Zhang and Liang / Front Inform Technol Electron Eng in press 1

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1 Zhang and Liang / Front Inform Technol Electron Eng in press 1 Frontiers of Information Technology & Electronic Engineering engineering.cae.cn; ISSN (print); ISSN (online) jzus@zju.edu.cn Bistable electrowetting device with non-planar designed controlling electrodes for display applications * Han ZHANG, Xue-lei LIANG Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing , China liangxl@pku.edu.cn Received Mar. 19, 2018; Revision accepted May 13, 2018; Crosschecked Abstract: Bistable electrowetting display (EWD) is a promising low power electronic paper technology, where power is consumed only during switching between two stable states, but not required for state maintenance once switched. In this paper, a bistable electrowetting device with non-planar designed controlling electrodes was fabricated by fully conventional photolithography processes. The device has potential for video speed with gray scale controllable display applications. The novel electrode design allows the realization of a lower driving voltage and higher contrast between the two stable states than those of previously reported bistable EWDs with planar electrodes. Key words: Bistable electrowetting, Non-planar, Controlling electrodes, Low voltage, High contrast CLC number: 1 Introduction Electrowetting (EW) is an effect of tuning the contact angle of a liquid droplet to a solid surface by applying an electric field (Frieder and Jean-Christophe, 2005; Jones, 2005). The contact angle can be tuned over 90 o by EW (Lao, 2008) with actuating speeds on a time scale of milliseconds (Hayes and Feenstra, 2003). This capability has made EW the most powerful and popular tool for manipulating tiny amounts of liquids, including generating, moving, merging, splitting and mixing of droplets in microsystems such as digital microfluidic systems (Cho et al., 2003; Huh et al., 2003), adjustable liquid lenses (Smith et al., 2006; Hou et al., 2007; Kuiper and Hendriks, 2004) and lab-on-a-chip devices (Huh et al., 2003; Pollack et al., 2002). EW controlled Corresponding author * Project supported by the National Key Research and Development Program (No. 2016YFA ) and the National Natural Science Foundation of China (No ) Zhejiang University and Springer-Verlag GmbH Germany, part of Springer Nature 2018 droplets can be used as optical switches, which have potential for display applications (Hayes and Feenstra, 2003; Heikenfeld et al., 2009; Zhou et al., 2009; Chen et al., 2011; Shui, et al., 2014). Therefore, a new kind of display technology, electrowetting display (EWD), has emerged in recent years (Heikenfeld et al., 2009; Zhou et al., 2009; Chen et al., 2011; Deng et al., 2017). Compared to existing display technologies, EWDs have advantages over LCDs including low power consumption, wide viewing angles, and high brightness in direct sunlight that is comparable to traditional paper (Frieder and Jean-Christophe, 2005; Deng et al., 2017; Blankenbach et al., 2011). EWDs also surpass the commercialized electronic paper (e-paper) technologies, such as those based on electrophoretic technology (Comiskey et al., 1998), in their capability for video speed and full color mode operation (Hayes and Feenstra, 2003; Blankenbach et al., 2008; You and Steckl, 2010). Prototype EWDs have been reported by research groups and companies (Deng et al., 2017; Blankenbach et al., 2011), and commercialization of this new display technology is being actively pursued (Deng et al., 2017; Jung et al.,

2 2 Zhang and Liang / Front Inform Technol Electron Eng in press 2012; Bitman et al., 2012). Among the developments in current EWD research, the demonstration of a bistable EWD, in which a droplet in the pixel has two stable states, is particularly exciting (Blankenbach et al., 2011; Bitman et al., 2012; Rawert et al., 2010; Yang et al., 2010). Power is consumed only during the switching of the pixel, but is not required to maintain the pixel state once switched. Thus, the number of times that a pixel needs refreshing in a conventional display (Dai, 2010) is reduced drastically, and less power is needed for the bistable display, resulting in longer battery life. Therefore, bistable EWDs are considered a potentially significant technology for e-paper products. In bistable EWDs, droplets are confined in either of two distinct stable states which are electrically switchable (Blankenbach et al., 2011; Bitman, et al., 2012; Rawert et al., 2010; Yang et al., 2010). The two confining states of liquids are usually realized by specially designed pixel structures. Thus, a more complicated design of the device structure is needed, which causes difficulties in the fabrication process. Because of these challenges, research on bistable EWDs lags far behind that of conventional EWDs. The reported performance of EWDs is also not as good as that of normal EWDs (Blankenbach, et al., 2008). Therefore, more effort is needed to push bistable EWD technology towards commercialization. Recently, a bistable EWD was reported by Charipar et al. (2015) in which planar controlling electrodes are used to manipulate liquid switching out-of-plane between two stable states. They also demonstrated multi-color operation by combining two mono-color pixel layers. In their device fabrication process, a laser-based technique was employed to pattern the planar controlling electrodes, which simplified the fabrication process. However, this laser patterning process has very low efficiency and the equipment cost for mass production is very high, which hinders its commercialization. Moreover, there is a very thick dielectric layer with low permittivity between the bottom and top electrodes in their planar electrode design, which means that a very large operating voltage, ~ V, is needed to switch the pixel. Therefore, both the fabrication process and the performance of the device need improvement. In this work, a bistable EW device with a non-planar controlling electrode structure was designed and fabricated by a fully conventional photolithography process. The switching voltage between the two states was reduced and a better pixel on/off contrast was obtained due to the novel non-planar electrode design. These results push bistable EWDs a step closer to commercial application. 2 Experimental methods The design of our bistable EW device includes a non-planar electrode architecture as shown schematically in Fig. 1(a, b). Polar (water) and non-polar (colored oil) liquid media were sealed between top and bottom glass plates. Separated thick dielectric steps (hereafter called pixels) were fabricated on the bottom plate, forming channels between the pixels. Colored oil can stay stably either on top of the pixels or in the channels, and switching between these two states is controlled by pixel electrodes (G1) and channel electrodes (G2). In our design, pixel electrodes are on top of the thick dielectric step, i.e., high above the bottom glass plate. U-shaped channel electrodes sit on the bottom plate directly with their two wings standing adjacent to the sidewall of the pixel and extending to its upper edge. The fabrication processes of the non-planar electrode EW devices are illustrated schematically in Fig. 1(e-j). A layer of SU-8 photoresist, ~20 μm thick, is first spin-coated onto the bottom glass plate and then patterned into cuboids by photolithography. The cuboids serve as the step dielectrics of the pixel. To fabricate the pixel and channel electrodes, an 80 nm-thick Al layer is deposited onto the substrate by magnetron sputtering. Then, photoresist (Microposit S1813) is spin-coated and patterned again. The exposed Al is wet etched, while the Al protected by photoresist is left untouched. After photoresist stripping, the control electrodes, G1 and G2, are obtained. To prevent electrolysis, control electrodes in EW devices are usually covered by a dielectric layer. In this work, a 800 nm-thick Si 3 N 4 layer was grown by PECVD on top of electrodes at 80 o C. Then a hydrophobic layer, CYTOP (Asahi, CTL-809M), was spin-coated to yield a ~115 nm-thick layer. The sample was then baked at 180 o C in a vacuum oven for 10 min. to fully cure the SU-8 and enhance the adhesion of the CYTOP layer. The devices were dosed under

3 Zhang and Liang / Front Inform Technol Electron Eng in press 3 water using a syringe filled with colored oil (dodecane with dissolved blue dye) which filled the channels by capillary action. The blue dye (Keystone AU- TOMATE BLUE 8AHF) was purified before being dissolved in dodecane according to a previously reported method (Zhou et al., 2009). Finally, the bottom plate was sealed underwater using epoxy to a top glass plate with unpatterned ITO as top electrodes. For comparison, EW devices with a planar electrode structure, the same as those reported by Charipar et al. (2015), were also fabricated (Fig. 1c-d). The pixel and channel electrodes were in the lowermost layer on the bottom plate, and the dielectric steps were on top of pixel electrodes. The fabrication processes of planar electrode EW devices are similar to those of non-planar devices, as described above, except that in planar devices the pixel and channel electrodes are patterned before the SU-8 steps. Fig. 1 (k, l) shows optical images of fabricated planar and non-planar electrodes, respectively. 3 Results and Discussion Electrode fabrication is obviously the most critical part of the fabrication process of our non-planar designed EW devices. According to Fig. 1, both pixel and channel electrodes are designed to cross the ~20 μm-high SU-8 steps and remain isolated from each other. Thus, Al film must cover the top surface of the SU-8 cuboids through its sidewall continuously down to the channel surface. To accomplish this, magnetron sputtering was chosen for Al deposition, as it is known for its ease of deposition on uneven surfaces. The electrodes were patterned by combining conventional photolithography and wet etching techniques. This is because we wanted to develop a fabrication process compatible with mass production of the non-planar bistable EW devices. Fabrication of non-planar electrode structures is quite routine in micro-electromechanical systems. However, it was challenging for us because of our lack of experience. Fig. 1 Schematic of the structure and working principle of non-planar (a,b) and planar (c,d) bistable EW devices, where voltage is applied to the pixel electrode G1 (a,c) and the channel electrode G2 (b,d). (e-j) Fabrication flow of the non-planar electrode structure. The red arrow in (j) indicates where the oil flows to the pixel from the channel, while the yellow arrow indicates the reverse case. The optical images are fabricated non-planar (l) and planar (k) electrode structures, respectively. The scale bar is 300 μm. The box area in (k) corresponds to the position of an SU-8 cuboid on the electrodes

4 4 Zhang and Liang / Front Inform Technol Electron Eng in press To isolate the pixel and channel electrodes, photoresist should be patterned first for selective wet etching of the unwanted Al film area. To coat photoresist all over the uneven surface, the S1813 photoresist was spin-coated at a relatively low speed, ~300 rpm. Fig. 2a shows top view SEM images of the non-planar electrodes. A zoom-in SEM image at a corner of a pixel is shown in Fig. 2b, which confirms that Al film was deposited continuously and uniformly across the high pixel steps. Fig. 2 c and d show the Al etching results, which demonstrate the pixel and channel electrodes were isolated successfully in the pixel, channel and sidewall areas. Electrical measurement also confirmed the separation of pixel and channel electrodes. In contrast, patterning of planar metal electrodes is much easier by conventional lithography (Fig. 1k). The channel and pixel electrodes were isolated following the outline of the rectangular pixel, but have one corner notched off for both non-planar and planar designs (Figs. 1 and 2). This design helps guide the fluid off the pixel surface, as demonstrated by Charipar et al. (2015). CYTOP (Fig. 1), the oil, when settled, is prevented from creeping along the device surfaces by the water/oil surface tension, and remains stable irrespective of the surface it occupies (Charipar et al., 2015). However, the situation changes once an electric voltage is applied. The colored oil is initially dosed into the channel area of the device via capillary force, and then settles down (Figs. 3 a and a ) due to the water/oil surface tension. The remaining images in Fig. 3 depict the switching processes of the bistable states. Figs. 3 b and b correspond to the off-state of the pixel, where oil stays on the pixel stably without voltage applied to the pixel and channel. Once a voltage is applied to the pixel electrode (G1 in Fig. 1), water wets the pixel and pushes oil into the channels (Figs. 3 c and c ). The pixel is switched to the on-state, and water remains there stably after the voltage on G1 is removed (Figs. 3 d and d ). When an operating voltage is forced on the channel electrode (G2), the non-polar oil does not respond to the applied voltage, while water (the polar medium) is pulled towards the bottom of the channel. This pushes the oil out of the channel to the pixel surface (Figs. 3 e and e ). When the voltage is removed, the water remains in the channel. This is because energy is required for the water to overcome the physical barrier at the edge of the pixel surface. After being pushed onto the pixel, the oil relaxes and spreads over the pixel surface. As Fig. 2 (a) SEM images of fabricated non-planar electrode structure. (b) Zoom-in SEM image of the red box area in (a). This image shows Al was deposited continuously across the SU-8 step. (c) Zoom-in image of the yellow box area in (a). (d) Zoom-in image of the yellow box area in (c), which shows the pixel and channel electrodes are isolated on the sidewall of a pixel After CYTOP coating, the water contact angle on this hydrophobic surface was measured to be ~115 o, which is good for EW operation (Zhang et al., 2017). The oil is immersed in a water medium in an EWD device, which wets the CYTOP surface. Since the entire surface of the bottom plate is covered with Fig. 3 Optical images of the devices switching between their two stable states. (a-f) Non-planar and (a -f ) planar EW devices, respectively

5 Zhang and Liang / Front Inform Technol Electron Eng in press 5 discussed before, the oil remains on the pixel surface stably without the applied voltage and the pixel reaches a stable off-state (Figs. 3 f and f ). Thus, the oil can stay stable either in the channel or on the pixel, and these two states can be switched by applied voltages. These results indicate a bistable system was realized in our non-planar EW devices. Note that because there is residual oil on the pixel, the on-states during operation of the device (Figs. 3 d and d ) are not as good as the initial on-states (Figs. 3 a and a ). This problem is caused by imperfections in the devices, which should be improved in the future. Bistable operations were realized in both the planar and non-planar devices (Fig. 3). However, their performances were very different. The switching voltages of the planar designed devices were usually in the range of 70~140 V, while those for the non-planar devices were much lower at 40~60 V. This is easily explained by differences in the design of the electrodes. In the planar design, in addition to the Si 3 N 4 dielectric layer, the thick SU-8 on the pixel electrode and the non-polar oil layer on the channel electrode reduce the electric field that the water experiences at the interface. Thus, relatively high voltages are required for the water to wet with CYTOP and push the oil out of, or into, the channel. In the non-planar design, the pixel electrode is raised to the top of the SU-8 cuboids. Therefore, the voltage required for driving the oil into the channel is determined only by the thin Si 3 N 4 dielectric layer, and is lower than that required by the planar device. In the case of driving oil from the channel to the pixel, water starts to wet with the CYTOP at the wing of the U-shaped channel electrodes when voltage is applied, as shown by the magenta arrows in Fig. 1a. Then, water is pulled down into the channel along the wing as voltage increases, and oil is pushed to the pixel along the sidewall part of the pixel electrode (as indicated by the red arrow in Fig. 1j). The required driving voltage is also determined by the thin Si 3 N 4 dielectric layer according to Fig. 1a, which is also lower than that of the planar design. The results shown in Fig. 3 confirm the advantages of our non-planar electrode design. The two stable states of the device correspond to the on- and off-states of an EWD. The switching speed and contrast ratio between the on- and off-states are important parameters for a display. Unfortunately, we currently lack appropriate equipment for measuring such parameters. However, we attempted to gain some information about these properties. The bistable operation of our non-planar EWDs was recorded by an accessory CCD camera of an optical microscope. Then, the recorded images were analyzed frame-by-frame to obtain the switching speed. The average brightness of the pixel area was extracted from the frame images, so that the on/off contrast ratio could be estimated. Fig. 4 shows the extracted brightness changes during successive switching by pulse voltages. Once a voltage pulse is applied to the channel electrode, oil in the channel beads up transiently causing a spike in the brightness, and a certain volume of oil is pushed onto the pixel. This switching was completed between two single frames of the video for some pixels, which corresponds to a switching speed of <40 ms (25 frames per second). These results demonstrate our non-planar EW devices have potential for video speed display applications. When the applied voltage is removed, the oil drop relaxes on the pixel surface immediately, resulting in a sharp decrease in brightness and then a gradual decay to a stable value. The total volume of oil that is pushed onto the pixel depends on the number of voltage pulses applied to the channel electrodes, Fig. 4 Pixel brightness change when switched off (a) or on (b). The spike was not captured for some pulses which were too fast. Insets are images of the pixel when the oil is relaxed

6 6 Zhang and Liang / Front Inform Technol Electron Eng in press causing a step change in the average pixel brightness (Fig. 4). These results indicate the gray scale of the pixel can be tuned by voltage on G2 with an appropriate wave form. Corresponding results from the reverse switching are shown in Fig. 4b, where a sharp increase of brightness was observed with a similar switching speed. The contrast ratio was estimated by the extracted maximum darkness and brightness of the two stable states, and 10-20% improvements were observed for the non-planar device compared to the planar device. Though only a rough estimation of the contrast ratio, this confirms that the non-planar devices have a higher contrast ratio than the planar devices. This conclusion can be easily confirmed on inspection of the results shown in Fig. 3. We believe the difference in the on/off contrast ratio is due to the design of the device. The thick SU-8 cuboids (with a relatively low dielectric constant) on the pixel electrodes in the planar design are relocated under the pixel electrodes in the non-planar design. Therefore, the dielectric layers on both the pixel and channel electrodes are thin, uniform and have a high dielectric constant. These factors improve the electric field exerted on the liquid, and hence lower the switching voltage. According to the thickness and dielectric constant of the dielectric layer used in the two designs, the estimated switching voltage of the planar design should be about 4~5 times that of the non-planar design (Verheijen and Prins, 1999), which is roughly consistent with our experimental results. Since the required voltage for driving the oil from the pixel to the channel for the planar device is much higher than that of the non-planar device, there is usually more residual oil on the pixel of the planar device, which lowers the brightness of the on-state (Figs. 3 c, c, d and d ). Charipar et al. (2015) showed that the driving voltage of the planar device can be lowered by reducing the SU-8 thickness. However, a thinner SU-8 leads to a shallower channel and a smaller volume of dosed oil, which reduces the darkness of the off-state and hence lowers the on/off contrast. Though a wider channel can store more colored oil and is helpful for pixel contrast, the pixel aspect ratio is compromised. These problems are easily solved by our non-planar device design, where the driving voltage is determined mainly by the thickness of the Si 3 N 4 dielectrics. Thus, a very deep channel is applicable, which will benefit the darkness of the off-state and hence the on/off contrast without compromising the aspect ratio. The concept of a non-planar bistable EW device was demonstrated in this study. However, a lot more work is needed to complement this idea. The fabrication process should be optimized. For example, photoresist is spin-coated onto the thick structures on the bottom plate at low speed. Though it can be coated to the full surface, the thickness of S1813 is not uniform, especially at the corner of the pixels, which causes difficulties for UV exposure and development. So we doubled the exposure dose to fully expose the photoresist. However, this overdose exposure causes the obtained etching width to be larger than the design. This will have an impact on the patterning results of the electrodes and hence the performance of the fabricated devices. Sometimes, we found the isolation between the pixel and channel electrodes was unsuccessful, leading to device failure. This problem can be solved by dip coating, which is used for coating photoresist uniformly on uneven surfaces (Brinker, 2013). A dip coater has been ordered, so that both photoresist and CYTOP can be uniformly coated in our future fabrication. We believe this will improve the device performance and fabrication yield. Though the switching speed can be <40 ms, we still observed slower switching (~100 ms) in some pixels. This indicates the uniformity of our devices should be improved. Moreover, the oil can be pushed onto the pixel quickly, the oil droplet relaxes more slowly, and the pixel gray scale is related to the number of voltage pulses (Fig. 4). So 3-4 successive pulses were used to switch the pixel on and off to the gray scale shown in Fig. 3. These results indicate the operating voltage signal should be optimized, e.g., by using a high frequency (khz) wave. Further work is planned to improve the overall performance of our bistable EW device to address all these concerns. In this study, we fabricated a bistable EW device with non-planar controlling electrodes through a fully conventional lithographic process, and demonstrated its potential for low power display applications. Due to its novel non-planar design, the switching voltage was reduced and the contrast ratio improved com 4 Conclusions

7 Zhang and Liang / Front Inform Technol Electron Eng in press 7 pared to previously reported bistable EWDs with planar electrodes. These achievements strengthen our research on bistable EW devices, and lay a solid foundation for future development. References Bitman, A., Bartels, F., Rawert, J., Blankenbach, K., Production Considerations for Bistable Droplet Driven Electrowetting Displays. SID Symposium Digest of Technical Papers 43(1): Blankenbach, K. et al., Sunlight Readable Bistable Electrowetting Displays for Indicators and Billboards. SID Symposium Digest of Technical Papers 42(1): Blankenbach, K., Schmoll, A., Bitman, A., Bartels, F., Jerosch, D., Novel highly reflective and bistable electrowetting displays. J Soc Inf Display 16(2): Brinker, C. J., Dip coating. In: Theodor S., et al.(eds.), Chemical Solution Deposition of Functional Oxide Thin Films. Springer Vienna, Vienna, p Charipar, K. M., Charipar, N. A., Bellemare, J. V., Peak, J. E., Pique, A., Electrowetting Displays Utilizing Bistable, Multi-Color Pixels Via Laser Processing. J Disp Technol 11(2): Chen, C. Y. et al., A 3.5-inch Bendable Active Matrix Electrowetting Display. SID Symposium Digest of Technical Papers 42(1): Cho, S. K., Moon, H. J., Kim, C. J., Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits. J Microelectromech S 12(1): Comiskey, B., Albert, J. D., Yoshizawa, H., Jacobson, J., An electrophoretic ink for all-printed reflective electronic displays. Nature 394(6690): Dai Y., Design and operation of TFT LCD panels. Tsinghua University Press, Beijing, China. Jung, H.-Y. et al., Development of Driver IC with Novel Driving Method for the Electrowetting Display. SID Symposium Digest of Technical Papers 43(1): Deng, Y., Tang, B., Henzen, A. V., Zhou, G. F., Recent Progress in Video Electronic Paper Displays based on Electro-fluidic Technology. SID Symposium Digest of Technical Papers 48(1): Frieder, M., Jean-Christophe, B., Electrowetting: from basics to applications. Journal of Physics: Condensed Matter 17(28): R Hayes, R. A., Feenstra, B. J., Video-speed electronic paper based on electrowetting. Nature 425(6956): Heikenfeld, J. et al., Electrofluidic displays using Young-Laplace transposition of brilliant pigment dispersions. Nat Photonics 3(5): Hou, L., Smith, N. R., Heikenfeld, J., Electrowetting manipulation of any optical film. Appl Phys Lett 90(25): Artn Huh, D. et al., Reversible switching of high-speed air-liquid two-phase flows using electrowetting-assisted flow-pattern change. J Am Chem Soc 125(48): Jones, T. B., An electromechanical interpretation of electrowetting. J Micromech Microeng 15(6): Lao, Y., Ultra hight transmission electrowetting displays. MS Thesis, MS thesis, University of Cincinnati, Cincinnati, America. Kuiper, S., Hendriks, B. H. W., Variable-focus liquid lens for miniature cameras. Appl Phys Lett 85(7): Pollack, M. G., Shenderov, A. D., Fair, R. B., Electrowetting-based actuation of droplets for integrated microfluidics. Lab Chip 2(2): Rawert, J., Jerosch, D., Blankenbach, K., Bartels, F., Bistable D3 Electrowetting Display Products and Applications. SID Symposium Digest of Technical Papers 41(1): Shui, L. et al., Microfluidics for electronic paper-like displays. Lab Chip 14(14): Smith, N. R., Abeysinghe, D. C., Haus, J. W., Heikenfeld, J., Agile wide-angle beam steering with electrowetting microprisms. Opt Express 14(14): Verheijen, H. J. J., Prins, M. W. J., Reversible electrowetting and trapping of charge: Model and experiments. Langmuir : the ACS journal of surfaces and colloids 15(20): Yang, S., Zhou, K., Kreit, E., Heikenfeld, J., High reflectivity electrofluidic pixels with zero-power grayscale operation. Appl Phys Lett 97(14): Artn You, H., Steckl, A. J., Three-color electrowetting display device for electronic paper. Appl Phys Lett 97(2): Artn

8 8 Zhang and Liang / Front Inform Technol Electron Eng in press Zhang, H., Yan, Q. P., Xu, Q. Y., Xiao, C. S., Liang, X. L., A sacrificial layer strategy for photolithography on highly hydrophobic surface and its application for electrowetting devices. Sci Rep-Uk 7: p Zhou, K., Heikenfeld, J., Dean, K. A., Howard, E. M., Johnson, M. R., A full description of a simple and scalable fabrication process for electrowetting displays. J Micromech Microeng 19(6): Artn