Tiny mirror arrays project the right image

Chip-sized arrays of mirrors have become a shining star in the world of electronic-image projection. Learning how these arrays and some newer microdisplay technologies work will help you appreciate how smaller really can be better.

Jim Lipman -- EDN, 2/17/2000

The days of projecting static textual and graphic information with incandescent lamps through an optical-lens system is quickly leaving us. The computer age has seen a proliferation of PC-based presentations, including animation, along with novel projection components.

A major innovation in microdisplays is the digital micromirror device (DMD). The DMD is a true microelectromechanical system (MEMS), with individually hinged mirrors mounted on a chip containing the mirror drivers. However, we are also seeing other types of microdisplay systems, fueled by evolving MEMS and liquid-crystal technologies. Understanding the DMD and these other microdisplay schemes will help you "see the light."

Mirror, mirror on the chip

Texas Instruments developed the DMD, an array of tiny mirrors, each mirror corresponding to one pixel, mounted over an SRAM array, one mirror for each SRAM cell. The mirrors measure 16 µm on a side with a linear pitch of 17 µm in both x and y directions. The mirrors, sometimes more than 2 million of them on a single chip, sit 2 µm above the silicon, attached by a center hidden support post (Reference 1). Hinges on two diagonal corners of the mirror allow the mirror to tilt when electrostatically attracted by one of two opposite diagonal corners of the SRAM cell. The datum—a one or a zero—stored in an SRAM cell tilts its mirror ±10°. A tilt in one direction causes that pixel to be diffracted and displayed; a tilt in the opposite direction means that the mirror is not displaying information (Figure 1). Nonactive mirrors in the array are in an off position (parallel to the silicon).

Signals using PWM control the mirrors through the SRAM. The PWM signals can switch the mirrors as many as 5000 times/sec. Each pixel diffracts light into or out of a projection lens. PWM also allows the DMD to create gray-scale images by varying a mirror's on and off times. A DMD-based system uses a color-filtering system with the incident light source to display color images (Figure 2). Display systems can use one, two, or three DMDs; the greater the number of DMDs, the brighter the image for a given light source (Reference 2). In a DMD color system, only one-third of the incident light, going through a rotating red/green/blue color filter hits the mirror array at any time. In a triple-DMD system, one DMD continuously receives each primary color. This technique results in a brighter display and one that can show higher saturated colors. Three-DMD systems can produce a displayed image with brightness as high as 12,000 lumens.

A big advantage of DMD-based projection systems over LCDs is that the DMD mirrors reflect light, whereas liquid crystals transmit polarized light through themselves. A simple way of looking at the two microdisplay technologies is to think of a DMD as a light switch and an LCD as a light valve. The reflective switch handles light more efficiently than the transmitting valve, resulting in a brighter display. Another advantage of DMDs over LCDs is that the mirrors in a DMD can be closer together than the individual liquid-crystal panes in an LCD. The closer spacing of a DMD, with a "fill factor" as high as 90%, results in a display image with higher resolution than LCDs with a lower fill factor. ("Fill factor" refers to the amount of the received image that the DMD projects.) One other advantage of DMDs is that they are less temperature-sensitive than liquid-crystal assemblies, a useful feature in display applications using heated light sources.

More on the microdisplay horizon

In contrast to DMDs, which are 2-D arrays of reflecting mirrors, is another diffractive MEMS microdisplay device in development: the grating light valve (GLV) from Silicon Light Machines. The GLV uses reflective ribbons, each about 3 µm wide and 100 µm long, suspended above a silicon substrate (Figure 3a). The technology anchors ribbons, composed of silicon nitride coated with aluminum, to the silicon and holds the ribbons in tension such that, when a voltage does not address them, they form straight lines parallel to the silicon's surface. When a voltage between the ribbon and a conducting surface on the silicon addresses the ribbon, it flexes, diffracting an image's pixel toward a display surface, such as a screen (Figure 3b). A deflection equal to one-fourth-wavelength of the incident light means that the pixel is fully on. No deflection corresponds to an "off" pixel. Deflections between these two limits create gray-scale images.

Each ribbon corresponds to one pixel, with a row of ribbons on a substrate representing a single scan line of a 2-D image. Each ribbon is also longer than a pixel's width, creating a 100% diffracting region in the middle of the ribbon. By scanning the substrate containing the row of ribbons, you get a full-image display. Light-display efficiency is around 70%, and the ribbons can switch in as little as 20 nsec, which is three orders of magnitude faster than a DMD mirror (Reference 3). Similar to a DMD system, three GLV arrays with a color-filtering subsystem display a color image.

Microdisplays are also fueling interesting support-device development. An example is the application-specific integrated lens (ASIL) from DigiLens (Reference 4). The ASIL is an electrically switchable, polymer-dispersed liquid crystal sand- wiched between two substrates. The substrates, coated on the polymer side with electrically conductive, transparent, indium-tin oxide (ITO), can be glass or, for lower cost and flexibility, plastic. An ASIL is laser-programmed to define a diffractive holographic property, such as color filtering or light deflection, for a monochromatic light source. By applying a voltage of around 40V ac to the ITO layers, you can turn off the diffractive property of the ASIL, providing a clear optical path.

One useful function for the ASIL is replacing the synchronized rotating wheel of a microdisplay color-filtering system. By stacking three ASILs, one each for red, green, and blue, in the light path of a microdisplay-projection system, you can display color images (Figure 4). In place of the large, noisy, power-hungry wheel, the small, static color-filtering system uses relatively low control voltages to synchronize the colors in a polychromatic display.

Author info

You can reach Jim Lipman at 1-925-606-1370, fax 1-925-606-1563, e-mail ednjim@earthlink.net.

REFERENCE

1. Younse, Jack, "Projection Display Systems Based on the Digital Micromirror Device," Proceedings of Micromechanical Structures and Microelectromechanical Devices for Optical Processing and Multimedia Applications, Oct 24, 1995, pg 64.

2. Yoder, Lars, "DLP: The State of the Art in Projection Display," Texas Instruments White Paper, May 1996.

3. Amm, DT, and RW Corrigan, "Grating Light Valve Technology: Update and Novel Applications," Proceedings of the Society for Information Display Symposium, May 1998, Paper 5.2.

4. "Application Specific Integrated Lenses (ASIL) Debut," Microdisplay Report, Volume 2, No. 11, November 1999, pg 1.