Digital Micromirror Device

Digital Micromirror DeviceTM (DMD) is a registered trademark of Texas Instruments. Digital Light ProcessingTM (DLP) is another technology developed by Texas Instruments that is mainly used for display applications. It has made significant inroads in the projection and display market. DLPTM technology (Fig.2.11) is a powerful and flexible technology and has already taken momentum on data and video projectors, HDTVs, and digital cinema products. Note that at the heart of these display solutions is DMDtm, a semiconductor-based light switch array of thousands of individually addressable and tiltable mirrors representing pixels.

DMD™ is considered to be a spatial light modulator (SLM), which reflects light that is produced externally. It is very attractive for many applications, including volumetric displays, holographic data storage, lithography, scientific instrumentation, and medical imaging, since the technology provides excellent resolution and brightness, high contrast and colour fidelity, and fast response times. These attributes are extremely important to many high-quality displays, printing and imaging applications. Liquid Crystal Displays (LCDs) are another familiar variety of SLMs.

In summary, it can be said that DMDTM is an array of semiconductor-based digital mirrors that precisely reflects a light source. Further, DMDTM enables DLPTM and displays images digitally. Rather than displaying digital signals as analog signals, DMDTM directs the digital signal directly to the screen.

The number of tiny mirrors depends on the size of the array. DMDTM are micromechanical silicon chips, which measure less than 5/8 of an inch on each side. It contains more than a quarter of a million tiny, movable aluminium mirrors and a wealth of electronic logic, memory and control circuitry. Each 16-^m2

mirror of the DMD™ consists of three physical layers and in between them two air-gap layers so that the mirror can be tilted +10 or -10 degrees, when subjected to electrostatic voltage. That is, if voltage is applied to the addressable electrodes, the mirror can tilt ±10 degrees signifying ON or OFF in a digital fashion.

There is a light source (Fig. 2.11) and display (screen) unit within the projection system. The micromirrors are mounted in such a way that the tilts are either toward the light source (ON) or away from it (OFF), creating a light or dark pixel on the screen, respectively. The bit-streamed image code to be displayed first enters the semiconductor which then directs each mirror to switch ON and OFF up to several thousand times per second. When a mirror is switched ON more frequently than OFF, it reflects a light gray pixel and a mirror that is switched OFF more frequently, reflects a darker gray pixel.

CMOS Substrate ^^^

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CMOS Substrate ^^^

- Projection Lens

Shaping Lens

Mirror Support Post Landing Tips

Torsion Hinge

Mirror Support Post Landing Tips

Torsion Hinge

Electro

K- Yoke

Hinge Support Post

Electro

K- Yoke

Hinge Support Post

Metal 3 Address Pads

Condensing Lens

Metal 3 Address Pads

To SRAM

Fig. 2.11. (a) A section of DMS showing two mirrors: (b) The three layers of a DMD mirror; (c) The projection details of a DMD™ utilizing DLPTM technology (Courtesy: Texas Instruments Inc., Source:); http://electronics.howstuffworks. com/projection-tv5.htm, http://www.ti.com/sc/docs/products/dlp/spie-paper.pdf)

In practice, the incoming light hits the three mirror pixels. The outer mirrors that are turned ON reflect the light through the projection lens and this is passed on to the screen surface. The mirrors are responsible for producing pixel images and are geometrically square in nature. The central mirror is made to tilt so that the OFF position is achieved. This mirror is responsible for reflecting light away from the lens to a light absorber so no light can reach the screen at that particular pixel, producing a dark pixel. In the same way, the remaining mirror pixels reflect light to the screen or away from it, depending on the input coded signal that is fed to the DMD™ chip. By using a colour filter system and by varying the amount of ON time of each of the mirror pixels a full-colour digital picture can effectively be projected onto the display screen. In this way, the mirrors in a DLPTM projection system can reflect pixels, even up to 1,024 shades of gray, to convert the video signal entering the DMD™ device into a highly detailed grayscale image.

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