Copper-rubber interface delamination in stretchable electronics

Next generation microelectronic devices will be flexible, rollable and capable of extreme elongations. The latter aspect enables novel applications such as skin-like human body sensors. Stretchable electronics consist of stiff microchips on a highly compliant substrate, circuited by metal wires which must be highly stretchable. Stretchability is achieved using mechanistic patterns. Ensuring interface integrity between the metallic lines and the substrate forms, however, a huge engineering challenge, making interface integrity the key limiting factor in stretchable electronics development. In the present work, interfacial delamination in the copper/rubber model interface system is characterized meticulously from (i) 90º peel tests with high-speed video imaging to obtain adhesion energies and rubber-lift geometry dimensions, (ii) microscopic visualization of the progressing delamination front using in-situ scanning electron microscopy, and (iii) modeling of peel tests using a cohesive-zones enhanced finite elements model. High adhesion energies are achieved for rough copper surfaces with deep ??valleys??. For these interfaces, energy dissipation upon delamination is dominated by the formation, stretching, and rupture of ~20¼m-long rubber fibrils. Fibrillation is triggered by hampering the debonding of rubber from the copper surface through an interplay of local surface area enlargement, complex mixed-mode loading, and mechanical interlocking in the roughness ??valleys??