Reflecting on the Mirrors

Like an inexplicably mixed-up dream, after hours of inundation from today‘s media-enriched environment, a girl has a conversation with an elephant in a field full of daisies. She opens a box from which a brilliant beam of light shoots towards the sky, and she whispers, “It‘s amazing. It‘s the mirrors” [1]. In this advertisement, the little girl lets the viewer in on the secret of Digital Light Processing (DLP) television, which uses millions of microscopic mirrors to project images onto the screen. Millions of mirrors work in harmony to create vivid pictures, yet the entire array can fit within a box only centimeters wide. To a person unfamiliar with how this might work, the DLP television does seem like a cryptic black box of magic. Actually, the principles behind how these televisions work are rather straightforward. And now, there may even be a better and simpler way to achieve similar results.

The most complicated aspect of these televisions is the manufacturing and integration of the systems due to the emerging field of microelectromechanic​al systems (MEMS). These tiny devices are unconventionally fabricated, etched with light and chemicals rather than tools. Still, the Digital Micromirror Device (DMD) may become one of the more easily understood applications of MEMS in daily life due to its simpler mechanical principles. Small-scale devices have become much more prevalent in recent years due to wireless technology, increased memory capacity, and shrinking volume.

DMD technology was originally developed by Texas Instruments in 1987, and it has recently become a major player in the high-definition television (HDTV) market with the incorporation of color and an aggressive advertising campaign. Conceptually, the micromirror has the most potential for larger, brighter displays, surpassing the more standard liquid crystal and plasma reflecting systems. These other types of systems are limited against high-powered light sources because absorption of light in these materials generates a lot of heat within the display [2]. Micromirrors have the added benefit of higher optical efficiency, meaning almost all the light is reflected and not absorbed. The result is a brighter picture for a given light source compared to liquid crystal or plasma displays. If the picture is brighter, there is more headroom to make the display larger, since the projection can expand farther before becoming dark.

There are numerous little mirrors involved in creating a picture. For each pixel on a screen, there is a corresponding mirror with a length on the scale of several microns. Today’s pinnacle of performance calls for a resolution of 1,920 x 1,080 pixels, resulting in over 2,000,000 tiny mirrors behind the screen, moving thousands of times per second.

For every pixel, there is a set of electrodes that controls the direct rotation of the mirror using electrostatic forces. Each mirror rotates to reflect the proper amount of light for the image of the moment. Technically, the DLP mirror’s rotation is binary (rotated 10° toward the light source into the ‘on’ position), or rotated 10° away from the light source into the ‘off’ position. If the mirror is in the ‘on’ position, a completely white pixel is displayed on the screen. The ‘off’ position reflects a black pixel. To produce shades of gray, the mirror moves back and forth between the on and off positions at different speeds [3].

When creating a color picture, the same principle is used for each additive component of light: red, green, and blue. White light from the signal lamp is split by a prism into the three primary colors, and then these beams are routed to different DLP chips. The three separate chips handle each of the respective colors of light and reflect the desired amount of their color. If a purple section needs to be projected onto the screen, mirrors from the red and blue arrays will shine for that same area and pass the light to the projection lens to be put on the screen. The human eye blends these two flashes together to create the proper color [3].

The sole micromirror chip currently available in televisions from various makers is the DLP system made by Texas Instruments. But a new variant on the technology has been developed by Daewoo Electronics, and it is poised to push the competitive market in DMD projection. At Daewoo’s Actuated Mirror Array Research Center in Seoul, Korea, Kuy-Ho Hwang and other engineers have devised an alternative to the digital concept by DLP. The on/off status of the DLP mirrors sounds simple enough, but the controls necessary to produce the hundreds of other intensities are very complex and costly. Daewoo Electronics took a more simplistic approach by creating shades of gray based on the tilting angle of the mirror. Instead of only having on/off states, the Thin Film Actuated Mirror Array (TFAMA) has a continuous spectrum of tilting angles (Fig. 1).

Despite its microscopic size, the basic mechanics of this device are exactly the same as large beam structures like diving boards. If one end of the beam is fixed, and a force is applied at the opposite end, a deflection will occur that is proportional to the applied force [4]. For example, a 300 lb person on the end of a diving board will make the end bend downward twice as far as a 150 lb person would. Cantilevered beams are a well-known type of structure, so the amount of deflection that the structure undergoes from an external force is easily determined. This knowledge allows precise control and manipulation of the beam system. When the mirror moves in various angles, its position changes to a different color and intensity. To set the TFAMA apart from DLP, Daewoo made its design highly dependent on unique materials.

Piezoelectric material (PZT) is placed between the electrodes in each mirror’s structure. These types of materials have a particular property so that when placed in an electric field, dimensions of the material will expand or contract depending on its orientation.

When a current is applied to the electrodes in the TFAMA structure, the piezoelectric material shrinks in the x-direction and expands in the y-direction, resulting in the beam tilting upwards. Hwang and his team found that the tilt angle was directly proportional to the voltage driving the circuit.

Using a gate or stop between the mirror component and the projection lens allows incremental amounts of light to pass through, creating the many shades of gray between black and white. A particular amount of current in the electrodes corresponds to a specific tilting angle of the mirror, which gives a unique intensity of gray (or color, if the light is split). At the time of publication, the TFAMA could produce 16.7 million colors, which is on par with the single-chip DLP [2], but still far fewer than the 35 trillion colors achievable by the three-chip DLP previously described.

With three separate arrays of TFAMA to handle the primary colors, similar results could be produced. But the main concern is still in manufacturing these small devices since precision is so much more important on the micro-scale.

Machining these small structures requires building them up layer-by-layer, involving very carefully chosen materials and methods. MEMS devices are built up by repeated cycles of growth, patterning, and etching.

A silicon base lies beneath the entire array, and it is allowed to ‘grow’ by oxidation. With each successive piece of the structure, a layer of material is deposited on top of the previous layers before being cut out. The patterning and etching are done using materials sensitive only to the topmost layer so that the lower layers—which have been completely shaped—do not become altered. The electrodes are made of platinum, and they hold between them the PZT, and atop the electrodes sits an aluminum mirror.

The most crucial issue in manufacturing the mirror arrays is getting them to be as flat as possible. If the mirror deviates from being completely flat, the optical efficiency drops, and the picture brightness—the main selling point for micromirror projectors—is compromised. Problems arise in making the mirror surfaces flat because the final process in machining the structure requires slight removal of top layers. Taking away this material, even in small amounts, leads to stress and warping of the finished product.

One possible way to resolve the issue is to make numerous, smaller mirrors that are not large enough to warp in this way. As long as a group of smaller mirrors acts together to move like one larger mirror that corresponds to one pixel, the effect should be equivalent. Still, this would require more manufacturing time and cost, which draws away from the allure of such devices.

The amount of brightness put out by micromirror devices allows for a more brilliant HDTV display with longer life and lower cost. Higher brightness in the picture gives more room for larger screens with vivid pictures. There is no chance of burned-in images on the screen—which is typical of traditional Cathode Ray Tube and newer Plasma displays—since there is no tube or phosphor involved. Replaceable light sources reduce the cost of maintenance. And the base price of digital micromirror powered televisions is lower than that of LCD or Plasma televisions of the same size and resolution.

On the near horizon is Daewoo Electronics’ new interpretation of Texas Instruments’ present standard DLP micromirror device, and televisions powered by this technology may drop prices even further with their simpler electronics. MEMs devices may be the key to maximizing television performance and meeting the ever increasing public demand for screen size and picture quality.