Digital Micromirror Device technology

Digital Micromirror Device, or DMD is an optical semiconductorthat is the core of DLP projection technology, and was invented by Dr. LarryHornbeck and Dr. William E. ‘Ed’ Nelson of Texas Instruments (TI) in 1987.The DMD project began as the Deformable Mirror Device in 1977, usingmicromechanical, analog light modulators. The first analog DMD productwas the TI DMD2000 airline ticket printer that used a DMD instead of alaser scanner. A DMD chip has on its surface several hundred thousandmicroscopic mirrors arranged in a rectangular array which correspond to thepixels in the image to be displayed. The mirrors can be individually rotated±10-12°, to an on or off state. In the on state, light from the bulb isreflected onto the lens making the pixel appear bright on the screen.
In the off state, the light is directed elsewhere (usually onto a heatsink), making the pixel appear dark. To produce greyscales, the mirror is toggled on and offvery quickly, and the ratio of on time to off time determines the shadeproduced (binary pulse-width modulation). Contemporary DMD chips canproduce up to 1024 shades of gray. See DLP for discussion of how colorimages are produced in DMD-based systems. The mirrors themselves aremade out of aluminum and are around 16 micrometres across. Each one ismounted on a yoke which in turn is connected to two support posts bycompliant torsion hinges. In this type of hinge, the axle is fixed at both endsand literally twists in the middle.
Because of the small scale, hinge fatigue is not a problem and tests have shown that even 1 trillion operations does not cause noticeable damage. Tests have also shown that the hinges cannot be damaged by normal shock and vibration, since it is absorbed by the DMD superstructure. Two pairs of electrodes on either side of the hinge control theposition of the mirror by electrostatic attraction. One pair acts on the yokeand the other acts on the mirror directly. The majority of the time, equal biascharges are applied to both sides simultaneously. Instead of flipping to acentral position as one might expect, this actually holds the mirror in itscurrent position. This is because attraction force on the side the mirror isalready tilted towards is greater, since that side is closer to the electrodes. Tomove the mirror, the required state is first loaded into an SRAM cell locatedbeneath the pixel, which is also connected to the electrodes. The bias voltageis then removed, allowing the charges from the SRAM cell to prevail,moving the mirror. When the bias is restored, the mirror is once again heldin position, and the next required movement can be loaded into the memorycell. The bias system is used because it reduces the voltage levels required toaddress the pixels such that they can be driven directly from the SRAM cell,and also because the bias voltage can be removed at the same time for thewhole chip, meaning every mirror moves at the same instant. Theadvantages of the latter are more accurate timing and a more filmic movingimage.