Mechanics of Thin Films on Polymer Substrates Produced by Roll-to-Roll Manufacturing

lexible electronic devices are currently being developed using polymer substrates. Such devices present advantages like conformability, are light-weight and are expected to be manufactured in roll-to-roll (R2R) schemes accounting for cost-reduction. These advantages may be compromised by the high levels of thermo-mechanical stresses often present in R2R processing, and associated risk of mechanical failure of the fragile devices. A key challenge is therefore to develop a stable process, especially due to the strong mechanical contrast between the compliant polymer substrate and the brittle inorganic device layers. We divide our presentation in three parts. First, a study of the mechanical properties of inorganic films on polymer substrates manufactured in R2R coating units will be presented. Second, we will explore the influence of the mechanical anisotropic effects naturally induced by the R2R processing on film integrity, with focus on residual stress. Third, we shall compare the results obtained in R2R processed structures with those of conventional S2S (sheet-to-sheet) methods. Our study is based on different cases including ITO-based transparent conducting electrode thin films deposited on PET substrates. This work reflects the importance of an accurate control of the mechanical properties and internal stress state of thin films on polymers, based on in-depth understanding of anisotropy issues in R2R processed structures. Finally we will present strategies to improve the mechanical limits in flexible electronic devices.