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TI Fellow details digital micromirror apps

Posted: 02 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? digital light processing? digital micromirror device? MEMS in displays?

TI has more applications up its sleeve for the digital micromirror.

Solid-state physicist and Texas Instruments (TI) Fellow Larry Hornbeck won an Emmy Award for his invention of the digital micromirror device (DMD), the MEMS 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 3D metrology systems. It is also enabling 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 that TI has still more applications up its sleeve for the digital micromirror.

EE Times: When TI began its MEMS development in 1997, 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. We got started in MEMS back then because of a U.S. Department of Defense (DOD) contract to make a spatial-light modulator using deformable mirrors. This was an analog technology that the DOD wanted to use for optical computing.

I guess you had to invent the fabrication techniques you needed to even get started with MEMS.
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.

As your micromirrors became more successful, did you dedicate a fab to MEMS?
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.

So you finish the whole chip, but leave an area open to add the MEMS last?
That's a radical departure from the way others do MEMS. I believe that 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 200C so that the metallization, transistors or any of the finished CMOS circuitry are not affected when we add MEMS to a chip. To this day, this forms our standard of manufacturing MEMS micromirrors, but it was a radical departure at that time.

For MEMS vendors, the holy grail is seamless integration of MEMS structures onto the same CMOS chips as the circuitry to which they interface. I think TI was ahead by integrating MEMS with CMOS from the start.
This is one of the pillars we are resting on as a basis for our success in DLP.

But at first, you were trying to make analog micromirrors.
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 that we were not going to be successful. The uniformity just wasn't there, our analog voltages were too high (as much as 30V), and there still wasn't enough of a mirror deflection angle. This was all because we were trying to make analog micromirrors. So the second radical departure from anything that anyone had done before was to go digital.

What year was that?
I invented the DMD 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.

Are there other benefits to going digital?
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.

Did you apply the digital design to the page-printing application you mentioned earlier?
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 decided to use a linear DMDan 840 x 1 array of micromirrors. The first product, the DMD2000 airline ticket printer, went to market in 1990.

What influenced TI to move in the HDTV direction?
In 1989, the Defense Advanced Research Projects Agency started an initiative to spur the development of HDTV technology in the United States, and TI was awarded a multimillion-dollar contract to develop a prototype high-definition DMD chip.

That helped you in the transition from printing to light projection?
Well, that was only the beginning. Rank-Brimar, a Rank Corp. subsidiary, was looking for a way to project HDTV 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 HDTV, which sounded strange in 1991 because back then, TV was analog, nobody was doing anything with MEMS at that level, and there wasn't a product, standard or anything. So what we decided to do was to go for digital HDTV, but to begin with projectors because we could already build them and had a customer. By 1996, we had our first DLP products.

How did TI get into the digital-cinema business?
In 1997, the year after we introduced business projectors to the market, we launched the first high-brightness three-chip DLP systems for large-venue applications. DMD is naturally suited to these applications because it can 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 we eventually 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 I were awarded Emmys in 1998 by the Academy of Television 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.

What's next?
TI has begun equipping digital cinemas with 3D versions of its DLP Cinema projection technology. At the other end of the spectrum, it's showing a prototype of a tiny DLP chip that's small and cheap enough to add a projector to a handheld. This enables relatively large displays to be projected from, say, a cellphone.

Just use your imagination for the future of digital micromirrors. We are developing reference designs for almost anything a DLP system could possibly be used for. We have important announcements in new application areas that we plan to make very soon.

- R. Colin Johnson
EE Times

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