DLP pioneer tells how TI did it with mirrors

'Second radical departure'
EE Times: But in the beginning, 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.