A team of researchers at the IBM T. J. Watson Research Center (Yorktown Heights, NY) recently designed and manufactured experimental liquid-crystal-on-silicon spatial light modulators (SLMs) that meet or exceed the SXGA (1024 × 768 pixel) and UXGA (1600 × 1280 pixel) standards. They used three 2048 × 2048-pixel arrays to build a 28-in.-diagonal color monitor. Though the technology is not yet commercially available, the researchers say it demonstrates the potential of small, high-resolution display elements.1
Putting liquid crystal (LC) directly onto a reflective silicon backplane has major cost advantages over many other types of display. The circuitry is made using standard silicon processes but does not contain any particularly small features. Thus, the display manufacturers can take advantage not only of the economies of scale involved in silicon fabrication, but also of older, less-fashionable, low-resolution silicon processes, such as those designed for 1.2-µm features. Equipment designed for this resolution is often significantly underused, so LC-display companies are likely to have the pick of fabrication houses to work with.
Though many other companies are moving into this area, the recent work at IBM has leap frogged most of the competition. Reflective LC modulators have been fabricated with a pixel count almost three times that of the nearest rivals, and these experimental devices have been built into a working system.
Despite the high pixel count, the basic design of the 5-cm-diagonal silicon backplanes is fairly standard. Each 17 × 17-µm pixel has its own transistor, storage capacitor, and electrode, and the mirror surface is made reflective by chemical-mechanical polishing. Row—or gate—drivers are integrated into the chip, and gate lines driven from both ends for electrical efficiency. The number of contacts required was halved to 1024 on a side by incorporating demultiplexing circuits into the device. The liquid crystal used was a nematic twisted to 45°.
In the projection display, a polarizing beamsplitter separates the incoming white light—from a 100-W lamp—from that coming out of the system. Red-, green- and blue-filtered modulators are mounted on prisms that route and recombine the light. The image is then projected onto a flat-surface diffusing screen. According to Paul Alt of IBM, aligning the modulators was relatively straightforward—though the optimal overlap of a third of a pixel does slip toward the edges. The display had a luminance of 100 cd/m2 and a contrast ratio of 3.8 at two line pairs per millimeter (in green) and was driven as a monitor for a personal computer running Windows NT.
Though the experimental system was probably no smaller than a cathode-ray tube with the same diagonal, it is dramatically lighter—most of the 20 × 20 × 20-in. cube is air. Also, Alt claims that the subjective appearance of natural images displayed on it is better than the raw performance data would suggest. He suggests that the combination of high pixel count at high spatial resolution, good contrast at very high spatial frequencies, and proper color balance might explain this. Though white-light applications are important, Alt sees pairing liquid-crystal-on-silicon modulators with monochromatic sources—such as vertical-cavity surface-emitting lasers and light-emitting diodes—as promising opportunities for the future.
REFERENCE
1. P. M. Alt, 17th International Display Research Conference, Toronto, Canada, Paper M1.4 (Sept. 1997).