Link between cracking mechanisms of trilayer films on flexible substrates and electro-mechanical reliability under biaxial loading

The propagation of cracks from a top layer in trilayer systems (Cr/Cu/Mo) on a polyimide substrate is studied experimentally by in situ synchrotron x-ray diffraction under equi-biaxial loading. The results show that depending on the thickness of the ductile Cu middle layer (100 or 500 nm), the propagation can be a direct vertical path through all layers or a more complex path. These effects are analyzed by monitoring the individual stresses of each layer along with electrical resistance and resulting crack patterns. Cracks starting from the upper Cr layer propagate instantaneously through the whole system for a 100 nm Cu layer but are strongly deflected in a 500 nm Cu layer, thus delaying the global fracture of the system measured by the increase of electrical resistance. Mechanisms are proposed and allowed to anticipate the electro-mechanical performances of stretchable systems constructed of several layers.