A Variable Focus Microlens Using EWOD on a Tapered SU-8 Structure

PIs: Victor M. Bright, Kamran Mohseni
Student: Yuan-Jen Chang (Richard)

Project field/specialty: Microlens, SU-8

Project Description:

Tunable microlenses, with variable focal length, have attracted significant interest in optical and biomedical applications. Such microlenses do not require any mechanical moving parts. An attractive technology for tunable microlenses is Electrowetting on Dielectric (EWOD). In EWOD, the surface energy of a liquid droplet on a dielectric layer is modified by application of an electric field. Consequently, the droplet's contact angle can change, which results in droplet shape change, and therefore change of focal length. Microlenses using EWOD with metal or glass structure have been applied in the design of miniaturized optical systems, such as mini-cameras for portable electronic devices. SU-8 is a photosensitive material commonly used to form thick, high aspect ratio polymeric microstructures with vertical sidewalls. Due to the thickness of a SU-8 layer, the exposure dosages are different at the top and at the bottom of the layer. A SU-8 structure with a smooth sidewall at a certain angle can be achieved by UV overexposure. Since SU-8 is an epoxy-based negative-tone photoresist, the hole developed after exposure tends to be smaller at the top than at the bottom. SU-8 has favorable mechanical properties, high thermal stability, and high dielectric constant. Therefore, SU-8 is not only suitable as the structure but also as a dielectric material in the liquid lens using EWOD. We demonstrate a variable focus microlens consisting of two immiscible liquids on a tapered SU-8 structure. The densities of the liquids are chosen to be close to each other. Using SU-8 photoresist as a structural material, a wide range of sizes of microlenses can be fabricated. In addition, the SU-8 structures are easily integrated into other microsystems.

The movie below shows the change in the contact angle and surface area of a droplet of water as an electric field is applied.

Funding Source: University of Colorado at Boulder

Publications:

• Y. J. Chang, K. Mohseni, V. M. Bright, "A Tunable Microlens Using Electrowetting on SU-8", 2nd International Conference on Transport Phenomena in Micro and Nano Devices, Barga, Italy, June 11-15, 2006, Accepted for publication
• Y. J. Chang, V. M. Bright, E. Schonbrun, and K. Mohseni, "A Variable Focus Microlens Using EWOD on Tapered SU-8 Structure", Proceedings of Solid-State Sensors, Actuators, and Microsystems Workshop, Hilton Head, South Carolina, June 4-8, 2006, pp.256-259

Last Updated: February 2008