Deposition method and performance of SiO₂ as a dielectric material for beam steering electrowetting devices

Electrowetting has the potential to be applied to steering and concentrating sunlight using liquid lenses. However, the performance of electrowetting devices depends on the careful selection/fabrication of their constituent dielectric material. The dielectric material should have high optical transparency, chemical stability, high relative permittivity, be relatively thin and have a high breakdown voltage. Recent studies noted that inorganic dielectric materials such as SiO₂ are more suitable for beam steering compared to organic dielectric materials such as fluoropolymers or parylene C for example. When optical transmission is not required, electrowetting devices are fabricated by thermally growing SiO₂ on top of silicon wafers. However, this deposition method is not suitable for beam steering electrowetting devices as the silicon substrate is not optically transparent. In this paper, we investigated the performance of SiO₂ as a dielectric material for beam steering electrowetting devices. Specifically, we measured the contact angle modulation capability of SiO₂ produced by electron-beam and sputtering deposition methods and correlated the results with morphological defects of the SiO₂ layer introduced by the deposition methods. We investigated the effect of annealing and the performance with ionic and DI water. Finally, we compared the performance of SiO₂ with other dielectric materials such as PDMS and Ta₂O₅ in air/water and silicone oil/water medium. (paper)