Microstructural influence on the cyclic electro-mechanical behaviour of ductile films on polymer substrates

Highlights: • Electro-mechanical failure of flexible systems is not well-defined. • Combined in-situ testing methods are used to determine lifetimes. • Cu films with different microstructures and interlayers on PI are investigated. • Coarse grains and interlayers reduce lifetime. • New definitions of electro-mechanical failure are presented. - Abstract: When ductile metal films on compliant polymer substrates are strained in tension catastrophic failure can be suppressed by the substrate, thus allowing for their use in flexible electronics and sensors. However, the charge carrying ductile films must be of an optimum thickness and microstructure for the suppression of cracking to occur. Studies of strained films on polymer substrates tend to have more emphasis on the electrical properties and thickness effects than on the film microstructure or deformation behaviour. To address both the electrical degradation and deformation behaviour of metal films supported by polymer substrates two types of combined electro-mechanical in-situ tests were performed. First, is a combination of in-situ resistance measurements with in-situ confocal scanning laser microscopy imaging of the film surface during cycling. The 4 point probe resistance measurements allow for the examination of the changes in resistance with strain, while the surface imaging permits the visualization of extrusion and crack formation. Second, is the combination of in-situ resistance with in-situ X-ray diffraction measurements of the film stresses during cycling. The combination of electrical measurements, surface imaging, and stress measurements allow for a complete picture of electro-mechanical behaviour needed for the improvement and future success of flexible electronic devices.