Texas Instruments Beats the Drum for its 'Digital Light Processing' MEMS
The first such projectors were unveiled about a year ago, led by the LP420 from In-Focus Corp. Now, it is being joined by smaller, yet more powerful projectors, the UP-800 from Plus Corp., and the Davis Litebeam. These projectors provide up to 600-lumen brightness, and rich color saturation on the equivalent of an SVGA screen, but are physcially about the size of a laptop computer and weigh less than 10 lb. The products are defining a new sector of the projector marketplace: ultraportables. TI expects to work with these and other vendors of projection equipment (including large-screen televisions), as well as vendors of high-resolution digital printing devices. To this end, TI has formed a Digital Imaging Venture Project to continue development of the technology.
At the heart of TI's Digital Light Processing technology is the Digital Micromirror Device (DMD), a static random access memory (SRAM) chip with an array of 508,000 (848 x 600) hinged, microscopic mirrors attached to its upper surface. Each mirror is equivalent to a single pixel in the projected image. Color light is reflected by the mirrors: the relative amount of time each mirror is in the 'on' or 'off' position when red, green or blue light shines on it determines the hue and shade of the pixel it generates. The unique DMD pixel geometry and high aperture ration of 90+0/0 enable an image to be projected which is almost seamless compared to other technologies.
According to TI literature, the DMD light switch (Figure 1) is a MEMS structure that is fabricated by CMOS-like processes over a CMOS memory. Each light switch has an aluminum mirror, 16 mm square, that can reflect light in one of two directions depending on the state of the underlying memory cell. With the memory cell in the (1) state, the mirror rotates to +10 degrees. With the memory cell in the (0) state, the mirror rotates to -10 degrees. By combining the DMD with a suitable light source and projection optics, the mirror reflects incident light either into or out of the pupil of the projection lens. Gray scale is achieved by binary pulsewidth modulation of the incident light. Color is achieved by using color filters, either stationary or rotating, in combination with one, two, or three DMD chips.
Figure 1: Two DMD pixels (mirrors are shown transparent)
The DMD pixel is a monolithically integrated MEMS superstructure cell fabricated over a CMOS SRAM cell (Figure 2). An organic sacrificial layer is plasma etched to produce air gaps between the metal layers of the superstructure. The air gaps free the structure to rotate about two compliant torsion hinges. The mirror is rigidly connected to an underlying yoke. The yoke, in turn is connected by two thin, mechanically compliant torsion hinges to support posts that are attached to the underlying substrate.
Figure 2: DMD pixel, exploded view
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 torques as in earlier TI devices, the rotation angle is precisely determined.
The address electrodes for the mirror and yoke are connected to the complementary sides of the underlying SRAM cell. The yoke and mirror are connected to a bias bus fabricated at the Metal-3 layer. The bias bus interconnects the yoke and mirrors of each pixel to a bond pad at the chip perimeter. An off-chip driver supplies the bias waveform necessary for proper digital operation. The DMD mirrors are 16 mm square and made of aluminum for maximum reflectivity. They are arrayed to form a matrix having a high fill factor (approximately 90%) for maximum use of light.
In terms of chip processing, TI literature says that the DMD is a surface micromachined MEMS device, using aluminum as the mechanical material compared with the more common choice, polysilicon. It has benefited directly from being IC-compatible in several ways. The underlying memory cell is a conventional 5-volt, six-transistor SRAM cell. No special circuits or high-voltage transistors were developed. SRAMs were chosen because of their light insensitivity, even at the highest illumination levels used in theater projection systems. The DMD superstructure uses conventional IC materials, aluminum for the electrical and mechanical elements, and DUV-hardened photoresist for the sacrificial materials that define the air gaps. The metal is sputter deposited and plasma etched. No special deposition or etching techniques were developed for either material. The only non-standard material is a special aluminum alloy for the hinge. But even this alloy is deposited and etched in the conventional IC fashion. In essence, says TI, the DMD superstructure is just a fancy multilevel metalization structure.
The fabrication of the DMD superstructure begins with a completed CMOS memory circuit. A thick oxide is deposited over Metal-2 of the CMOS and then planarized using a chemical mechanical polish (CMP) technique. The CMP step provides a completely flat substrate for DMD superstructure fabrication, ensuring that the projector's brightness uniformity and contrast ratio are not degraded.
Through the use of six photomask layers, the superstructure is formed with layers of aluminum for the address electrode (Metal-3), hinge, yoke, and mirror layers and hardened photoresist for the sacrificial layers (Spacer-1 and Spacer-2) that form the two air gaps. The aluminum is sputter-deposited and plasma-etched using plasma deposited SiO2 as the etch mask. Later in the packaging flow the sacrificial layers are plasma-ashed to form the air gaps.
After the air gaps are formed, the DMD superstructure is too delicate to survive the conventional saw and cleaning process used to separate the chips from one another. Therefore, before the sacrificial layers are removed, the wafers are partially sawed along the chip scribe lines to a depth that will allow them to be easily broken apart later. The partially sawed and cleaned wafers then proceed to a plasma etcher that is used to selectively strip the organic sacrificial layers from under the DMD mirror, yoke, and hinge layers. Following this process is a so-called passivation step wherein a thin, self-limiting, anti-stick layer is deposited to lower the surface energy of the contacting parts of the DMD superstructure and to provide lubrication. This passivation step, in conjunction with the electronic reset sequence, ensures reliable operation for the life of the device.
Before separating the chips from one another, each chip is tested for full electrical and optical functionality by an automated wafer tester. The chips are then broken apart along the partially sawed scribe lines, placed in packages, and wire bonded. A plasma cleaning step and second passivation immediately prior to window attachment ensures a high-quality anti-stick layer. A lid with a high-quality optical window is welded to the package weld ring to ensure a hermetic and clean environment for the DMD. Several elevated temperature burn-in tests are then performed before the finished DMD parts are qualified.
Figure 3 presents photomicrographs of completed DMD chips after spacer removal. A 3 x 3 array of mirrors is shown in the upper left corner. To the right is another 3 x 3 array, where one mirror has been removed to reveal the underlying mechanical structure. The bottom photographs show closeups of a DMD wafer that was removed from the process flow after completion of the yoke layer. Spacer-1 has been etched to give a realistic view of the mechanical structure underlying the mirror.
In summing up the DMD MEMS technology TI says that conventional, surface-micromachined MEMS technology uses polysilicon as the mechanical material and phosphosilicate glass (PSG) as the sacrificial layer. The number of mechanical elements is typically 1-10. Contrast this with DMD technology where the number of mechanical elements is ~106, the mechanical material is aluminum and the sacrificial layer is photoresist. A summary of the distinguishing features of DMD MEMS technology is given in Table 1.
| Table 1. Distinguishing features of DMD MEMS technology | |
|---|---|
| Number of moving parts | 0.5 to 1.2 million |
| Mechanical motion | Makes discrete contacts or landings |
| Lifetime requirement | 450 billion contacts per moving part |
| Address voltage | Limited by 5 volt CMOS technology |
| Mechanical elements | Aluminum |
| Process | Low temp., sputter deposition, plasma etch |
| Sacrificial layer | Organic, dry-etched, wafer-level removal |
| Die separation | After removal of sacrificial spacer |
| Package | Optical, hermetic, thermal vias |
| Testing | High-speed electro-optical before die separation |
More information is available at TI's Web site, ti.com.
By Nick Basta