Evaluating the Robustness of Top Coatings Comprising Plasma-Deposited Fluorocarbons in Electrowetting Systems

Abstract

Thin dielectric stacks comprising a main insulating layer and a hydrophobic top coating are commonly used in low voltage electrowetting systems. However, in most cases, thin dielectrics fail to endure persistent electrowetting testing at high voltages, namely beyond the saturation onset, as electrolysis indicates dielectric failure. Careful sample inspection via optical microscopy revealed possible local delamination of the top coating under high electric fields. Thus, improvement in the adhesion strength of the hydrophobic top coating to the main dielectric is attempted through a plasma-deposited fluorocarbon interlayer. Interestingly enough the proposed dielectric stack exhibited (a) resistance to dielectric breakdown, (b) higher contact angle modulation range and (c) electrowetting cycle reversibility. Appearance of electrolysis in the saturation regime is inhibited, suggesting the use of this hydrophobic dielectric stack for the design of more efficient electrowetting systems. The possible causes of the improved performance are investigated by nanoscratch characterization.

Keywords:

• Electrowetting
• plasma fluorocarbon deposition
• hydrophobic coating
• nanoscratch
• adhesion strength

Acknowledgements

The research leading to these results received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement No. (240710). The authors wish to thank Drs E. Gogolides and A. Tserepi at the Institute of Microelectronics, NCSR ‘Demokritos’, for their expert advice in plasma-deposited fluorocarbon films and cleanroom processing. The authors also wish to thank Drs Panagiota Petrou and Sotirios Kakabakos from the Institute of Radioisotopes and Radiodiagnostic Products of NCSR ‘Demokritos’ for providing access to their optical microscope. The authors would like to thank Prof. T. Krupenkin and Dr J. Ashley Taylor of the University of Wisconsin-Madison for kindly providing samples with Cytop^® hydrophobic top coating.