The crystallinity and mechanical properties of indium tin oxide coatings on polymer substrates

We present the relationship between the microstructure and mechanical strength of indium tin oxide (ITO) on flexible substrates. With varying thickness (h_f), ITO is deposited on polyethylene terephthalate (PET) by dc magnetron sputtering. The microstructure of ITO is controlled by substrate surface conditions and sputtering parameters. The maximum substrate temperature during deposition is limited to 80 °C due to the low glass transition temperature (T_g) of PET. The crystallinity and surface roughness (R_rms) are analyzed by high resolution x-ray diffraction, high resolution transmission electron microscopy, and AFM. The crack resistance of ITO is evaluated by uniaxial tension test. The experimental results reveal that, at a fixed h_f, the degree and quality of crystallinity of ITO are highly improved by increasing sputtering power and the substrate temperature. As the crystallinity is improved, the ratio of preferred growth orientations of (222) to (400) is increased and the degree of peak shifts to lower 2θ is decreased. They indicate that the crystallinity is improved as the lattice damage is reduced and film density is increased. The tension test results confirm that, up to a certain h_f, the strength of ITO can be significantly enhanced by improving the microstructures.