Electronic Design

An LCoS-Based Rear-Projector Display May Supplant TFTs In Larger Sizes

Small thin-film-transistor (TFT) displays are cropping up everywhere. TFT displays with diagonal dimensions of 20 in. or more, however, are less common because they're more expensive. Their costs can soar as high as four or five figures.

Philips Flat Display Systems, San Jose, Calif., thinks it has a solution to these high prices. Applications that require larger displays, such as desktop monitors, multimedia projectors, and large-screen TVs, can use liquid-crystal-on-silicon (LCoS) displays instead of TFT technology. Specifically, the company will use LCoS as a light engine in an LCD projector.

Toward this end, Philips will work with Hana Microdisplay Technologies Inc. (HMTI), Twinsburg, Ohio, to manufacture LCoS panels. Luc Bauer, HMTI president and CEO, says that his firm is one of the few companies today that can provide contract manufacturing of LCoS display panels.

Designers configure the display around an LCoS that may be no more than 0.8 in. along the diagonal. Then, they couple it to a rear-projection system with magnification on the order of 25 or so. This produces a display that costs a lot less than a direct-view TFT display, despite the costs of the projection system. Researchers say that it's similar to the display devised by Optical Coating Laboratory (OCL) Inc., Santa Rosa, Calif. (see the figure).

OCL's LCoS rear-projection display uses an ultra-high-pressure mercury lamp with a hemispherical reflector as its light source. The light passes first through a blue lens that collimates the beam. Next, the homogenizer ensures that the beam is uniform. The polarization-conversion system removes UV and IR light components to protect the LCDs and to reduce the operating temperature. The beam then enters the polarizing beam splitter, which deflects the beam downwards to the three-component, color-splitting/combining Philips prism.

The three primary color comp-onents—red, green, and blue—are then sent to the three LCoS LCDs. The LCDs are rectangles with a 4:3 aspect ratio (16:9 for HDTV) on the order of 0.8 to 0.9 in. diagonally. Each LCD modulates a primary color. These modulated beams are reflected upward and returned to the polarizing beam splitter. The converged image then passes through the polarizer, the projection lenses, and the fold mirror. Finally, it's projected onto the screen.

Admittedly, this type of rear-projection display isn't going to be as thin as a flat panel. It may be 8 to 9 in. deep, versus the TFT flat panel's 0.5 in. Still, it's much thinner than a CRT of comparable screen size. Also, the silicon transistors are mounted in silicon instead of glass, opening LCoS manufacturing to a plethora of benefits.

LCoS fabrication can take advantage of the huge advances in silicon wafer processing, which is an industry that's constantly improving its technologies. Conventional semiconductor wafer-processing equipment will fill the bill instead of the costly, customized equipment necessary for handling very large substrate TFT display manufacturing.

LCoS displays can be developed with silicon design tools that are available virtually everywhere. It's not necessary to descend to the costlier 0.18- and 0.13-µm geometries. The 0.5- to 0.8-µm range will do just fine, providing ample resolution.

Researchers already are anticipating the cost savings. Philips says a 25-in. TFT monitor isn't available yet. But a 15-in. flat-panel monitor costs $1000, and an 18-in. monitor costs nearly $2500. It's certainly likely, then, that a 25-in. monitor would cost upwards of $4000. Bauer, meanwhile, believes an LCoS version will cost $2000 or less.

Under the agreement's terms, Philips and HMTI will collaborate on the manufacture of LCoS panels at HMTI's 18,000-ft.2 LCoS manufacturing facility in Ohio. Both parties will work together, sharing proprietary display specifications as well as manufacturing and process expertise in their endeavor to produce high-quality LCoS panels.

"By utilizing HMTI's established manufacturing facility, rather than building our own facility, we can focus our resources on the other critical design factors necessary to commercialize this breakthrough technology," says Wald Siskens, general manager of Philips' Flat Display LCoS group,

David E. Mentley, senior vice president of Stanford Resources Inc. in San Jose, Calif., sees the benefits. "You can wind up with resolutions on the order of 1920 by 1080 pixels that might be used for a high-definition TV—millions of pixels on a small light modulator— without pushing the silicon part of the technology at all," he says. He isn't so sure, though, that LCoS will supplant active-matrix TFTs. "LCoS is viewed as the ultimate, low-cost solution for high-resolution, miniature displays and projection engines...\[but\] it may not be so simple a task to reduce these things into practice."

TAGS: Components
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