Impact of Substrate Rigidity on the Structure of Multilayer Nanoscale ITO Films: Implications for Flexible Electronic Devices

Polymeric substrates have become increasingly important in the recent drive in technology to produce flexible displays and mechanically adaptable devices. Multi-nanoscale layer coatings are often necessary for specific device applications, and these complex coatings are often fabricated by sputtering onto the substrate. Correlating fabrication conditions to bilayer structure and performance is currently investigated by monitoring microstructure via X-ray diffraction and microscopy, but these techniques do not provide an understanding of interfacial structure between subsequent layers. The work presented here investigates the impact of depositing increasingly thick bilayer films of indium tin oxide (ITO) and tungsten (W) on flexible (poly(ethylene terephthalate) (PET)) and rigid (silicon) substrates by utilizing complementary characterization methods of X-ray and neutron reflectivity to study the nanoscale structures (depth profile and interfacial breadth) between layers. Sputtering a W layer onto ITO that resides on a silicon substrate increases the density of the supporting ITO layer as the nanoscale thickness of the W layer increases. Interestingly, an increase in the thickness of ITO leads to a decrease in density and compositional changes, which are dependent on the supporting substrate. Additionally, the interlayer interfaces of the multilayer on PET broaden relative to those in the multilayer that is on the silicon substrate. The compositional, density, and interfacial nanostructure fluctuations are attributed to the difference in rigidity/hardness between the two substrates and the role of substrate rigidity on the sputtering process. This fundamental study defines the influence of substrate properties on coating composition, density, and interfacial structure at the nanoscale—all of which play important roles in the application specific properties and function of the targeted bilayers. The findings from this study have implications on the nanoscale structure in flexible functional thin films used in a wide range of applications such as flexible television and smartphone displays.