Influence of Microstructure on Mechanical and Electrical Properties of Indium Tin Oxide Thin Films Deposited on Polymer Substrates

The electrical and mechanical behaviors of 400 nm thick indium tin oxide (ITO) thin films on polyimide substrates were investigated by means of electrical resistivity and synchrotron X-ray diffraction (XRD) measurements during equibiaxial tensile tests. Different microstructures were obtained with a change of the sputtering gas (argon or xenon), the deposition temperature (RT or 100 °C), or the oxygen partial pressure. The prepared ITO films are adhesive, crystalline even at room temperature deposition, and exhibit an electrical resistivity between 0.5 × 10 –3 and 1.5 × 10 –3 Ω·cm. It was observed that the use of argon during deposition leads to increased compressive residual stresses, resulting from the “atomic peening” effect, which induces point defect formation within the film. Results from deformation experiments highlight a dependence of ITO’s elastic anisotropy on deposition conditions, closely associated with the preferential growth orientation of the film. The use of the Van der Pauw method during tensile loading allows for accurate identification of its piezoresistive response, characterized by a slightly negative gauge factor (−5.1 to −2.9). The mechanical and electrical integrity of ITO thin films, essential for their integration in flexible and transparent electronics, was assessed through the determination of the apparent and the intrinsic crack onset strain (COS), the latter accounting for the contribution of residual stresses. We show that compressive residual stresses enhance mechanical strength, but when they originate from point defects, as in this study, their beneficial effect is limited, thereby compromising the electrical and mechanical integrity of the films.