Fabrication of tapered SU-8 structure and effect of sidewall angle for a variable focus microlens using EWOD

Electrowetting on dielectric (EWOD) is a promising technology for variable focus liquid microlens. When an electrical field is applied across a liquid droplet and a dielectric layer, the contact angle of the liquid can be tuned with different applied voltages. Therefore, the focal length of a microlens would vary by simply applying an electrical field without any moving mechanical components. In this paper, SU-8 is employed as dielectric layer and structural material in the design of a variable focus liquid microlens using EWOD. A tapered SU-8 structure is created by UV overexposure to confine the droplet and provide the droplet-centering mechanism. The angle of the sidewall is controlled by the width of the exposed area. Contact angle variation of water droplets on SU-8 is measured and favorably compared with the Lippmann–Young equation. The working microlens is demonstrated using images of numbers and laser beam focusing.