CAMBRIDGE, Mass. Today, startup companies are touting microelectromechanical systems (MEMS) as a new growth area for integrated circuits (ICs). According to Analog Devices Inc., however, ADI has been pioneering MEMS chips since before their first accelerometer chip, back in 1989. Since then, ADI has shipped hundreds of millions of accelerometers for automobile applications, and just this year broke into consumer electronics by shipping more than one million three-axis accelerometers for Nintendo's Wii video-game controller.
Going forward, according to Robert Sulouff, director of business development at ADI's dedicated MEMS facility, in Cambridge, Massachusetts, the company's MEMS chips will take on applications as varied as microphones; acoustics; medical diagnostics; drug delivery; RF-switches, -resonators, "oscillators, and "filters; as well as enabling a new breed of ultrahigh-precision measurement and testing equipment.
Here, Sulouf describes the history and future of MEMS at ADI, in this question-and-answer session with EE Times' contributor, R. Colin Johnson.
EET (R. Colin Johnson): Who inspired ADI to jump on the MEMS bandwagon?
ADI (Robert Sulouff): It started in the 1987 to 1988 timeframe, when a Boston University professor stopped by and talked with Steve Sherman [the first recipient of ADI's now-annual "Founder's Innovation Award"]. From that introduction we began experimenting with adding MEMS to our integrated circuits.
EET: When you invented the accelerometer, were you thinking about this phenomenal airbag-trigger application?
ADI: No, at that time there was a lot of talk about smart automotive suspensions to control the quality of the ride, and accelerometers were viewed as a good sensor for that. It wasn't until several years later that the mechanical airbags became a real dominant problem--just as we were sampling our first accelerometers.
EET: I see, and the auto makers could immediately see how superior it was, so they all switched over?
ADI: That's right. It was one sensor instead of a collection of them around the vehicle. It was an accelerometer instead of a switch. And most important, the systems implementation cost was maybe one-tenth that of the mechanical solution.
EET: Yes, and cost is what it all boils down to--if you have the reliability too.
ADI: Right. It had the continuous monitoring reliability, and it was cheaper and easier to use.
EET: Can you outline the process steps ADI uses to add MEMS mechanical structures to CMOS chips?
ADI: Our strength and specialty is fabricating the MEMS mechanical structure alongside its electronics on the same CMOS chip. We first process a bipolar CMOS [BiCMOS] chip, with typical transistors, using all of the high-temperature processes, and we reserve an area in the center of the die for the MEMS sensor portion. After making this standard IC, we add the MEMS using a special polysilicon--an extra thick material that you would normally use to put down CMOS gates.
EET: And how do you keep from turning the rest of the chip into soup while you are adding the MEMS?
ADI: (laughs) Well, you've hit-on the really important point--you have to allow for the [high] temperatures. The way we do it in the current implementation--and which we have been perfecting over the last 14 years of production--is we make large transistors using an older process that can take high-temperature annealing. Our BiCMOS, before it has any metal or anything else on it, can take 1000 degrees [centigrade] for three hours. After we add the MEMS, we go back and finish the integrated circuit by putting down the contacts, the metal, the passivation, and then we remove the sacrificial material underneath the MEMS structure so it's free and clear.
EET: So the sacrificial material is removed after the metallization step?
ADI: Yes, that's correct--it's the last step in the process.
EET: So that must be a low-temperature process.
ADI: Yes it is, we put down a photoresist, then use HF [hydrofluoric acid] for about an hour to remove all the oxide underneath the polysilicon.
EET: And is that a part of your secret formulae, which is surrounded by patents?
ADI: The entire process has a whole collection of patents [associated with it]. Some of the really critical ones cover something that people really struggle with: the ability to hold together all those small mechanical structures while you are going through these wet etches. There are two parts of the "secret sauce" there. The first part is to use a soft photoresist [photoresist that is softened by ultraviolet light passing through the transparent areas of the mask] to hold all the fingers and structures apart from one another in a wet solution, because, if you allow them to just float around, whenever you pull it out of the liquid they stick to one another with surface tension. The second part [of the secret sauce] is to add an oxygen-plasma dry etch, just to remove the photoresist. And for an extra measure afterward, before we dice up the wafer, we put down a vapor coat--just a few atoms thick--to keep it from sticking while in use.