untitledTransparent color pixels using plastic MEMS technology for electronic papers
Y. Taii1,2a), A. Higo1,2, H. Fujita1, and H. Toshiyoshi1,2
1 Institute of Industrial Science, the University of Tokyo
4–6–1 Komaba, Meguro-ku, Tokyo 153–8505, Japan 2 Kanagawa Academy of Science and Technology
3–2–1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213–0012, Japan
a) taii@iis.u-tokyo.ac.jp
Abstract: We present a new type of image display device that could be used as, for instance, over-sized rewritable color posters by using a simple MEMS fabrication technique. The mechanism of making colors is based upon the optical interference of the Fabry-Perot interferome- ter. A thin PEN (polyethylene naphthalate) film with metal half-mirror was laminated over a glass substrate with an optical cavity, and the electrostatic deformation of the film controlled the color of the trans- mitting light. Color pixels of three primary colors (red, green and blue) were successfully demonstrated with driving voltage ranging from 80 to 120Vdc. Keywords: flexible display, electronic paper, Fabry-Perot interfer- ometer, PEN, plastic MEMS Classification: Micro- or nano-electromechanical systems
References
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[6] T. Oguchi, M. Hayase, and T. Hatsuzawa, “Driving Performance Im- provement of the Interferometric Display Device (IDD),” IEEE/LEOS Int. Conf. Optical MEMS and Their Applications, pp. 107–108, 2001.
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system and the CIE 1964 supplementary standard colorimetric system,” 1952.
1 Introduction
Pulp papers have advantages compared with electronic displays in terms of, for instance, thin thickness, light mass, and portability. Clear visibility of pa- per is the most significant attraction to viewers, who tend to read materials on printed papers rather than on computer displays of poorer image reso- lution and contrast. However, a problem is that the consumption of papers does not show a sign of reduction in this information technology age, and it is still giving strong impact to the forestry resources. For this reason, electronic papers are expected to have the performance equivalent or superior to those of the conventional papers before replacing the vast amount of pulp papers.
Various kinds of electronic papers or thin image displays have been de- veloped to meet the wide range of application requirements; for instance, the electronic ink display based on encapsulated electrophoresis developed by E-Ink Corporation delivers superior wide view angle and low power con- sumption [1]. Organic electroluminescence is suitable for making flexible full color displays for motion pictures [2].
Besides crucial requirements such as clear visibility, mechanical flexibility, and low power-consumption, electronic papers need to meet the demands for low-cost productivity and the scalability to large display area. In this report, we present a Fabry-Perot interferometer type of image display element that could be used for over-sized rewritable color posters, by using a simple MEMS (Micro Electro Mechanical Systems) fabrication technique.
MEMS versions of the Fabry-Perot interferometers are commonly used for wavelength tunable lasers [3] and optical modulators [4]. Fabry-Perot interferometers have also been employed in a MEMS display of reflection type [5, 6], in which solid structures are used for micromechanically movable parts and that the structures are developed by delicate photolithography steps. In contrast to this, our approach aims at the mechanical flexibility in future by using a thin plastic film for the deformable Fabry-Perot membrane and also at the scalability to larger display area.
2 Design of MEMS Transparent Color Pixels
Figure 1 shows the layered structure of the developed MEMS transparent color pixels. A bitmap array of the miniature Fabry-Perot interferometer
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constitutes the color images illuminated by a transmitting white backlight. A thin PEN (polyethylene naphthalate) [7] film with an aluminum semi- transparent mirror is placed on the glass substrate with an air-gap in be- tween. The aluminum layers work as optical reflectors and as electrodes for electrostatic operation of the PEN membrane. The air gap spacer is simply made by a photoresist layer of 600 nm thick.
For fabrication, an aluminum reflector/electrode and a color-developing silicon dioxide layer (310 nm thick) were first deposited on a glass substrate by vacuum evaporation and sputtering, respectively. The aluminum layer was only 12 nm thick for acceptable optical transparency. In the next step, photoresist patterns of square openings (600 um x 600 um in area, 600 nm in depth) were defined by the photolithography for the air cavity. On a 16- um-thick PEN film, another metal reflector of 12-nm-thick aluminum was deposited. The PEN film was finally laminated onto the glass substrate.
Fig. 1. Layered structure of the transparent color pixels. An electrostatically deformable plastic membrane with an aluminum mirror is used in the form of the Fabry-Perot interferometer for color development.
3 Operation of Color Development
Figure 2 illustrates the OFF and ON states of the device in operation along with the transmission spectrum obtained by the numerical simulation using OPTAS-FILM [8]. The pictures in Figure 2 show the top view color images taken with the optical microscope with a color CCD camera. When no voltage was applied and the PEN membrane stayed at the flat rest position, the transmitting light looked dark gray to the eyes as shown in Figure 2 (a). When the voltage was applied between two aluminum layers, on the other hand, the upper film was electrostatically attracted to the bottom substrate to squeeze the air gap, resulting in the change of the color. The transmitted light interferes only in the silicon dioxide layer, and it made green color as shown in Figure 2 (b). The attached movie shows the ON/OFF operation
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of an 800-um-size pixel under the voltage of 100 Vp-p of 0.2Hz sinusoidal wave. At operating voltage of 90V to the 600-um-size pixel, the membrane was brought into contact to make a color with the effective area of 62% with respect to the total pixel area. By increasing the applied voltage to 118 volts, the colored area increased to 68%. Aperture at the given voltage (118V) was found to depend upon the pixel size; the 200-um-size pixel exhibited maximum 20% aperture, while the 800-um-size pixel made 78%.
Fig. 2. Schematic illustration of the OFF and ON state of the deformable membrane. (a) Gray color with a suspended membrane with no bias, and (b) green color in electrostatic pull-in at 90 Vdc. A movie file attached.
We designed the thickness of the silicon dioxide to be 310 nm for the green pixels, and the pixels of other colors were designed by changing the silicon dioxide thickness; 240 nm for a blue pixel and 370 nm for a red pixel. Figure 3 shows the three primary colors on the CIE chart [9] that shows the purity and the gamut of the color compared with the standard cathode ray tube (CRT) display. The color was measured by the display tester (Yokogawa Electric. Corp., 3298F). The CIE color coordinates were determined to be (x, y) = (0.31, 0.38) for the pixels under zero voltage, (0.31, 0.56) for the green pixel in operation, (0.36, 0.23) for the red, (0.19, 0.21) for the blue. The backlight was a light bulb of continuous white spectrum of (0.34, 0.37).
The developed green pixel was found to have the purity and the gamut equivalent to those of CRT. On the other hand, the red and blue pixels were found to be still poor in color purity. Optical analysis using the optical spectrum analyzer discovered that the red pixel had two transmission peaks in the visible wavelength range (a red peak at 670 nm and a blue peak at 450 nm), which is believed to be the cause of the degraded color purity. More vivid red and blue colors are currently under development by re-designing
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the layered structure of color-making silicon dioxide. The pixel at the OFF state did not show deep black but gray, as shown in Figure 2 (a), because of the white back light of continuous spectrum. Real black color could be made by using a back light of narrow peaks at red, green, and blue wavelengths such that none of them could pass the transmission channels at the pixel’s OFF state.
Fig. 3. Measured color gamut in the CIE chart for the obtained green, red, and blue pixels. The col- ors are designed by the silicon dioxide thicknesses: 310 nm for green, 370 nm for red, and 240 nm for blue.
4 Conclusion
We have developed a new type of transparent display pixels based upon the MEMS Fabry-Perot interferometer mechanism using an electrostatically deformable plastic film. Three primary colors were successfully demonstrated by pre-defining the thicknesses of the color-making layer of silicon dioxide. The solid glass substrate could be potentially replaced with the flexible plastic film to make a totally flexible electronic paper.
Acknowledgments
The authors thank Teijin DuPont Films Japan Limited for providing us with the PEN films used in this work. This study has been performed in the Op- tomechatronics Project (April 2005 – March 2008) with Kanagawa Academy of Science and Technology (KAST).
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