A Full-Color SXGA TN AMLCD For Military Head-Mounted Displays and Viewer Applications

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1 A Full-Color SXGA TN AMLCD For Military Head-Mounted Displays and Viewer Applications Ollie Woodard, a Jason Lo, a Murshed Khandaker, a John Gassel, a Frederick Herrmann, a Hiap Ong, a Bor Yeu Tsaur, a and Colin Reese b a Kopin Corporation, Westborough, MA b U.S. Army RDECOM CERDEC Night Vision& Electronic Sensors Directorate, Fort Belvoir, VA ABSTRACT We report progress in developing a 0.97-in diagonal AMLCD with a full color SXGA resolution. A 1280x1024X3 dot array was developed with integrated color pixel filters to create an SXGA color pixel array. These displays are fabricated on 8-inch SOI wafers and transferred to glass wafers to produce transmissive liquid crystal displays. Improvements have been made in this AMLCD to bring the fabrication process to manufacturing. Fabricating a 3.9 million pixel dot display on a 1-inch die required a new display design and fabrication in an 8-inch wafer line. The 8- inch process provided enhanced process capabilities and tighter design rules to achieve good performance and reasonable starting yields. An ASIC driver and ultra thin efficient backlight were developed to miniaturize the display module and to reduce total power to < 750mW for soldier mounted applications. Total package size is less than 0.5 in 3. The ASIC will also drive Kopin SVGA and VGA color displays singly or in pairs for binocular applications. An end-to-end 8-inch wafer process was established at the wafer foundry and at Kopin. A 3-year manufacturing technology insertion program (Mantech) has begun to optimize the 8-inch line processes and the SXGA color display. Meeting yield and performance goals will reduce display cost and enable systems performance goals. Key results include vivid, high-resolution color, wide viewing angles and low power operation. Performance data and specifications will be presented. Keywords: Active Matrix Liquid Crystal Display (AMLCD), TN transmissive displays, color filter, Helmet Mounted Display (HMD), Flat Panel Display (FPD), Weapon Sight, and Image Fusion. 1. INTRODUCTION Kopin s CyberDisplay microdisplays are the world s densest transmissive Active Matrix Liquid Crystal Displays (AMLCD s) using the smallest pixels. The size of the pixel can be made very small because the pixel transistors are made of high-quality single-crystal silicon. To realize the transmissive displays out of opaque silicon, Kopin employs its patented technology to lift-off the thin IC layer onto a glass plate. Kopin s CyberDisplay AMLCD s have been used in military head-mounted and sensor viewer applications. These include monochrome AMLCD s with VGA, SVGA and SXGA resolutions. For the next generation of military systems requiring day/night operations with image fusion, Kopin and the U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) are co-operatively developing high-resolution, full-color displays. We previously reported the design and development of SXGA monochrome displays and SXGA color Multiple Doman Vertical Alignment (MVA) displays with color filter technology [1, 2, 3]. Although the normally black MVA display is promising to deliver a very high contrast ratio with a wide viewing angle, the transmission was rather low. In this paper, we report a newly Head- and Helmet-Mounted Displays XIII: Design and Applications, edited by Randall W. Brown, Peter L. Marasco, Thomas H. Harding, Sion A. Jennings, Proc. of SPIE Vol. 6955, 69550B, (2008) X/08/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol B-1

2 developed full-color SXGA display using twisted nematic (TN) liquid crystal, together with an ASIC driver and miniature backlight. 2. DISPLAY NEEDS AND REQUIREMENTS A full-color SXGA (1280x1024 resolution) AMLCD is needed to support of the next generation of military systems. Future dismounted soldier vision systems will include multiple waveband sensors (i.e., FLIR and image intensifier) mounted on the soldier s head for navigation and targeting. The imagery from these sensors will be processed using image fusion (with either optical or digital color image fusion techniques) to deliver one combined image to the soldier s HMD. In addition to the image-fused sensor video, the HMD could display color maps or color icons indicating threats, warnings, heading, and waypoints. The color SXGA display requires low power consumption to help increase the mission duration and reduce the need to carry extra system batteries, thus reducing the weight a soldier must carry and the logistical burden to support the activities of the warfighter. The 1W target was exceeded with a total system power <750mW. Prior development of a sequential color SXGA microdisplay resulted in too high power (3W) for a soldier-based display. The new TN SXGA display based on color filter technology has substantially lower power consumption, while having the potential to support high brightness for aviation applications (with a higher power backlight). The SXGA AMLCD could also be inserted into a broad range of commercial products. Dual use is important to establish a cost-effective, long-term, domestic source for SXGA color displays. Target specifications for the color SXGA display were established in collaboration with NVESD. Table 1 shows target specifications and measured performance parameters if they do not yet meet the specifications. Table 1: Target Specifications for Color TN SXGA Display Parameters PARAMETER TARGET (STATUS) Display type Color AMLCD transmissive twisted nematic Resolution SXGA (1280x1024) full color Color 24-bit full color R,G,B color dots Pixel pitch 15 µm (H) x 15µm (V) 1700 DPI Sub-Pixel Pitch 5 µm (H) x 15 µm (V) for each R, G, B dot Display area 19.2mm x 15.6mm Frame rate 60Hz ASIC drive electronics 12 mm x 12 mm x 0.5 mm BGA Interface 51 wires total for video, clocks and power Backlight Integrated module System power <1W (<0.75W) Temperature range -37 C to + 65 C Luminance Range ft-l Flicker -40 db Pixel Cross talk < 5% Image retention < 16 msec Response 35 C < 15 msec ( < 32msec ) Warm-up time < 15 sec (< 2 minute) Viewing Angle >50 o cone ( >40 o cone) Contrast ratio 100:1 (> 60:1) Color performance > 50% NTSC ( > 30% NTSC) Aperture >60% ( 47%) Transmission >5% ( 2%) Yield 10x improvement (TBD) Proc. of SPIE Vol B-2

3 Improvements will be made in the display design, the fabrication process and by an improved pixel structure. The color SXGA display has identical screen dimensions as the monochrome SXGA display to make color upgrades possible with minimal product packaging changes. System interfaces, physical and functional partitioning as well as overall packaging and thermal management requirements were also assessed. A controller ASIC and an ultra-thin efficient backlight were developed for the SXGA color display to miniaturize the display module and to reduce power to low levels required for man-portable applications. The ASIC was also designed to drive two SVGA or two WVGA displays for binocular and 3-D stereoscopic applications. 3. COLOR FILTER TECHNOLOGY Each 15-µm-square color pixel of the SXGA display consists of three 5 µm x 15-µm red, green and blue rectangular color dots. Color filters are added at the IC foundry by patterning directly over the display circuitry. In this way, the filters are properly aligned before display assembly, and end up on the input side of the display pixel as illustrated in Figure 1. Kopin s circuit transfer process results in the encapsulation of the color filter material between the display circuit and the input glass to provide enhanced color stability. The performance of color-filter displays using IC foundry-deposited color filters was previously reported [2]. Color filters are used in Kopin s QVGA, VGA, and SVGA color displays as well as other high-volume commercial display products. The color gamut achieved with the IC-foundry color filters compares favorably with the SMPTE C standard as shown in the CIE plot of Figure 2. Color filter thicknesses can be adjusted for each display design to optimize display color and brightness. 0.6 Glass LC TFT & black matrix 0.3 Green filter Glass Red filter 0.2 CIE Chart 0.1 LED's SMPTE C Color Filters Figure 1: Graphical cross section of red and green pixel cells showing light transmission direction. Figure 2: Color filter gamut compared to SMPTE C standard and LEDS used for backlights 4. DISPLAY DESIGN LCD modeling was used to test designs and to improve display operation. Kopin s 1-dimensional LCD software based on the ONG 2x2 optics model [3] was used to make a fast estimation on transmission versus voltage, viewing angle and pixel geometry. The Autronic 2-dimensional LC modeling software was used to predict performance and optimize LC cell gap, with data on crosstalk, contrast, gamma, response time and viewing angle. The Autronic software was also used to simulate light transmission, LC orientation and equi-potential fields for this TN pixel profile. Proc. of SPIE Vol B-3

4 Figure 3 shows schematically the 2-dimensional modeling results of the display pixel structure. The applied pixel voltages from left to right are +4V(red), -4V(blue), 0V(purple), and -4V(blue), simulating column inversion. A horizontal LC alignment with 20-degree pretilt is used for both the top and bottom.!5q% Transmission 55O.Onni 7 7.L.._Li u.uums I / 1.OOms / j± 2.OOms/ r>e. U.1: EzQQrn / trr. 30OOms /.. 'I /j Figure 3: Pixel cross-section: Autronic TN 2D modeling results on light transmission and response time (Top). Equipotential lines and LC directors (bottom). Drive is 4V electrode voltage with column inversion. Color dot width is 5 µm and cell gap is 3 µm. The upper plot shows pixel transmission at selected times (0, 1, 2, 3 & 30msec) after voltages are applied to the pixel electrodes to dynamically model the LC response time. The lower plot shows the profiles for LC orientation, equivalent potential lines, and LC directors. Modeling helped us to understand the basic physics for the TN operation and to optimize LC cell design. Figure 4 illustrate the fall time of pixel 2 from the 2-D model data. Modeled transmission vs. pixel voltage applied is also be used to plot gamma Pixel 2 Transmission Fall Time Model Transmision 50.00% 45.00% 40.00% 35.00% 30.00% 25.00% 20.00% 15.00% 10.00% 5.00% 0.00% msec Figure 4: Pixel 2 fall time from 2DiMOS model Proc. of SPIE Vol B-4

5 5. DISPLAY FABRICATION SXGA displays are fabricated in a high-speed, low-power CMOS process utilizing single crystal silicon-on-insulator (SOI) starting material. Kopin s display fabrication process has been described previously [1]. Since the same basic fabrication process is used on all Kopin displays including high-volume commercial displays, the manufacturing line is maintained to run smoothly and ready for low-volume military display runs. The SXGA color display was initially designed for and first processed in a 6-inch wafer foundry and in Kopin s 6-inch display assembly facility. The largest and most complex display released previously and manufactured in the 6-inch line was the monochrome SXGA display developed for the Comanche program, now used in the JSF program and others. Because the color SXGA display has red, green and blue sub-pixels for each color pixel, it has 3-times the number of pixel elements and support circuitry as the monochrome SXGA display. The 6-inch design had low transmission, low contrast ratio and low yield. In addition, the displays exhibited vertical line image artifacts caused by column-to-column coupling and unequal voltage drops in the 16 parallel video lines, part of which were made of poly-silicon because of the routing constraints. The resulting images had column-to-column gray scale differences, resulting in vertical gray stripes. These two problems created an impossible choice. Making the lines narrower to increase the spacing made the lines too resistive and uneven voltage drops created non-uniform video levels on the pixel columns. Making the video lines wider to reduce voltage drops reduced the line-to-line spacing and increased coupling between them. Increasing the die size to make more space was not an option because the display had to be the same size as the SXGA monochrome display. The fabrication of color SXGA displays requires the use of advanced 8-inch silicon processing with tighter design rules and multiple metal layers to help the routing issue. The use of narrow, low-resistance metal lines for the vide lines can eliminate coupling and voltage drops. Tighter design rules allowed smaller devices, making room for increased pixel apertures. Kopin has installed an 8-inch process line in its Westborough display plant and is processing 8-inch wafers at its foundry partners facilities. The use of a thinner silicon layer on the SOI wafer for the 8-inch processing requires many changes in the foundry processing parameters. A major benefit of thinner silicon is substantially reduced circuit sensitivity to light, allowing higher luminance level operation. Kopin s display assembly process also needed major changes to accommodate the larger 8-in wafer. New 8-in machines and fixtures were installed and qualified before displays were produced. The full-color SXGA TN/LCD has been successfully fabricated in the 8-inch line with good display performance. A Manufacturing Technology Improvement Program (Mantech) supported by NVESD will refine the process and improve the display performance over the next three years. Displays produced during the Mantech program will be delivered to Night Vision and to systems company s applications for integration. 6. PIXEL DESIGN A new design of the color pixel mosaic was required to optimize performance by taking advantage of the tighter 8 design rules. The new pixel has a larger (47%) aperture ratio by using a smaller pixel transistor. Figure 5 shows a 15 µm x 15 µm color pixel design consisting of three 5µm x 15µm red, green and blue sub-pixels. The pixel array has 3840 x 1024 active color dots. A parallel program is developing a new pixel design that will increase the pixel aperture and double the pixel transmission. This pixel technology and new processing equipment required will be integrated into the Mantech program in the Proc. of SPIE Vol B-5

6 7. DISPLAY CIRCUIT DESIGN Figure 5: 15-µm color pixel design for TN display. Figure 6 shows a block diagram of SXGA TN display. The display features capacitive-coupled video inputs with integrated DC restore circuitry for reduced interface voltage swing. This approach allows operation with a single 3.3 V power supply for low power operation of digital and analog circuits as well as the use of the CMOS drive ASIC. A sleep mode is provided to simplify system power management. Integrated scanners drive the pixel array. Two bi-directional horizontal data scanners switched the video inputs onto the column lines. The bi-directional vertical scanners selected rows one by one, driving from both ends of each row line. The input level shift circuits accept digital control signals with 3.3-volt levels. DC Restore Data Scanner 12 VIDxH VIDxL VEE HCK0 HCK1 HST I DWN HEAT SLEEP* TOUT1 TOUT2 Level Shift Control Logic Power Control Select Scanner VEE VCOM Select Scanner Low V Detect Data Scanner Figure 6: SXGA Color TN Block Diagram. Proc. of SPIE Vol B-6

7 An automatic internal power down reset circuit is used to equalize charge in the pixel array before power is removed from the display to prevent image retention and/or flicker upon restoration of power. An internal heater is also integrated into the display to support a fast warm-up mode. The heater operates without affecting display operation to allow image displays within 15 seconds after turn on at -37ºC. An internal temperature-sensing device very close to the LC allows direct control of the LC temperature. The control circuit is outside the ASIC. Because of the large die size of SXGA display with many video inputs, it was very important to maintain low and uniform RC delay among all video paths to prevent image artifacts from being shown on the display. The large pixel resolution also required high-speed level shifter and horizontal data scanners, and further required large driving ability to realize fast transition edges of internal signals. Failure to do so would have resulted in image shadowing, narrower voltage margin, and/or slower clock rate. Long row and column lines presented another challenge to designers for pixels to settle down within a short horizontal retrace time. The 8-in design rules and process capabilities facilitated a display design that has very good images without visible artifacts. 8. DISPLAY PERFORMANCE Results presented here are early display results from the first wafers processed in the 8-inch line. These first displays present very good images that look considerably better than the performance specifications might indicate. The Mantech program will optimize all process steps and to tweak the design for optimum performance and to meet all of the target specifications. 8.1 Transmission and Contrast Ratio Transmission and contrast ratio are both affected by cell gap. Video voltage levels greatly affect the contrast ratio. The variation of the LC alignment layer over the large 8-inch wafer is also a factor. The scatter diagram of Figure 7 shows a wide variation of contrast and transmission for displays from two wafers. A goal of the Mantech program is to improve performance and to stabilize performance parameters. Transmission vs Contrast (2/15/08) 8 inch Lot MPP0R-.99- Wafer7-F7 & Wafer 10-A7 3.5% 3.0% 2.5% Transmission 2.0% 1.5% 1.0% 0.5% W7 Transmission W10 Transmission 0.0% Contrast Figure 7: Transmission vs. contrast ratio scatter plot for 8-inch line fabrication. Proc. of SPIE Vol B-7

8 8.2 Display Response Time Display response time is dependent on the LC material parameters, temperature, cell gap and drive voltages. Figure 8 shows rise and fall response times vs. temperature for black-to-white and white-to-black transitions. Normal operating temperature is 35ºC, which will keep these response times under 15 ms. Steps between gray levels can require longer times. Improvements are required to meet the 15 msec response time specification for all grayscale steps.. SXGA Color Display Response Time vs. Temperature Response Tims (ms) Display #1 Tr Display #1 Tf Display #8 Tr Display #8 Tf Display #12 Tr Display # 12 Tf Temperature C Figure 8: SXGA color display response time vs. temperature for 3 displays. 8.3 Display Viewing Angle Figure 9 illustrates the contrast ratio vs. viewing angle of incidence from normal. The display has a peak contrast ratio of 100:1 at 10 degrees off normal, and 67:1 at the normal incidence angle. The >40-degree cone angle of the 20:1 contrast ratio iso-curve (show in red) provides the viewer with a good image SXGA Color Lot mppor W8 Die Figure 9: SXGA color display contrast ratio vs. viewing angle from normal and azimuth Proc. of SPIE Vol B-8

9 Display assembly variations in the LC gap and LC alignment affect the centering of the contrast ratio iso-curves. The Mantech target is a 50-degree viewing angle cone 8.4. Display Module Components The SXGA display, ASIC drive electronics, and backlight make up a compact and power-efficient module. Figures below show the display, the thin backlight and the ASIC BGA chip. The BGA package can be mounted on a small PC board about the size of the display. The board will have the display connector, the system connector, the heater control circuit and any other user options. The total height of a stacked package its will be less than 1 cm. r Figure 10: SXGA Color Display (0.97 diagonal). Figure 11: Thin backlight. Figure 12: Framed SXGA color display with backlight -.I::, Figure 13: SXGA color display drive electronics ASIC in BGA package (12 x 12 mm). Proc. of SPIE Vol B-9

10 9 CONCLUSION Early improvements in brightness, contrast ratio, viewing angle, operating temperature range and yield have been achieved by moving display design and fabrication from a 6-inch line to an 8-inch line. Although the SXGA color display is still a work in progress, its image quality is very good. Sample quantities of displays have been delivered for integration into military systems. A three-year Mantech program to establish a mature 8-inch line is in progress and will enable the TN SXGA color displays to meet target specifications with high yield for reduced costs. Process improvements for higher display performance are being developed in parallel with the Mantech program. Integration of the improved process into the SXGA color Mantech program will begin in ACKNOWLEDGEMENTS We appreciate the technical work carried out by many Kopin personnel in Westborough, Taunton, and Scotts Valley. Display assembly is done in the Kopin Westborough display line. The 8-inch display line development at Kopin s Westborough facility was partially supported by U.S. Army RDECOM CERDEC Night Vision & Electronic Sensors Directorate, Fort Belvoir, VA and by the Commonwealth of Massachusetts. 11. REFERENCES [1] Ong, H.L.,N. Cheong, J. Lo, M. Metras and O. Woodard, SID 03, p. 680 (2003). [2] Ong, H.L., N. Cheong, C. Reese, O. Woodard, SPIE Aerosense p. 301 (2004), A, A New Normally Black, High Contrast, Wide Symmetrical Viewing Angle AMLCD for Military Head Mounted Displays (HMD's) and Other Viewer Applications [3] Ong, H.L., Ngwe Cheong, Jason Lo, Marty Metras, and Ollie Woodard, and Ronald P. Gale, SID 05, p. 646 (2005) Proc. of SPIE Vol B-10