Nanomechanical and nanotribological properties of hydrophobic fluorocarbon dielectric coating on tetraethoxysilane for electrowetting applications

Low voltage electrowetting (EW) systems are typically made of stacks of an insulating dielectric layer underneath a hydrophobic top coating. Of importance here is the strength of adhesion of the coating on the dielectric, investigated through nanoidentation and nanoscratch testing. Improvement of the adhesion strength of the hydrophobic top coating to the main dielectric was attempted through a fluorocarbon interlayer and the stack exhibited improved adhesion strength proven by nanoscratch testing. The difference between the scratch and post-scratch curve corresponds to the elastic recovery of the films, making nanoscratch testing a reliable technique for defining the elastic and plastic regions of thin coatings. The friction mechanisms in accordance with applied load were determined. Additionally, nanoindentation measurements were performed in order to define the hardness and the elastic modulus of the multilayer structure (thickness of layers of few nm). Moreover, the elastic recovery of the stack was investigated and the residual imprints were revealed through SPM imaging. Nanoscratch can be used for a plethora of tests, where a single scratch is useful for critical load, film adhesion and mar studies. Nanoscratch data, coupled with in-situ images, provide detailed information concerning a materialʹs behaviour under simultaneous normal and lateral stresses. The purpose of this work is to investigate the mechanical integrity of an EW device design i.e. contribution of each layer of the stack to the total mechanical integrity. This design is advantageous since it exhibits resistance to dielectric breakdown, higher contact angle modulation range and improved reliability in multiple EW tests.