Solid-state physicist and Texas Instruments fellow Larry Hornbeck won an Emmy for his invention of the digital micromirror device, the microelectromechanical system at the heart of TI's digital light processing (DLP) technology for projection displays. In addition to advancing the state of the art for digital cinema, front projectors and HDTV, Hornbeck's MEMS micromirror is enabling 3-D metrology systems, confocal microscopes that eliminate the out-of-focus "haze" normally seen around fluorescent samples, and holographic storage systems that write data in three dimensions instead of just two. Hornbeck told EE Times' R. Colin Johnson that TI has still more applications up its sleeve for the digital micromirror.
EE Times: In 1977, when Texas Instruments began its MEMS development,were you focusing on a particular application, such as light processing?
Larry Hornbeck: Even back then, we were interested in using MEMS to modulate light. I had only been at TI for a few years after receiving my PhD from Case Western University as a solid-state physicist. We got started in MEMS back then because of a Defense Department contract to make a spatial-light modulator using deformable mirrors. This was an analog technology that the DOD wanted to use for optical computing.
EE Times: I guess you had to invent the fabrication techniques you needed to even get started with MEMS.
Hornbeck: In 1981, MEMS meant bulk micromachining of single-crystal silicon, which made the devices expensive to manufacture. But the universities were already experimenting with surface micromachining of polysilicon, which was much more economical and has become the traditional way of doing MEMS today.
EE Times: As your micromirrors became more successful, did you dedicate a fab to MEMS?
Hornbeck: We've never needed a special fab, but have always made our MEMS fabrication compatible with our conventional CMOS manufacturing areas. We finish out all the transistors and metallization layers for interconnecting the transistors, and then we use a low-temperature process to put MEMS on top of the completed CMOS chip.
EE Times: So, you finish the whole chip but leave an area open to add the MEMS last?
Hornbeck: That's a radical departure from the way others do MEMS--I believe we are still the only company that does it this way. The way I implemented the process was by choosing aluminum alloys for the mechanical elements and conventional photoresist to act as a sacrificial spacer. All of this is done at temperatures under 200°C, so that metallization is not affected, the transistors are not affected, none of the finished CMOS circuitry is affected when we add MEMS to a chip. To this day, this forms our standard basis of manufacturing MEMS micromirrors, but it was a radical departure at the time.
EE Times: For MEMS vendors, the holy grail is seamless integration of MEMS structures onto the same CMOS chips as the circuitry to which they interface. All told, I think TI was ahead by integrating MEMS with CMOS from the start.
Hornbeck: This is one of the pillars we are resting on as a basis for our success in DLP.
EE Times: But at first, you were trying to make analog micromirrors.
Hornbeck: Yes, we struggled for several years trying to get enough uniformity and optical efficiency to do simple xerographic printing with a linear array of 2,400 analog micromirrors. But by 1986, it became apparent we were not going to be successful. The uniformity just wasn't there, our analog voltages were too high--as big as 30 volts--and still there wasn't enough mirror deflection angle. This was all because we were trying to make analog micromirrors. So the second radical departure to anything that anyone had done before was to go digital.
EE Times: What year was that?
Hornbeck: I invented the digital micromirror device in 1987 and applied for a patent that, when issued, formed the basis for all subsequent DMD architectures. Instead of continuing to develop analog MEMS micromirrors that depended upon a delicate balance between electrostatic attractive forces and the restoring forces of a flexure, I developed a micromirror that would flip between two digital states, where contact was made to stop the micromirror in the positive and negative directions. This technique made it easier to control the angles compared with our analog micromirrors, which had no stops.
EE Times: Are there other benefits to going digital?
Hornbeck: Yes. By operating the micromirrors in a bistable mode, we could go to much lower operating voltages, since the micromirrors could be triggered into either stable state. So this new digital architecture enabled larger rotation angles with better uniformity and lower operating voltages compared with analog micromirrors. That's been the basis ever since for our success with MEMS.
EE Times: Did you apply the digital design to the page-printing application you mentioned earlier?
Hornbeck: Actually, the first commercialization of DMD was in an airline ticket printer. We had a very successful impact printer for airline tickets in those days, but the industry was converting from the old-style red carbon copies to individual coupons. Printing individual coupons upped the speed requirement, and our impact printers couldn't keep up. So to maintain market share we decided to go to higher-speed xerographic printing. Instead of using a conventional polygon scanner, TI made the decision to use a linear DMD--an 840 X 1 array of micromirrors. The first product, the DMD2000 airline ticket printer, went to market in 1990.
EE Times: The HDTV application for DMD seems quite a stretch from the ticket printer application. What influenced TI to move in the HDTV direction?
Hornbeck: In 1989, the Defense Advanced Research Projects Agency started an initiative to spur the development of HDTV technology in the U.S., and TI was awarded a multimillion-dollar contract to develop a prototype high-definition DMD chip.
EE Times: That helped you in the transition from printing to light projection?
Hornbeck: Well, that was only the beginning. Rank-Brimar, a subsidiary of the Rank Corp. in the U.K., was looking for a way to project high-definition TV in very large formats for theaters and auditoriums. In 1989, they invested money to help us develop prototype three-chip DMD projectors. By 1991, TI itself decided to start a corporate venture project where we brought together the critical mass of people and resources to fund our own initiative, which we called the Digital Imaging Venture Project. Its goal was to develop high-definition television, which sounds strange for 1991, because back then TV was analog, nobody was doing anything with MEMS at that level, there wasn't a product or a standard or anything. So what we decided to do was to go for digital high-definition TV, but to begin with projectors because we could already build them and had a customer. By 1996, we had our first DLP products.
EE Times: What kinds of projectors were available back then?
Hornbeck: In 1994, a traditional projector weighed 35 to 40 pounds, was relatively dim, and cost between $15,000 and $18,000. So we thought we could dramatically impact the weight, brightness and cost of projectors. Front projection became the means to our early success. We started off with just three customers in 1996--InFocus, nView and Proxima--and now we have 75. Epson had an LCD projector at that time that was our main competitor.
In those days, we provided a complete digital light engine to OEMs, because no engineers were familiar with designing digital light projectors. Now we provide chip sets and software to our OEMs. The chip sets consist of one or three DMDs, an ASIC for image processing and formatting, and a waveform chip to drive the DMD. The software, DLP Composer, allows OEMs to design customized projectors.
Today, DLP has about 50 percent of the worldwide front-projection market, with over 350 products available. DLP is the market leader in 1,080p HDTV technology for displays 40 inches and above.
EE Times: How did TI get into the digital-cinema business?
Hornbeck: In 1997, the year after our introduction of business projectors to the market, we introduced the first high-brightness three-chip DLP systems for large-venue applications. DMD is naturally suited to these applications because of its ability to take the heat loads from very bright projection lamps. We started talking to movie makers about what kind of technology would work for them, and eventually we won them over.
In the meantime, our three-chip systems were being used by the TV broadcast studios as monitors behind news anchors and for game shows because of their color stability. And that's how it came about that both TI and myself were awarded Emmys in 1998 from the Academy of Televisions Arts & Sciences. TI got its Emmy for DLP TVs, and I got mine for the invention of digital micromirrors. I have my Emmy at home on a shelf. It makes for quite a conversation piece.
EE Times: What's next?
Hornbeck: TI has begun equipping digital cinemas with 3-D versions of its DLP Cinema projection technology, and at the other end of the spectrum it's showing a prototype of a tiny DLP chip that is small and cheap enough to add a projector to a handheld device, enabling relatively large displays to be projected from, say, a cell phone.
Regarding the future of digital micro- mirrors, just use your imagination. We are developing reference designs for almost anything a DLP system could possibly be used for, and we have several important announcements in new application areas that we plan to make very soon.
See related image