Electro-mechanical performance of thin gold films on polyimide

Thin metal films on compliant polymer substrates are of major interest for flexible electronic technologies. The suitability of a film system for flexible applications is based on the electro-mechanical performance of the metal film/polymer substrate couple. This study demonstrates how a 10 nm Cr interlayer deteriorates the electro-mechanical performance of 50 nm Au films on polyimide substrates by inducing the formation of cracks in the ductile layer. Combined in-situ measurements of the film lattice strains with x-ray diffraction and electrical resistance with four point probe of the Au-Cr and Au layers during uniaxial straining confirmed different electro-mechanical behaviours. For Au films with a Cr interlayer the film stress decreases rapidly as cracking initiates and reaches a plateau as the saturation crack spacing is reached. Crack formation and stress drop correspond to a rapid increase in the film resistance. Without the interlayer the Au film stress reaches a maximum around 2% engineering strain and remains constant throughout the experiment. The film resistance is unaffected by the applied elongation up to a maximum strain of 15%, giving no sign of cracking in the metal layer. The outstanding electro-mechanical performance of the gold film indicates that adhesion layers, like Cr, may not be necessary to improve the performance of ductile films on polymers.