The effect of the surface physical properties of polymer substrates on the adhesion and cracking of transparent conductive oxide (TCO) coatings

This work presents the effects of surface physical properties of compliant polymer substrates on the properties of stiff oxide coatings. In this study, polyethylene terephthalate (PET) is selected as the substrate and indium tin oxide (ITO), which is one of the most extensively used transparent conductive oxide (TCO) materials for electrodes in display applications, is sputter deposited on PET varying thickness (h _f). Before deposition, PET surface profiles such as roughness and morphology are modified by Ar ion beam treatment. At a fixed beam power, treatment time is carefully controlled to vary only the physical profiles, excluding chemical states of the surfaces. PET and ITO surfaces and interfaces are analyzed by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. By fragmentation test, the adhesion and cracking of ITO are evaluated. Our investigation reveals that depositing atoms penetrate into the PET surface due to the low density of PET and the initial surface profiles of PET are not maintained during deposition due to its high compliance. When the effective surface area is highly increased by the ion beam treatment, the polymer surface becomes stiffer since the number of penetrating atoms increases. The rough surface features on the stiffer substrate are less deformed during deposition; hence, interfacial interlocking that improves the adhesion becomes effective throughout the deposition.