Organic ultrathin film adhesion on compliant substrate using scratch test technique

The mechanical properties of interfaces and more precisely the adhesion are of great importance to understand the reliability of Organic Thin Film Transistor (OTFT) on compliant substrate. Since these devices are flexible, they will undergo a lot of mechanical stress during their useful life. Many adhesion test techniques have been developed to measure adhesion energy of thin films but they are hard to implement in the case of submicronic organic thin film deposited on flexible substrate. Recently, the feasibility and repeatability of the scratch test technique as a tool for testing the adhesion and the damage behaviour of ultrathin film on polymeric substrate have been demonstrated. However, direct comparison of critical load between samples was not straightforward since different failure mechanisms were induced. In the present work, we have performed nanoscratch experiments on submicron thin film deposited on flexible substrate. The use of a tip radius of 5 μm enabled to induce a unique delamination mechanism by localizing and maximizing the stress closer to the interface. We have observed an increase of the critical load on samples processed with an adhesive plasma treatment prior to thin film deposition; confirming the effectiveness of this treatment. We have also performed mechanical ageing tests on specimens and proved that the scratch test technique is sensitive enough to monitor the degradation of the interface properties. Finally, we have discussed some existing energy models. Taking into account some limitations, Laugier’s model gives an upper bound of adhesion energy.