Arrayed electrowetting microwells

K. Zhou, J. Heikenfeld
2008 Applied Physics Letters  
Colored oils and aqueous solutions have been electromechanically pumped in and out of arrayed microwells. The microwells comprised pyramidal pits in Si substrates that were coated with an aluminum electrode and a hydrophobic dielectric. These substrates were then suspended between volumes of water and oil. When colored oil was placed behind the substrate, surface tension forces caused the oil to completely fill the microwells and provide brilliant red coloration to the array. With application
more » ... ϳ30-50 V, electrowetting drove the colored oil behind the substrate and the reflection was made dominantly white. Arrayed electrowetting 1 display pixels were first reported in 2003 by Hayes and Feenstra. 2 These pixel structures utilize a parallel plate capacitor configuration of water/ colored-oil/hydrophobic dielectric/electrode. At no voltage, interfacial surface tension forces between the water/oil/ dielectric result in a water contact angle Y of ϳ170°. Therefore, the oil which has a very small complimentary contact angle of ϳ10°, is spread across the surface and provides coloration. Applying voltage to this system causes the water to electrowet the hydrophobic dielectric, the oil contracts to ϳ20% of its original area, and surface coloration is thereby altered. 3 Challenges for these devices include limited contrast ratio ͑i.e., the oil is always visible on the surface͒ and a short optical absorption length of ϳ5 m in display resolution pixels ͑ϳ100 m͒. These challenges limit the whitestate reflectance and the colored-state saturation, respectively. Reported herein is a novel structure consisting of arrayed microwells. As shown in Fig. 1 , electrowetting is utilized to drive colored liquids in and out of microwell arrays and result in vivid change in the perceived surface coloration. These devices provide a simple means to provide tunable color surfaces, and may prove additionally useful for switchable retroreflectors or photonic crystals. First presented herein are fabrication and experimental details. Next, electrowetting and optical characteristics as a function of voltage are reviewed. Lastly, brief speculation on possible applications and future challenges are discussed. A variety of techniques, such as microreplication, can be utilized to create microwells of tens to hundreds micrometer size in white plastic composites. Solely for purpose of simplicity, Si micromachining was utilized to create the microwell arrays tested herein. 500 m thick ͑100͒ Si wafers were first patterned with a square grid of thermal oxide on the front of the wafer and a continuous coating of oxide on the backside of the wafer. Next, a highly Si/ SiO 2 selective 25% tetramethylammonium hydroxide etch solution was utilized to anisotropically etch pyramidal microwells into the Si substrate. The process was carried out in a temperature controlled oil bath at 75°C and with a cooling top to prevent evaporation of the etchant. The etch process was allowed to progress through the entire wafer and provided sloped ͑111͒ sidewalls of 54.7°. The resulting microwells were ϳ850 m wide on the front side. After microwell formation, the remaining thermal oxide was removed in buffered oxide etchant. Next, an ϳ80 nm Al reflector was evaporated onto the wafer. To provide a dielectric for electrowetting operation, ϳ1 m of Parylene C ͑ r ϳ 3, E bd ϳ 2 MV/ cm͒ was deposited using a Specialty Coating Systems 2010 Lab-Coater. This process involves dimer vaporization, vapor pyrolysis, followed by room-temperature monomer deposition and polymerization on the substrate surface at ϳ0.1 Torr. To impart proper hydrophobicity on the surface, the sample was dip-coated with Cytonix Corp. FluoroPel 1601 V solution, and baked at 120°C for 20 min to form a ϳ0.1 m thick a͒ Electronic mail: FIG. 1. ͑Color online͒ Operation of electrowetting microwells: ͓͑a͒ and ͑b͔͒ photographs and diagrams of operation with colored oil; and ͑c͒ diagrams of operation with colored water. A 3 mm capillary ͑not shown͒ was connected at the outside edge of the two liquid volumes to allow liquid counter flow during operation. See Ref. 12 for supplemental videos of operation for Figs. 1͑b͒ and 1͑c͒.
doi:10.1063/1.2898890 fatcat:xhvbzfigmjgfdejhxulx42vpoy