Digital Micromirror Device (
The applications of large screen, high brightness electronic projection displays include (1) electronic presentations, (2) entertainment, (3) status and information and (4) simulation (e.g., training, and games). So far the electronic presentation market is being driven by the pervasiveness of software that has put sophisticated presentation techniques into the hands of the average PC user. Normally three of projection display techniques are used widely, i.e. oil film, CRT-LCD, and AM-LCD. Oil film projectors is developed in early 1940s and have been the workhorse for applications that require projection displays of the highest brightness. But the oil film projector has a number of limitations including size, weight, power, setup time, stability, and maintenance. In response to these limitations, LCD-based technologies have challenged the oil film projector. These LCD-based projectors are of two general types: (1) CRT-addressed LCD light valves and (2) active-matrix (AM) LCD panels. LCD-based projectors have not provided the perfect solution for the entire range of high-brightness applications. CRT-addressed LCD light valves have setup time and stability limitations. Most active-matrix LCDs used for high-brightness applications are transmissive and, because of this, heat generated by light absorption cannot be dissipated with a heatsink attached to the substrate. This limitation is mitigated by the use of large-area LCD panels with forced-air cooling. However, it may still be difficult to implement effective cooling at the highest brightness levels. In response to these and other limitations, as well as to provide superior image quality under the most demanding environmental conditions, high brightness projection display systems have been developed based on Digital Light Processing (DLP) technology. DLP is based on a microelectromechanical system (MEMS) device known as the Digital Micromirror Device.
"Digital Micromirror Device", or "DMD". Is developed by Larry J. Hornbeck of Texas Instruments Inc. This device combines aluminum alloy mirrors, silicon based electrostatic drives, and silicon microelectronics to create a "light switch". Because the entire device is micro machined on a single silicon chip, each mirror is so small that "dozens of them are covered by the tip of a standard straight pin". It is expected that these mirror devices will play a major role in the next generation of video displays and hard copy printers. An example of the device is shown below in Figure 1. This is a 1280 x 1042 Digital Micromirror Device. The central, reflective portion of the device consists of 1,310,720 tiny, tiltable mirrors. A glass window seals and protects the mirrors. Each mirror is 16 um on a side.
Figure 1. SXGA DMD device with black aperture.
dmd cell (mirror) structure
The DMD pixel is an integrated MEMS structure which is fabricated on a CMOS SRAM cell. Refer to the Figure 2, the mirror (aluminia) is rigidly connected to an underlying yoke. The yoke is connected by two thin, mechanically compliant torsion hinges (also aluminia) which are supported by the posts that are attached to the underlying substrate. Electrostatic fields are developed between the underlying memory cell and the yoke and mirror, creating an electrostatic torque. This torque works against the restoring torque of the hinges to produce mirror rotation in the positive or negative direction. The mirror and yoke rotate until the yoke comes to rest against mechanical stops that are at the same potential as the yoke. Because geometry determines the rotation angle, as opposed to a balance of electrostatic torque as in other micromirror devices, the rotation angle is precisely determined.
Figure 2. Schematic of DMD two pixels superstructure.
The address electrodes for the mirror and yoke are connected to the complementary sides of the underlying SRAM cell. The yoke and...
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