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Oscillator aims MEMS at the mainstream

Posted: 16 May 2006 ?? ?Print Version ?Bookmark and Share

Keywords:R. Colin Johnson? microelectromechanical systems? MEMS? SiTime? oscillator?

Since the '60s, microelectromechanical systems (MEMS) have struggled to enter the mainstream. With the success of selected MEMS applications, more players are entering the arena. But few have aimed at a larger market segment than SiTime Corp., whose MEMS-First oscillator lines are meant to be pin-for-pin compatible with quartz crystal oscillators.

"We plan to revolutionize the quartz-crystal industry by offering a silicon MEMS alternative that's completely compatible with standard CMOS processing," said Joe Brown, manager of strategic alliances at SiTime. "Vacuum tubes were replaced by transistors, and quartz crystals now have the opportunity to be displaced by silicon technology."

MEMS first became a high-volume industry with airbag sensors like the iMEMS accelerometers from Analog Devices Inc. and later with the digital light-processing chips from Texas Instruments Inc. Most recently, Akustica Inc.'s sensor-silicon MEMS microphone has attracted attention. Now SiTime is bidding to become an even higher-volume player, since virtually every electronic device produced today uses a quartz-crystal oscillator as its time base.

Gartner Inc. calls the market for analog quartz crystals relatively flat at over $1 billion yearly, with an average selling price of 15 cents on shipments of billions of units per year. Even while still in sampling mode, SiTime has started tapping that vast market by selling more than a million units of its prototypes. Full-volume fabrication of final production units from SiTime, which is fabless, is slated for later this year.

"The consensus seems to be that MEMS oscillators' performance is competitive with traditional crystals in most applications," said Stephen Cullen, a contributing analyst at In-Stat. "But I expect the initial applications to be in places where their smaller size can justify a price premium."

In the long term, SiTime and competitors like Discera Inc. and Innovative Biotechnologies International Inc., may sidestep the issue. Instead of competing with the low cost of quartz crystals, they would eliminate them by integrating MEMS time bases onto CMOS chips.

"The long-term advantage," Cullen said, "is the ability to integrate MEMS and CMOS silicon on the same chip, which should lead to further size reductions and ultimately lower overall cost. The endpoint could be a MEMS clock integrated into every chip that needs one, rather than deriving multiple clock signals from a single crystal."

SiTime intends to realize this single-chip solution by licensing its MEMS oscillators to consumer chipmakers themselves, so that the MEMS time bases can be integrated onto the CMOS chips alongside an application's electronic circuitry.

"Our technology is so straightforward that all the major semiconductor manufacturers, from LSI Logic to Actel to Cypress Semiconductor, have embraced it because it sits right in line with their own plans," said John McDonald, VP of marketing at SiTime.

The one-chip solution will require CMOS chipmakers to retool their existing dice to work with integrated MEMS time bases, which may take many years. As an interim solution, SiTime plans next to introduce a special two-chip solution that stacks its MEMS dice on top of standard CMOS dice to supply all their timing signals from above.

For major consumer chip manufacturers, both the one- and two-chip solutions have a hidden advantage over quartz crystals: the ability to have several clock frequencies feeding multiple PLLs on a single chip. Thus, all the timing signals for an entire chip could be packed into the piggyback MEMS die, reducing the complexity of most consumer chips made today and potentially offsetting the higher cost of MEMS oscillators when compared with single-frequency quartz crystals.

"By altering the geometry of the tuning fork," Brown said, "multiple frequencies from multiple oscillators can be embedded into the first four layers of a CMOS wafer, thus giving every chip access to as many different time bases as is required."

That would eliminate the need to have more than one time-base chip per electronic device, since they can all be built into a single chip. "For example," Brown said, "you could pack a real-time clock at 32kHz, plus a communications frequency oscillator operating at several hundreds of megahertz, plus you could add any number of other kilohertz or megahertz frequencies and all their PLLs on one monolithic piece of silicon. You can't do that with quartz."

Inside the parts
SiTime's oscillators are based on an internal MEMS resonator as an alternative to quartz-crystal resonators. Other companies have developed solid-state alternatives to quartz-crystals for RF and IF applications, such as SAW filters, ceramic filters and film bulk acoustic resonators. SiTime, for its part, says its MEMS resonators not only apply all those applications, but are less expensive to add to systems, are more reliable and smaller, and don't need customization.

SiTime's initial product, the SiT8002, is pin-for-pin compatible with Epson's SG-8002, but also has lower jitter, lower phase noise, lower power consumption and a smaller package, the company said.

To fabricate a MEMS oscillator, SiTime begins with a silicon-on-insulator wafer, then adds just four mask levels to craft a mechanical "tuning fork" or "beam" that is attached to the substrate on one side and is severed elsewhere by a deep trench etch process. By opening a 400nm gap for the 10?m-tall beams, whose width is determined by the desired frequency, the beam can freely oscillate when driven electrostatically by an electrode on one side of the beam. A capacitive sensor is then located at the other side of the beam to detect the oscillation frequency and drive the PLL circuitry that conditions the signal to mimic a traditional quartz-crystal time base.

To ensure long-term reliability, SiTime grows an epitaxial silicon cap above a polysilicon layer across its entire wafer, securely encapsulating the MEMS oscillator and providing an atomically smooth surface over which normal CMOS circuitry could be grown. The capping operationwhich is performed at 1,100C to drive out such possible contaminants as moistureisolates the structure from the environment so that it cannot affect performance now or in the future, ensuring the long-term stability that has traditionally eluded MEMS.

The other big pitfall that stumped previous MEMS resonator designers was poor temperature hysteresis and a temperature coefficient of 30ppm/C. SiTime claims to have solved these problems with the electronic circuitry it adds to the CMOS portion of its two-chip solution.

"Across the surface of our wafers," said Brown, "we see about a 0.8 percent deviation in frequency. So we connect our resonator to a CMOS die, which pulls in the frequency to

5ppm. The circuitry also performs temperature compensation and A/D conversion, and locks in the desired frequency with a PLL."

Brown also claimed that SiTime MEMS-First oscillators are "instantly on" with exactly the right frequency, while quartz crystals require a warm-up period as the rest of the circuitry waits for the clock to settle. He further said that scaling down the MEMS resonator for higher frequencies will be easy.

"One of the remarkable things about MEMS-First technology," Brown said, "is that as its size is scaled down for higher frequencies, its performance actually goes upwhich is the opposite of quartz, whose performance goes down for smaller sizes."

The company claims to have fabricated many geometries for its tuning-fork-like resonators using standard CMOS photolithographic techniques. Successful geometries include a four-beam structure attached only at the middle, a one-beam structure and a disk structure, all of which have different trade-offs regarding frequency and performance.

- R. Colin Johnson
EE Times

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