc were no longer dominated by the film thickness. The effects of other salient interface parameters including peak cohesive stress and Go are explored.
I. INTRODUCTION
The adhesion of thin-film structures is of significant importance for a range of modern technologies including microelectronic devices and actuators. As device length scales and associated film thickness decrease, the mechanical properties of the thin-film layers are significantly altered from bulk values. This in turn affects the resistance of interfaces to debonding even if interface chemistry and morphology are unchanged. 1,2 The measured interface fracture energy, Gc, has contributions from both the interface fracture process typically dominated by local chemistry and characterized by the “work-of-adhesion” or intrinsic interface fracture energy, Go, as well as other dissipative processes such as plastic deformation in adjacent ductile layers. The relationship between the fracture process and its attendant
a)
Currently at the IBM TJ Watson Research Center, Yorktown Heights, NY 10598. b) Currently at the Department of Mathematics, University of Pittsburgh, Pittsburgh, PA 15260. 2758
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J. Mater. Res., Vol. 15, No. 12, Dec 2000 Downloaded: 12 Mar 2015
stress fields and the extent of plasticity that surrounds the crack tip is a central problem in fracture mechanics. For the case of interface debonding in layered materials, it has been the subject of a number of experimental and modeling studies (see e.g., Refs. 1 and 3–8). Experimental evidence for the significant effect that metal layer thickness has on the interface fracture energy of thin-film structures is shown in Fig. 1.1 The structure is representative of a microelectronic interconnect containing a metallized Al–Cu layer sandwiched between brittle dielectric SiO2 oxides. The Al–Cu layer is separat
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