TENSILE AND FRACTURE-BEHAVIOR OF FREESTANDING COPPER-FILMS

We have studied tensile deformation and crack growth behavior of free- standing, electron beam evaporated copper films suspended over windows etched in a silicon substrate. In-situ straining experiments were per formed on these films using a transmission electron microscope. Extern al tests were conducted on a piezoelectric-driven microtensile system. Macroscopically, these copper films exhibited very low ductility (<1% ). Dislocation activity was limited in regions far From propagating cr acks. We discuss this in terms of how limited film thickness and fine grain size restrict dislocation glide distances and source operation d uring uniform loading. Near stable growing cracks, we observed conside rable local plasticity. We saw evidence of slip activity both within g rain interiors and in grain boundaries near such cracks. Although some dislocations moved very fast, others showed rates much lower than tho se typically measured for bulk copper. Fracture was both intergranular and transgranular, and occurred by linking of microcracks. Microcrack s formed within one grain diameter or so of the main crack, usually wi thin a grain boundary. Linking then look place by the easiest availabl e path. A thickness-limited fracture toughness relation adequately des cribes the observed behavior.