(SID2009)high image quality of ultra low power DMS based display technology
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37.4: High Image Quality of Ultra-Low Power Digital Micro-Shutter BasedDisplay TechnologyJignesh Gandhi, Je Hong Kim, Nesbitt Hagood, Lodewyk Steyn, John Fijol, Tim Brosnihan, Stephen Lewis, Gene Fike, Roger Barton, Mark Halfman, Richard PaynePixtronix, Inc., 100 Burtt Road, Andover, MA 01810, USAAbstractThe Pixtronix DMS™ (Digital Micro Shutter) display technology intrinsically provides exceptional image quality due to its optical architecture and device technology. This technology, based on MEMS micro-shutters formed on active TFT backplanes, has enabled the development of color sequential, time division gray scale, direct-view displays achieving breakthrough performance - including 150% NTSC (CIE 1976 u’v’ color space) color gamut, 24-bit color, > 500:1 contrast ratio and 170° viewing angle, all at ¼ the power consumption of comparable TFT-LCD display modules. In addition to this, these displays present horizontal contrast viewing angle comparable to that of OLEDs providing exceptional color saturation in large angle viewing directions. This paper will briefly describe the Pixtronix DMS™ display technology. It will also describe key display architecture elements of DMS technology responsible for the excellent optical performance. The optical characterization of prototype displays and discussion of the results will be presented. A non-emissive technology like DMS technology is able to provide performance close to that of emissive technologies like OLED at a power much lower than that of an LCD display.1.IntroductionTwo critical elements of Multimedia functionality in portable consumer electronics devices for the end user experience are the image quality of the display and the amount of time the display can be viewed per battery charge [1]. Today’s incumbent display technologies, such as TFT LCD and OLED, force mobile consumer electronics OEMs to make trade offs between power consumption and image quality.The Pixtronix DMS™ display technology, based on micro-machined light modulators, greatly helps this power / performance trade off by providing wide color gamut, high brightness, high contrast ratio and wide view angles, all at roughly ¼ the power consumption of competing TFT-LCD or AMOLED displays of the same size and luminance. There are four key technologies that enable the dramatic performance and reduced power consumption in the DMS™ Display Technology. [2, 3]2.Key Elements of DMS TechnologyThe first key element is a proprietary optical architecture (Fig. 1) which includes a light recycling backlight. This backlight enables a pixel which has a physical aperture of ~10-15% of the pixel area to allow 40-60% of the light out to the viewer. This high throughput is the main reason of reduced power consumption of the DMS™ Display Technology over LCD displays.In addition to being a highly recycling efficient backlight, the light source of the backlight is tricolor (red, green and blue) LED’s. These LED’s have high color purity and they enable a very large color gamut, well exceeding typical LCD and OLED color gamut. DMS modulator itself has rather flat transmission curves and hardly alters the color coordinates of the light emitted from the backlight. Hence the DMS display has inherently high color gamut.The second key element is the digital MEMS micro shutter element at the heart of each pixel. The inherent design of this mechanical element results in advantages in contrast ratio viewing angle for the DMS technology. The contrast ratio remains very high even at high viewing angles. This enables image quality with high color purity and saturation at large viewing angles (similar to that of OLEDs).The third element is the high speed of the MEMS device: This enables use of Field Sequential Color (FSC) and time division grey scale with rapid color change frequencies (~1 kHz) to eliminate any deleterious video artifacts such as color breakup and flicker. Digital nature of DMS display enables many inherent advantages of digital architectures. Among the many, we specifically mention high color purity in low gray scales which is an Achilles heel for LCD displays.The fourth element that we do not call out specifically here is the proprietary TFT circuit designs that drive these MEMS devices in a high frequency yet low power manner enabling the complex image formation on the DMS display.3.ResultsPixtronix has built and tested several 2.5” QVGA prototype units. Prototypes have been built using LTPS-PMOS, LTPS-CMOS, and amorphous silicon backplanes. Driver chips are bonded directly to the backplane The displays, consisting of MEMS on TFT, aperture plates and backlight units, form a module which is driven through flex by an application board that includes power supplies and a system controller. The results described below are characterization data of these prototypes.The Pixtronix DMS™ technology typically achieves performanceof 8-bit color depth with gamma of 2.2, full 60Hz video, a contrast ratio of > 500:1 in white (> 1000:1 in blue) and a color gamut of over 150% of NTSC or 170% of sRGB ( in CIE 1976 u’ v’ color space) on the demonstrated prototypes (Fig. 7). Displays have a viewing angle of >170° in the horizontal and >130° in the vertical direction (to CR 10:1) (Fig. 4). Typical display brightness is 170 nits of on-axis luminance and with brightness FWHM of over +/- 40 degrees. In these conditions, the prototype backlight typically dissipates 48 mW of power.Subsequent prototypes are expected to achieve >1000:1 contrast ratio and 170° vertical and horizontal view angle. The total power consumption of the entire system is expected to remain at less than ¼ the power of competing TFT-LCD displays.The optical characterization results described below are performed using standard optical equipments including ELDIM XL88 conoscope system. We have closely followed NIST guidelines and NIST traceable calibration standards for flat panel display measurements.3.1. Optical ArchitectureThe optical design of the DMS™ Display is unique in its backlight design and overall functionality [4]. Shown in Figure 1, the module is built from a thin film transistor back-plane with MEMS shutter structures added to it using a simple panel process flow using conventional TFT LCD equipment, called the MEMS Glass in Figure 1. A Cover Glass is then aligned and bonded in a process similar to LCD cell assembly that uses color filters. The cover glass is mirrored with highly reflective surface facing away from the MEMS and highly absorptive surface on the side facing the MEMS. The mirrored surface has apertures which are aligned to the MEMS shutters such that the shutters can controllably block the light coming through the apertures. Once the MEMS substrate and aperture plate are bonded, this cell is then placed over the backlight with the highly reflective side facing the backlight, thereby allowing light that does not initially strike openings in the aperture plate to recycling within the display. The resultant efficiency of the display demonstrated at the meeting is 50%. That is, when shutters are open, 50% of the optical power available or emitted from the backlight can be measured as emitted through the open shutters.CIE 1976D65u'NTSCsRGB Sample Gamut D65The backlight is a proprietary design and fabrication technique to form an efficient recycling backlight. It is an edge-lit backlight whose light source is two RGB LED packages. Fig. 2 shows themeasured color coordinates of a DMS display in the CIE 1976 u’v’ color space. The measured color gamut of a DMS prototype is 150% of the NTSC area.3.2. MEMS Light ModulatorThe DMS™ Technology is based inthe field of micro-electromechanical systems [5-8]. The modulator in the DMS™ Display technology is a novel laterally moving, thin-film shutter aligned to an aperture. It is supported and driven by two patented dual-compliant zipping actuator [9, 10]. The pixel has been demonstrated over the pixel density range of 100ppi to 300ppi. Each pixel has one shutter which is used to show all colors in the FSC time division multiplexed driving scheme. The shutter can have one or more slots and apertures. More the number of slots, shorter the travel required between the Open and Closed states.Figure 3(a) shows electron micrographs of a shutter array built over a TFT-based active backplane. Figure 3(b) shows schematic view of the device. The electrostatic actuator on either side is comprised of two narrow and flexible beams. This shutter is a mechanically digital device, with two states, open and closed. The force densities applied to each of the dual compliant actuators are well above the surface energies in the support structures, and hence the phenomenon of “stiction” that afflicts many MEMS devices is a non-issue for the DMS™ display technology. [2, 3] It is this unique rectangular structure of the shutter slots that provides DMS technology some of its key advantages. The rectangular design of the slots enables small shutter travel and reduced operating voltages. The light blocking shutter is oversized by certain amount of overlap in each direction compared to the cover glass slot underneath it. The ratio of this overlap with respect to the size of the slot in the respective direction is a critical150% of NTSC (170% of sRGB). Figure 3 (a) MEMS Shutter SEM Micrographs (b)Schematic drawing of DMS MEMS device.Shutter Flexible BeamsTransistors CapacitorSID 09 DIGEST • 533parameter for determining off-axis light blockage. Due to this, shutters in the long direction (horizontal direction) have very low light leakage in the off-axis direction. The contrast ratio in this direction remains very high (>100:1 typically) at all angles. Figure 4(a) shows a measured isocontrast plot of a DMS display. Figure 4(b) shows the viewing angle cross-sections of this plot for horizontal (Extraction at 0 – red curve) and vertical (Extraction at 90 – blue curve). The contrast in horizontal direction remains wellabove 300:1 up to 85 degree in either direction.This enables an image with excellent color purity in viewing directions close to the horizontal direction (as per the isocontrast plot). Figure 5(a) shows the color dispersion plots for R, G, and B color coordinates on CIE 1976 (u’v’) chart for various viewing angles in the horizontal direction. The cluster of data points centered near each color is the color coordinates for viewing angles from -85 deg to 85 deg in steps of 1 degree. Figure 5(b) plots the calculated color gamut (as a % of NTSC area in CIE 1976 color space) based on data of Fig. 5(a) as a function of horizontal viewing angle. This data clearly proves saturated image quality at all angles (in horizontal direction) very similar to that of OLED displays. Table 1 provides the same information in comparison with a high-end SFF LCD device measured with the same equipment.The shutters in the vertical directions (short direction) have more light leakage in off-axis direction. Proprietary thin film engineering and mechanical design is applied to reduce this light leakage and obtain vertical contrast viewing angle similar to LCD displays. As shown in the data above, the vertical viewing angle (defined by CR>10) is -60deg/+70deg (also shown by the yellow curve in the iso-contrast plot). The production devices are designed to achieve +/-85 deg viewing angle.CIE 1976v'BlackBodyWhite dispersionRed DispersionGreen DispersionBlue DispersionDMS Conventional LCD %NTSC CIE1976 149 %±2.5% 73% ±9.2%3.3.Digital FSC AlgorithmsField Sequential Color (FSC) operation is recognized as a means to avoid transmission loss through color filters. The MEMS modulator described above switches fast enough (~100usec) to allow >1000 color transitions per second and hence avoids the impairments normally associated with FSC-LCD. Digital nature of DMS display enables many advantages inherent in digital architectures. High color purity as a function of horizontal viewing angle established due to mechanical structure of the display carries itself even when the display is driven in gray scale. Since the gray scale is generated by keeping the shutter open or closed for a given time period, the color quality and viewing angle properties at low gray levels are very close to that in the full brightness modes. Figure 6(a) and (b) shows similar color gamut plot as a function of horizontal viewing angle. As it is clearly seen, even at 128 gray level (approximately 1/4th the peak brightness in a gamma 2.2 display), the color gamut is maintained well over 100% NTSC ratio. Average %NTSC CIE1976 in angular distribution is 141%±2.0%.This means that DMS displays have very small change in color point as a function of viewing angle for not only white but also for color images. DMS displays do not suffer from color change issues and gray scale inversion issues common to LCDs.Digital nature of DMS system also provides programmability of color bit depth and color gamut that is not available in any existing display technologies. E.g. bit depth and color gamut can be reduced for appropriate images or use modes, either via content adaptive algorithms or via user selectable modes. This results in additional power savings in addition to typical content adaptive backlight control also employable in DMS display.Table 1 Comparison of %NTSC variation (shown asstandard deviation) between DMS and LCD in thehorizontal viewing directions.Figure 5 (a) Chromaticity Diagram (b) Color gamut in horizontal view direction of DMS displayFigure 4 (a) Isocontrast Plot (b) CR extraction in horizontal(0) and vertical direction (90) of DMS display534 • SID 09 DIGESTCIE 19760.00.10.20.30.40.50.6u'v'BlackBody White dispersion Red Dispersion Green DispersionBlue DispersionFigure 7 below shows images of the QVGA prototypes at various angles as a visual testament of the measurements described above. Another inherent advantage of the mechanical nature of the DMS technology is better performance over wider temperature range overcoming yet another challenge of existing device technologies. Since DMS technology uses materials and equipment set similar to LCD fabrication, manufacturing of DMS technology is calculated to be lower cost than LCD [3].4. SummaryWith its 2.5” (diagonal) QVGA resolution engineering prototypes, Pixtronix has demonstrated fundamental breakthroughs in the areas of high optical transmission, low power consumption and exceptional video image quality. DMS technology inherently provides exceptional image quality in terms of color gamut andcolor purity as a function of viewing angle and gray levels. Image quality is comparable to that of an emissive display like OLED while consuming only fraction of the power compared to a non-emissive technology like LCD. Deployment of this technology in mobile multimedia display applications will dramatically improve the end user experience and directly contribute to the increased adoption of mobile multi-media devices and services.5.References[1] J. Bergquist, C. Wennstam, “Field Sequential-Color Displaywith Adaptive Gamut”, Proceedings of SID 2006, Paper #49.2 [2] N. W. Hagood et. al. A Direct-view MEMS display forMobile Applications, Vol 38, SID Symposium Digest, 2007, p 1278. [3] N. W. Hagood et. al. MEMS-Based Direct View DisplaysUsing Digital Micro Shutters, to be published in 15th International Display Workshop, 2008. [4] U.S. Patent # 7,365,897 and # 7,417,782[5] Marc Madou, Fundamentals of Microfabrication , CRC Press1997, ISBN 0-8493-9451-1 [6] P. Rai-Choudhury, ed., MEMS and MOEMS: Technologyand Applications , SPIE Press Monograph 2000, ISBN 0-8194-3716-6l [7] G. K. Fedder, R. T. Howe, T. K. Liu, E. P Quevy,“Technologies for Co-Fabricating MEMS and Electronics”, Proceedings of the IEEE, Vol. 96, No. 2, February 2008, pp.306-322 [8] J. Bustillo, R. Howe, R. Muller, “Surface Micromachiningfor Micro-electromechanical Systems”, Proceedings of the IEEE, Vol. 96, No. 8, August 1998, pp.1552-1574 [9] U.S. Patent # 7,271,945[10] R. Legtenberg, J. Gilbert, S. Senturia, “Electrostatic CurvedElectrode Actuators”, Journal of Microelectromechanical Systems, Vol. 6, No. 3, Sept. 1997, pp.257-265LeftRightFigure 7 Images of the DMS display prototype from various angles.Figure 6 (a) Chromaticity Diagram (b) Color gamut in horizontal angular view of DMS display at 128 gray levels.SID 09 DIGEST • 535。
