Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays
The mechanical integrity of tin-doped indium oxide (ITO) thin films sputtered onto a high temperature aromatic polyester developed for flexible display applications was investigated by means of tensile experiments equipped with electrical measurement, and carried out in-situ in an optical microscope. Attention was paid to the influence of ITO thickness, composition and crystalline microstructure, internal stress, annealing, and polymer substrate. It was observed that process-induced internal stresses were systematically compressive, and that tensile cracks in the ITO always initiated at pin-hole defect sites. A transition from stable to unstable crack growth was detected when crack length was several 100 times coating thickness. The occurrence of such a transition, which corresponded to an increase in electrical resistance equal to approximately 10%, indicated that crack propagation controlled the loss of functional performance of the device. It was moreover found that an improved surface quality of the polymer substrate, such as that obtained with planarization hard coats, was a major factor to increase the cohesive properties of ITO thin films. It was also observed that the intrinsic crack onset strain followed classic fracture mechanics scaling, in inverse proportion to the square root of ITO thickness.