Dielectric breakdown and failure of anodic aluminum oxide films for electrowetting systems

We study electrical properties and breakdown phenomena in metal/aluminum oxide/metal and electrolyte/aluminum oxide/metal contacts, with the aim to achieve a better understanding of failure modes and improve the performance of model electrowetting systems. Electrical conduction in anodic aluminum oxide dielectrics is dominated by the presence of electrically active trapping sites, resulting in various conduction mechanisms being dominant within distinct voltage ranges until hard breakdown occurs. Breakdown voltage depends on its polarity, due to the formation of a p-i-n junction within the oxide; such asymmetric behavior tends to disappear at larger oxide thickness. Electrolyte/dielectric contacts present an even more pronounced asymmetry in breakdown characteristics: a cathodic bias results in breakdown at low voltage, while under anodic bias high field ionic conduction starts before breakdown occurs. These phenomena are interpreted in terms of electrochemical reactions occurring at the surface: cathodic processes contribute to oxide dissolution and failure, while anodic processes result in additional oxide growth before breakdown.