BOSTON -- Initial technical reports are filtering out of two young companies working on flat-panel displays based on microelectromechanical-system (MEMS) technology. Silicon Light Machines Inc. (Sunnyvale, Calif.) disclosed details of its miniature Grating Light Valve (GLV) displays at the recent Photonics West conference, while Iridigm Display Corp. (Boston) plans to discuss its Interferometric Modulator (IMod) displays next month at the Society for Information Display conference.

MEMS promise highly efficient reflective flat-panel displays that demand few dollars and watts, and can be manufactured using conventional IC fabrication processes. The single MEMS display available today, the digital micromirror device (DMD) of Texas Instruments Inc. (Dallas), is successfully carving out a niche for itself in electronic projectors. The GLV and IMod displays use a simpler structure than the DMD and are hence more manufacturable, according to their developers. While individual pixels in a DMD are made up of tilting mirrors, both SLM and Iridigm make use of tiny membranes, or "microbridges," suspended over an air gap. By manipulating these membranes, the optical characteristics of the pixel is transformed.

Silicon Light Machines is a 1994 spinout from the Ginzton Laboratory at Stanford University. The company's GLV consists of parallel rows of aluminum-coated silicon-nitride "ribbons" suspended above a silicon-dioxide layer. When appropriate voltages are applied to a ribbon and an associated electrode, the ribbon is pulled down a minute distance by electrostatic force, creating an interference pattern that diffracts light. When the voltages are removed, the ribbon snaps back into place.

"When all the ribbons are in the same plane, incident light is reflected from their surfaces," D.M. Bloom of SLM said at Photonics West.

SLM is pursuing 20-micron-square pixels that use a set of four ribbons, each 20 microns long on a 5-micron pitch. Only two of the four ribbons can be manipulated; the other ribbons are fixed. The ribbon "pull-down distance" is determined during manufacture.

Iridigm, which stands for iridescent paradigm, was founded in 1994 by Mark Miles, the inventor of IMod and now chief technical officer of Iridigm, and Erik Larson, president. Electromechanically, the IMod appears similar to SLM's GLV -- when it's appropriately driven, the membrane collapses to fill an air gap -- but the IMods are square aluminum membranes rather than a set of elongated silicon-nitride ribbons. At the current stage of development, an IMod measures 25-microns square. The GLV and IMod differ optically, though both operate off incident light.

SLM's GLVs operate by diffraction, Iridigm's IMods by irridescence. The surface and membrane of an IMod are both coated with a stack of thin-film optical materials. According to Miles, the air gap between the surface and membrane forms an "optically resonant cavity" whose dimensions are tuned during manufacturing to reflect the peak wavelength of some particular color.

In its quiescent state, the IMod reflects light; when it's switched, the microbridge is lowered to the surface and the structure absorbs light. In short, when the bridge is up, the pixel is on, and when the bridge is down, the pixel is off -- exactly the opposite of a GLV. Alternatively, the IMods can be tuned to switch between two colors rather than on/off states. IMods are viewed through the glass substrate, while GLVs are viewed from the opposite side.

SLM's basic GLV can be manufactured with only two masks, but SLM has built GLV devices using a total of seven masks. "The simple GLV design should provide lower initial costs and higher yields compared with light-valve technologies that require more complex manufacturing," Bloom said.

The relative simplicity of microbridges also translates into great speed, thanks to "the small size and mass and small excursions of the GLV ribbons," Bloom said. At the geometries detailed above, switching time for a GLV is just 20 nanoseconds -- that's "roughly a million times faster than conventional LCD display devices and about 1,000 times faster than Texas Instruments' DMD," Bloom said. Such speed simplifies the generation of gray scale through pulse-width modulation.

The IMod is also fast. "These devices have already exhibited switching speeds in excess of 50 kHz," Miles will tell the SID conference. "Models indicate the potential for multi-MHz switching."

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