Kaichang Zhou

A full description of a simple and scalable fabrication process for electrowetting displays

Electrowetting displays provide a high white state reflectance of >50% and have attracted substantial world-wide interest, yet are primarily an industrially led effort with few details on preferred materials and fabrication processes. Reported herein is the first complete description of the electrowetting display fabrication process. The description includes materials selection, purification and all fabrication steps from substrate selection to sealing. Challenging materials and fabrication processes include dielectric optimization, fluoropolymer selection, hydrophilic grid patterning, liquid dosing, dye purification and liquid ionic content. The process described herein has produced pixel arrays that were switched at <15 V on active-matrix backplanes, and which have individual sub-pixel areas of <50 × 150 µm2. The majority of fabrication processes can conform to liquid-crystal style manufacturing equipment, and therefore can be readily adopted by many display practitioners. Also presented are additional tips and techniques, such as controlling the onset of oil film break-up in an electrowetting display. This paper should enable anyone skilled in displays or microfabrication to quickly and successfully set up research and fabrication of electrowetting displays.

Scalable fabrication of electrowetting displays with self-assembled oil dosing

Scalable fabrication of electrowetting displays with self-assembled oil dosing is reported. Electrowetting pixel structures consisted of flexible substrate/electrode/hydrophobic dielectric/hydrophilic grid. 20 electrowetting display prototypes were constructed with four pixel sizes ranging from 300×900to50×150μm2 and with five grid thicknesses ranging from 16.2to3.3μm. Self-assembled oil dosing was achieved by dip coating the substrate through an oil film suspended on water. The oil films in pixels were geometrically flat. Operating voltage increased with pixel aspect ratio. Electrical capacitance and optical absorption calculations confirm that oil thickness is within ±5% of the hydrophilic grid height.

Recent Progress in Arrayed Electrowetting Optics

Electrowetting devices can now be formed in arrays covering thousands of square centimeters of glass. New research is pointing the way toward exciting applications for laser radar, 3D displays, adaptive camouflage, electronic paper, retroreflector communication and lab-on-a-chip.

High reflectivity electrofluidic pixels with zero-power grayscale operation

Electrofluidic display pixels are demonstrated with zero-power grayscale operation for 3 months and with >70% reflectance. The color of the pixel is changed as electrowetting moves the pigment dispersion between a top and bottom channel. When voltage is removed, a near zero Laplace pressure and a hysteresis pressure of 0.11 kN/m2 stabilizes the position. For 450 μm pixels, an electromechanical pressure of 1.4 kN/m2 moves the pigment dispersion at a speed of ∼2650 μm/s. The predicted switching speed for ∼150 μm pixels is consistent with video rate operation (20 ms). The geometrically sophisticated pixel structure is fabricated with only simple photolithography and wet chemical processing.

Laplace barriers for electrowetting thresholding and virtual fluid confinement.

Reported are Laplace barriers consisting of arrayed posts or ridges that impart ∼100 to 1000 s of N/m(2) Laplace pressure for fluid confinement, but the Laplace pressure is also small enough such that the barriers are porous to electrowetting control. As a result, the barriers are able to provide electrowetting flow thresholding and virtual fluid confinement in noncircular fluid geometries. A simple theoretical model for the barriers and experimental demonstrations validate functionality that may be useful for lab-on-chip, display devices, and passive matrix control, to name a few applications.

Ultra-High Transmission Electrowetting Displays Enabled by Integrated Reflectors

A reflector technique for increasing the transmission performance of a back-lit electrowetting display is presented. The electrowetting display pixel structure consists of an opaque oil film that blocks light transmission. Electrowetting with 5-10 V breaks up the oil film and creates a transmissive area for the pixel. With real-world electrowetting materials and device constraints, the transmissive area typically reaches 60% to 80% of the pixel area. By integrating a simple thin-film reflector between the backlight and the remaining oil film area, the effective transmission can be boosted to >90%. This high efficiency is promising for battery-powered applications and for high-brightness sunlight-legible displays.

Electrofluidic displays: Fundamental platforms and unique performance attributes

— Electrofluidic displays transpose brilliant pigment dispersions between a fluid reservoir of small viewable area and a channel of large viewable area. Recent progress in the technology, a new multi-stable device architecture, and a novel approach for segmented displays that can display pigment without the optical losses of pixel borders is reported. The fundamental aspects of electrofluidics that make it compelling for the next generation of e-paper products is reviewed.

Flat electrowetting optics and displays

Flat electrowetting optics currently include pixel arrays for displays and prism arrays for beam steering. Electrowetting display pixels utilize a colored oil layer that provides high efficiency control of light transmission or light reflection. Electrowetting microprisms tilt the angle of the meniscus between liquids with different refractive index and thereby cause refraction of a light beam passing through the meniscus. Both of these technologies are projected to provide an order of magnitude increase in raw performance compared to liquid-crystal and other technologies. For example, transmissive electrowetting displays are expected to achieve >80% transmission, which far exceeds the ~8% transmission of a commercial liquid crystal display. Electrowetting microprisms have a clear roadmap leading to greater than +/- 45° of continuous beam steering, which surpasses the few degrees of beam steering achieved with electro-optic phased arrays. However, before widespread commercial application can be achieved, a variety of other challenges, such as low-voltage operation, must be solved. Many of these challenges are engineering problems, not fundamental scientific discoveries, and significant technological progress is expected for flat electrowetting optics.

Arrayed electrowetting microwells

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 of ∼30–50V, electrowetting drove the colored oil behind the substrate and the reflection was made dominantly white.

33.3: Flexible Electrofluidic Displays Using Brilliantly Colored Pigments

We have developed a novel electrofluidic technology for displays that employs brilliantly colored, well-saturated pigments in solution, modulated by electrowetting physics. We discuss the operating principles of the display, and demonstrate progress in key areas needed for realizing products, including fabrication on flexible substrates and performance from −28 °C to 80 °C.