Reliability of indium tin oxide thin films: Properties under mechanical and thermal loads

One of the primary applications of indium has been in the form of tin doped indium oxide (ITO). ITO is the preferred transparent conducting layer in many applications, such as liquid crystal displays, plasma displays, touchscreens, and photovoltaics. The high electrical conductivity, optical transmittance and moisture resistance make ITO a dominant transparent conductor. Recently, there has been a tremendous rise in production of portable electronic devices and a necessity of flexible devices is increasing more than ever. To produce flexible devices, flexible substrates should replace conventional rigid substrates. Although ITO has a low sheet resistance and very good transparency, ITO thin film deposited on a flexible substrate produces cracks at low tensile strains. This failure is due to the brittleness of ITO. Therefore, the mechanical stability of ITO on flexible substrates should be thoroughly considered in the design and manufacturing of flexible devices. Furthermore, the thermal expansion coefficient mismatch between the ITO film and the substrate creates cyclic internal stress on the film when the device is subjected to fluctuating temperatures. This could create micro-cracks and consequently damage the film as the micro-cracks propagate throughout the film over a prolonged exposure to the fluctuating temperatures. In this chapter, the effects of mechanical loads such as bending and tensile loads are investigated and discussed. In addition, the effect of mechanical and electrical properties of the ITO films exposed to thermal and isothermal conditions are discussed. The film-response to the mechanical and/or thermal loads is detailed in terms of different measures, namely electrical resistance, surface morphology, film composition, film crystallinity, transmittance and failure modes, such as cracks and thermal damage.