LCOS Devices for AR Applications

Transcription

1 LCOS Devices for AR Applications Kuan-Hsu Fan-Chiang, Yuet-Wing Li, Hung-Chien Kuo, Hsien-Chang Tsai Himax Display Inc. 2F, No. 26, Zih Lian Road, Tree Valley Park, Sinshih, Tainan County 74148, Taiwan Abstract Liquid crystal on silicon (LCOS) microdisplay takes the advantage of mature CMOS processes and reflective liquid crystal light valves to produce high quality, high information content displays. In this talk, I will introduce the recent technology developments of LCOS devices for augemented reality (AR) applications and how we forecast the market growth in the near future 1. Introduction Wearable technology is spreading all over the body. The growing use of embedded wearable devices connected to a smartphone is spawning a massive industry geared to fitness, health, entertainment and other goals, offering potential benefits to everyone from the newborn infant to the infirm elderly. Those new applications also raise new requirements on display system which is one of the main human machine communication channels. The display system for wearable technologies must be very compact and light weight. More than that, the system efficiency has to be extremely high such that the battery life can last for a long time. Clearly, Microdisplay is a good candidate to fulfill all the requirements. It is very compact and light weight. There are 3 kinds of microdisplay. Self Emission Microdisplay such as Micro-OLED [1, 2], Transmissive Liquid Crystal Microdisplay such as HTPS LCD or Cyberdisplay [3]. They can provide vivid and high resolution image. However, since the brightness is not high enough for outdoor activities, they are mainly applied on see-closed applications, such as personal theater. The other type of microdisplay is Reflective Microdisplay. It can be LCOS or Scanning mirror or and Mirror Array [4]. Such kind of microdisplay can deliver high brightness image easily. It is well-known that display having high brightness image (> 2,000 nits) is essential for outdoor application. In the following contents, we will introduce how LCOS devices approach high definition AR displays. 2. Color Filter LCOS Liquid-crystal-on-silicon (LCOS) is a kind of microdisplay that integrates mature silicon process and liquid crystal process. LCOS has very high resolution and maintains a large aperture ratio (>90%). However, it is monochrome so that it requires additional techniques to produce color image. In past few decades, several techniques are proposed: for example, three-panel designs, which make use of three panels for three primary colors [5, 6]; another method is time sequential color [7]. On system point of view, the simplest method to reproduce the desired color image is integrating color filter in LCOS (CFLCOS). Similar concept can be found in direct-view TFT liquid crystal display. However, CFLCOS solution actually induces much more technical issues than direct-view TFT. The pixel size of CFLCOS is much smaller than direct-view TFT LCD, i.e. <6um. In order to achieve sub-micron alignment accuracy, the color filters have to directly apply on silicon substrate rather than on ITO glass (direct-view case) as shown in Figure 1. Such approaches will induce low contrast issue since the charge sharing effect between the color filter and liquid crystal bulk reduce the effective liquid crystal driving voltage significantly. The normal practice to overcome such issue is to reduce the capacitance of liquid crystal bulk. One of effective method is increasing the cell gap as large as possible. However, such approach will also make Fringe Field Effect get worse. Figure 1. The structure of a typical LCOS The poor color situation is another main technical issue. The Fringing Field induced by neighboring pixel becomes significant or even dominated electric field. The light leakage

2 deteriorates the color saturation of the display image. One may use black matrix to avoid the light leakage. However, the aperture ratio will be suffered, particularly for those pixel < 10 overall efficiency drop dramatically. We had propose a new method named Fringing Field Color Filter to improve CFLCOS color saturation [8]. Other than investigating the liquid crystal mode or pixel arrangement, we try to improve the color saturation of CFLCOS by color filter directly. In Figure 2, the concept of Fringing Field Color Filter is illustrated. The shape of color filter can be patterned to some optimized shape which is exactly the same of the distorted pattern. Once the distorted light up pixel is almost completely covered by the same color, the color saturation should be improved significantly. Such method is very remarkable. It will not affect the contrast and reflectivity of the panel since there is no change of electrode, aluminum mirror and liquid crystal mode. Furthermore, the impact of fringe field on color saturation is no longer significant once such Fringing Field Color Filter is applied. In other word, the pixel size can become much smaller than before Figure 2 Top: Conventional color filter pattern. Bottom: Fringe Field Color Filter pattern. The pixel electrodes are shown in black rectangular box Experimental results show that color saturation can be ramped up to 48% from 40% (CIE31). The pixel size can be further shrinked down m. from It makes 18um 18um the to 8um 8um. In other words, the resolution of the CFLCOS can be improved by more than 5 times. 3. Double Mirror Structure The Fringing Field Color Filter can just solve part of the problem. Particularly, it is difficult to improve the reflectance of the mirror. Once the pixel size is shrunk down, the aperture ratio of the mirror will drop also. Such phenomenon is not avoidable for normal mirror design. As a result, low mirror reflectance will be found. Some LCOS manufacturer may improve the mirror reflectance by applying dielectric mirror on top of the aluminum electrode. However, it is not a good choice for CFLCOS. The low k material will cause voltage drop across the liquid crystal. In other words, the device will suffer from either low contrast or higher power consumption. Obviously such solution is not a choice to meet the requirements mentioned above. In order to solve the low aperture ratio issue, we proposed a double mirror structure[9]. It is a new mirror structure having 2 important functions. Firstly, it will not suffer from any aperture ratio problem, thus the overall reflectance of such mirror can remain high even the pixel size become much smaller. Secondly, it can further improve the fringing field problem. A Butterflylike pixel shape is designed which can compensate the deformation of liquid crystal. Thus the overall color saturation can be improved as well. The Structure of double mirror CFLCOS is illustrated in Figure 3. The liquid crystal is sandwiched between the ITO glass and color filter. The double mirror is located under the color filter. The double mirror composed of 3 layers structure: Top mirror a acts as a pixel electrode and reflector; b is the dielectric layer which acts as insulator and optical cavity control; c is the bottom mirror which functions as a reflector and common electrode. The voltage level of Bottom Mirror is the same with common electrode (VCOM) of the liquid crystal device (ITO electrode); d is the pixel gap where is filled with dielectric material. More than that, there is a height difference h between Top Mirror and Bottom Mirror.

3 Figure 3. Structure of CFLCOS with Double Mirror We simulate the reflected plane wave under different h using FDTD method. As shown in Figure 4, if the dimension of h meets certain range, the reflection and diffraction lose can be optimized for the electromagnetic wave with wavelength 550nm. It can be found that the reflection behavior of double mirror (Figure 4a) will act like a plane mirror rather than a pixilated surface (Figure 4a). The diffraction and absorption near the pixel gap region disappear for the double mirror case. Figure 4. The simulation results of reflection of (a) double mirror structure, (b) conventional mirror structure. Such kind of mirror is implemented on a LCOS device with 8um (Subpixel: 5.33um x 4um) pixel size and 86% aperture ratio. The reflectance of the double mirror is shown in Figure 5. By comparing the reflectance of a conventional mirror surface having pixel size of 13.5um (Subpixel: 9um x 6.75um) and aperture ratio 92%, the reflectance using Double Mirror Structure is even higher than the conventional large mirror case.

4 Figure 5. Reflectance of Double Mirror (circle line) and Conventional Mirror (square line) 4. Front-lit LCOS Now we have illustrate how CFLCOS is prepared to be a high definition microdisplay for AR applications. However, the optical engine for LCOS devices is still found too bulky. As shown in Figure 5(a), the optical engine of LCOS consists of a light coupling system (such as light guide plate or any other coupling method). It also needs a polarization beam splitter () to form proper image. It functions as a polarizer and analyzer. Such bulky system also means it is not easy to build. Thus the cost of manufacturing is quite high. A Front-lit system was introduced on top of a LCOS [10] as shown in Figure 5(b). The image can be formed by making use of a 1mm thick Front-lit flat plate. After get rid of over 6 ~ 8 mm thick Polarization Beam Splitter, the whole optical system becomes very compact and light weight. Such Front- Lit LCOS can fulfill all the requirements of wearable display. (a) (b) Traditional Optical Engine Front-Lit Optical Engine LCOS Panel Image system Light Rod Object End Reflector Light Rod Object End Reflector By carefully design the Optical Element, most of the out coupling flux can be controlled within +/-10 degree viewing cone. Therefore the brightness near normal direction can be very high. The typical brightness of a 0.22 panel at normal direction can be as high as >15000nit. Figure 7 shows a photo of our 0.22 nhd Front-Lit LCOS module. Figure 6. Light ray propagation inside the light guide plate 6-8mm Projection system Observer <1mm Observer Figure 5. (a) traditional optical engine; (b) front-lit optical engine Figure 6 shows that how light ray is propagated within the front light system. When the s- polarized ray A meets the Optical Elements in Light Guide Plate, the propagation direction of ray A will be changed to ray B. The Optical Element acts as a tilted reflector which can change the direction of ray A to ray B by reflection. We can make use of total internal reflection or dielectric mirror to achieve such kind of requirement. Now ray B is reflected by the reflective polarizer and then head towards LCOS (ray C). Since the propagation angle is no longer larger than the critical angle of total internal reflection, it can escape from the Light Guide Plate. Finally s-polarized light propagates downward and is modulated to p-polarized light ray D by the LCOS underneath. Now, the Reflective Polarizer acts as an analyzer. Ray D passes through the Reflective Polarizer. The image then can be formed by any projection optics. Figure 7. A photo of 0.22 nhd Front-lit LCOS 5. Conclusions High brightness and compactness Front-lit CFLCOS with is developed. The Fringing Field Color Filter and Double Mirror technologies were developed to enable small pixel pitch (<7.5um) for the CFLCOS devices. Accompanied with high efficiency white LED, the brightness can reach up to >15000nits with total module thickness < 3mm, which is extremely suitable for wearable AR display applications. Another advantage is that no colorbreak-up will be observed for such display module, it can introduce much more comfortable viewing experience for the users. 6. References [1] O. Prache, "Active matrix molecular OLED

5 microdisplays," Displays, 22, pp (2001). [2] K. Sarayeddine, E. Kartal, and F. Templier, "Applications of OLED Microdisplays," in OLED Microdisplays, pp (2014). [3] J. E. Melzer and K. Moffitt, Head-mounted displays: designing for the user (McGraw-Hill New York, NY, 1997). [4] D. Armitage, I. Underwood, S.-T. Wu, Introduction to Microdisplays (Wiley, 2006) [5] D Dove and M Kisco, Arrangement for Projection Displays Employing Reflective Light valves, US patent # 5,658,060, (1997) [6] M G Robinson, J Korah, G Sharp and J Birge, SID 00 Digest, p.92, (2000). [7] K.-H. Fan-Chiang, et. al. "LCOS Panel Using Novel Color Sequential Technology," SID Symposium Digest of Technical Papers, 38, pp (2007).. [8] Y.-W. Li, K.-H. Fan-Chiang, et. al., LCOS Using a Fringe Field Color Filter, SID Symposium Digest of Technical Papers, 43: pp (2012). [9] Y.-W. Li, W.-H. Chen, K.-H. Fan-Chiang, H-C. Kuo, and H.-C. Tsai. "Color Filter LCOS with Double Mirror Structure," SID Symposium Digest of Technical Papers, 45, pp (2014) [10] Y.-W. Li, C.-W. Lin, K.-Y. Chen, K.-H. Fan- Chiang, H.-C. Kuo, and H.-C. Tsai, "Front-lit LCOS for Wearable Applications," SID Symposium Digest of Technical Papers, pp (2014).