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(Color online) The rupture of a metal film is caused by strain localization. Local thinning leads to local elongation. (a) A freestanding metal film accommodates the local elongation as the ruptured halves move apart. (b) When the film is well bonded to a substrate, the local elongation in the film may be suppressed by the substrate. (c) Debond of the metal film assists rupture.
A -thick Cu film is well bonded to a polyimide substrate and is stretched to a strain of 10%. Except for isolated short cracks, the majority of the Cu film is intact.
A -thick Cu film is poorly bonded to a polyimide substrate and is stretched to a strain of 6%. Channel cracks start to appear at strains about 2%. The density of the channel cracks increases with the applied strain. The Cu strips between adjacent channel cracks easily peel off, indicating the poor adhesion between the film and substrate.
A -thick Cu film is well bonded to a polyimide substrate and is stretched to a strain of 30%. (a) Zigzag cracks appear in the Cu film. (b) Microcracks initiate at angles from the load direction, then coalesce to form the zigzag cracks. (c) Microcracks form by a mixture of local thinning and intergranular fracture.
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