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New Projection Technologies: Silicon Crystal Reflective Display and Grating Light Valve By Gary Mandle

Cathode Ray Tubes (CRT) have for the most part been the mainstay of displays for over a 100 years. Displays that just several years ago were only at a discussion level, are now being demonstrated. Two new technologies are on the brink of delivery and offer performance surpassing these older methods: Silicon Crystal Reflective Display (SXRD) technology and Grating Light Valve (GLV).

S

everal manufacturers have been developing reflective liquid crystal technologies. One most notable has been JVC, with its D ILA device. Sony has also seen several significant benefits in using liquid crystal on silicon (LCoS) technology and has made some changes to the manufacturing process, crystal material, and driving technology. The new devise is named silicon xrystal reflective display (SXRD).

Grating Light Valve Another technology soon to be delivered is the grating light valve (GLV). This offers a significant change in thinking, in that the light source incorporates lasers for each primary color, each generating its own primary frequency. The GLV device modulates this light source and uses it to raster an image.

SXRD Basic operation of SXRD imagers differs greatly from conventional high-temperature polysilicon (HTPS) liquid crystal display (LCD). Normal LCD devices are transparent and regulate light as it passes through the device. Polarizers are placed at each end of the LCD cavity. The polarizer closest to the light source prealigns the light entering the device. The crystal material is elastic and holds two states. The first (for A Si and P Si technologies) is a relaxed state that rotates the light and aligns it with the polarizer on the viewing face. If a charge is applied to the LCD material, the material moves to a tensioned state and straightens. The light then is not rotated and blocked by the polarizer at the viewing side of the device. 474

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The SXRD Device The SXRD device is a very different operation. Whereas HTPS is translucent, SXRD is reflective (similar to DLP). Light is prepolarized before entering the device by using methods placed forward of the device in the light chain. This increases stability and also allows higher MTF through the optical chain. Light then passes through an iridium tin oxide layer (ITO) and inorganic alignment layers on its Figure 1. Cross-section of the SXRD imager. way to the mirror surface at the back of the LCD cavity. drive technology incorThe iridium tin oxide layer is used to keep outside porates 12-bit prostatic charges away from the LCD cavity. The aligncessing, intended for ment layers are the static pads used to transfer the commercial use only. drive charge from the drive field effect transistor (FET) To achieve this level to the LCD material (Fig. 1). of performance at a The LCD material is a vertical alignment construction reasonable price, that uses a hybrid crystal structure. These materials manufacturing methhave been reengineered for the SXRD device so that a ods have been greatly significant improvement has been made to the speed in refined. Improvements changing LCD states. Optical motion of the crystal is include advancements not a twist, but more of a movement from horizontal to in silicon wafer design, Figure 2. Close-up of the vertical; horizontal being the relaxed state. new driver circuitry SXRD pixel. Movement is created when a static charge is present design using siliconat the driver pad. Potential differences between the pad on-glass (SOG) conand the opposing alignment layer cause the LCD matestruction, and new spacerless technologies, used in the rial to move to a tensioned state. Regardless of cell assembly processes. These advances give a cell whether the light is rotated by the crystal or not, it is cavity structure of less than 2 ␮m at a tolerance of betreflected back into the light chain. ter than ⫾3%. Pixel pitch is less than 8.5 ␮m with pixel gaps at This makes for a device that can perform in environaround 350 nm (Fig. 2). The depth of the cell cavity ments that require very large light levels. Cooling is perstructure is less than 2 ␮m, which allows for very low formed in the same manner as other reflective devices. flair and very high contrast. On/off pixel cycling (Tr + Tf) The SXRD device only turns the light; thus the reflecis better than 5 msec. Development of this technology tive efficiency is very high. With less light being convertstarted in 1996, with the first .78 in. 1.77 aspect devices ed to heat, less heat is transferred into the imager. A ready for production in 2003. The first deliverable prodheat sink is mounted to the back of the device, but is uct was a projector for consumer use, available in much smaller than those used in other technologies. 2004. Resolution was 1920 x 1080 at a device contrast The SXRD Projector ratio of better than 3000:1. Second generation 1.55 in. The imager is much different from the other current 1:85 aspect devices are in production at a resolution of technologies, therefore, the projector concepts must 4096 x 2160 at better than a 4000:1 contrast ratio. The SMPTE Motion Imaging Journal, December 2005 • www.smpte.org

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NEW PROJECTION TECHNOLOGIES: SILICON CRYSTAL REFLECTIVE DISPLAY AND GRATING LIGHT VALVE also be different. Prisms and beamsplitting optical blocks replace mirrors and dichroic filters that are normally used in typical LCD projectors. The first 4K models introduced will be less than 17 in. tall and 40 in. long. Figure 3 shows the system layout of the first-generation 4K projectors. The chassis of the projector is divided into four sections. The left section houses the lamp power supplies. Because the projector uses two lamps, two power supplies are required, which are mounted side by side and independently cooled. The center area of the projector holds the imagers, optical block, and lensFigure 3. Projector system layout. mounting hardware. The right section houses the power filtering, input processing, scan converter, final drive processing, and power supply for the signal path electronics. Finally, at the back of the projector are both lamp housings, incorporating xenon bulbs, and front-end optical processing (Fig. 3). • Light green depicts lamp housings and mechanicals • Cyan depicts the optical block • Red depicts light routing optics • Olive green depicts fans and cooling Figure 4. Light path showing reflective polarizer and SXRD • Brown depicts input processing imager. • Purple depicts lamp power supplies • Orange depicts the signal processing block fed to a second dichroic and is separated into blue and • Yellow depicts lens green. Each color is then fed to its perspective polarizThis projector uses a two-lamp system. For the 5Ker. Because the light is not polarized, it is reflected lumen model, two 1K-watt xenon lamps are used. For toward the SXRD device. Light then enters the SXRD the 10K-lumen model, two 2K-watt xenon lamps are chip and may or may not be rotated and reflected back used. The first advantage is having redundant light towards the same polarizer (Fig. 4). sources. If maximum brightness is not required, then If the SXRD imager has rotated the light, it remains one lamp can be used and the second can act as a hot in phase with the polarizer and passes through to the standby, or both lamps can be run at a lower brightnext step in the optical chain. If it is not rotated, it is ness, which would extend their life. The lamps are reflected back toward the light source. This means mounted facing each other, focusing into a prism, there is no need to absorb the excess light, reducing which directs the light into a lens chain that includes heat management problems and greatly improving the optics to even the light across its section. efficiency of the optical chain. This is also one method Dichroic mirrors separate the light into red and cyan. for achieving high contrast (2000:1 for the first modRed is then fed directly to the red polarizer. Cyan is els). Modulated light is then combined in a cross prism 476

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Figure 5. Projector light path.

only being considered, therefore, there hasn’t been much need for development of a 4K-signal format. There is still very little that can acquire or record it. In order to display this much information, it was necessary to build from devices, based on the 2K architecture. To make it simple, the decision was made to simply quadruple the number of inputs and feed the imager using four 2K-signal paths. This makes the signal path design more complicated, but keeps the overall cost down, because processing methods are based on 2K principles. The projector uses four SMPTE 372M inputs. Each 4:4:4 dual link connection can be fed to one of four or all quadrants of the SXRD chip. The first projectors were scheduled to deliver in September 2005. The first models will be 5K lumen and 10K lumen, respectively, and will follow the DC-28 DCDM Colorimetry recommendation.

Grating Light Valve

CIE 1931 Color Space Figure 6. Color space comparison.

Grating light valve (GLV) is a new device that operates as a spatial light modulator. The GLV falls into the same category as DLP, in that its operation is mechanical. These devices are called microelectrical mechanical system (MEMS). The modulation is accomplished using a physical moving imager. Unlike an LCD-based device that has no moving parts, the GLV uses simple metallic ribbons manipulated by a static source to reflect or refract incoming light. An interesting feature of this technology is that it uses lasers for its light source. David Bloom of Stanford University invented this version of GLV in 1992, and patents were issued in 1994 (other versions have been patented by different

and reflected out to the lens assembly (Fig. 5). Lamp sources are a new xenon design. This lamp technology offers very uniform spectrum as well as high brightness. Another advantage is that the spectrum is wide enough to allow gamut performance exceeding ITU 709 specifications. Plotting the gamut of different imaging technologies shows that, although not as good as film, xenon offers a much wider color space than what is used as the current standard (Fig. 6). Figure 7. The GLV imager. Until recently, a 4K-display was SMPTE Motion Imaging Journal, December 2005 • www.smpte.org

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NEW PROJECTION TECHNOLOGIES: SILICON CRYSTAL REFLECTIVE DISPLAY AND GRATING LIGHT VALVE companies). Silicon Light Machines was formed in 1994 and started development of the device for use in several different industries. In July 2000, Sony obtained licenses for its use in electronic displays. The HD version GLV measures 1.33 in. tall x .24 in. wide x .08 in. thick (Fig. 7). The imaging principle of GLV is based on coherent superposition of diffracted waves. It comprises of a silicon substrate layered with a set of six ribbons per pixel with pairing drivers. For HD display, the GLV is built using 6,480 individual ribbons (1080 x 6) each measuring 200 ␮m long x 3 ␮m wide, pretensioned to 0.8GPa. When voltage is applied between the upper aluminum surface and the lower electrode, the ribbon will sink 1/4 wavelength. Three ribbons act as bias ribbons, and the other three are active. Voltage is applied to every other ribbon so that the pixel set forms the grate pattern, which diffracts the incoming light. In the flat state, light is simply reflected back to the source. In the flexed state, it is diffracted at an incident angle away from the center of the device (Fig. 8). Ribbons are constructed of an aluminum base metal with a thin nitride coating for insulation. The aluminum thickness is in the range of 500 angstroms, with the nitride coating underneath being a little more than 1,000 angstroms thick. The ribbons are formed over a gap area that allows clearance for the flex of each ribbon. Tungsten is used as a static pad to draw the ribbon into its flexed state. Typically 18 V is the maximum used to achieve full brightness. Anymore than that, and refraction efficiency is degraded. This tungsten layer sits on a silicon base, using an oxide layer for conductive insulation. Drivers are attached using standard tape automated bonding (TAB) methods (Fig. 9). Figure 10 shows a cross section of the layers and mounting of the driver chips.

Moving Ribbon Air Gap Fixed Ribbon

Silicon Substrate

Aluminum 500AD Nitride 1000A Air1300A Tungsten 1000A

Up: Reflection Ribbons held up by tensile stress

Down: Diffraction Ribbons pulled down electrostactically

Oxide 5000A Silicon

Figure 8. GLV ribbon operation. Rows and columns driven from Alternate sides of array

One Pixel

Drive Line for moving ribbons

Figure 9. Close-up of GLV ribbons.

Dry Nitrogen

Clear Glass Lid

Digital Driver Chip

Module Substrate

Figure 10. The GLV construction.

The GLV Projector The construction of the GLV projector is much different from that of LCD, DLP, or SXRD. These technologies load the picture information into a fixed matrix image, but the GLV uses a horizontal scanning method. Each primary color uses a separate optical path, separate optics, and different color laser sources. Laser light is fed to a collimating lens element that fans out the laser source light to align with the GLV device. This 478

Figure 11. GLV projector light path.

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Linear GLV arry

Direction of optical scan

Figure 12. GLV scanning path.

focuses each pixel to a 25 ␮m x 25 ␮m area on each ribbon. The diffracted image from the GLV is collected at a second lens group and focused onto a spinning mirror that rotates at the vertical frame rate but scans horizontally across the picture (Fig. 11). Image processing feeds the GLV device as a vertical row and successively sends that information as one pixel for all vertical lines. This means that during the horizontal scan, each horizontal pixel for all rows is loaded. All 1080 pixels are loaded one at a time as the mirror scans horizontally. This means that all of the ribbons are loaded 1,920 times for each rotation of the mirror (Fig. 12). The first prototype projectors are already achieving amazing performance levels. The first and most important feature is a much improved color gamut. Because the light source is single-frequency laser, the primaries are at the outer edge of the perceptual range defined by the standard observer. This gamut is the widest of any display built so far. Current resolution is still at 1920 x 1080, but future development will enable higher resolutions. Contrast ratios are exceeding 2000:1 in the current prototypes. One very important feature of this type of display is that the aspect ratio of the picture can be changed with no loss of resolution. With matrix displays, the image must be processed to fit the imager’s horizontal and

vertical pixel count. This either leads to geometric distortions if the screen is filled, or some pixels must be cropped to maintain the proper image. With GLV, the display will scan across the horizontal. The horizontal information can be timed to sync with geometric requirements, allowing both 1:85 and 2:35 aspect ratios with the same horizontal and vertical pixel counts. In addition, the screen may not necessarily need to be flat. This presents solutions to display problems that were not practical with current technology and enables usage in areas such as medical and simulation, along with production applications.

Conclusion SXRD is now delivering in consumer products as 1920 x 1080 matrix imagers; 4096 x 2160 4K imagers were scheduled to deliver in September 2005. GLV is still in development. These technologies surpass performance of most displays currently used and have prospects of going even further. Both are targeted to different uses. SXRD is directly designed for digital cinema projection, and GLV is targeted toward the postproduction process.

Bibliography GLV images courtesy of Bob Monteverde, Silicon Light Machines. Kubota, Shigeo, “The Grating Light Valve Projector,” Optics & Photonics News, Sept. 2002. Siew, Ronian H., “Partial Coherent Imaging Using Grating Light Valve,” Agfa Corp. SXRD images courtesy of Michihiro Tobita and Taka Hayasaka, Sony Corp. Umeya, Shinijro and Matthews, Edwards, “ETD Project Completion,” Sony Corp. Presented at the 146th SMPTE Technical Conference and Exhibition in Pasadena, CA, October 20-23, 2004. Copyright © 2005 by SMPTE.

SMPTE Motion Imaging Journal, December 2005 • www.smpte.org

THE AUTHOR Gary Mandle joined Sony in 1985 as an electronic engineer involved in camera design of stabilization and CCDbased iris systems, which resulted in several patents. For the past eight years, he has been the product manager for Sony’s BVM evaluation grade monitors and large-venue projection products.

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