 |
April 10, 2006 — By his own admission, MEMS industry veteran Kurt Petersen didn’t think MEMS resonators were a viable commercial product. For starters, moisture would eventually creep into the packaging, undermining performance. And to make matters worse, the packaging itself would make it difficult if not impossible to compete in a price-driven market.
“I think I built some of the first MEMS resonators in the 1970s,” Petersen said. “I never thought they would be competitive with quartz.”
But that was then. This is now. And in the interim Petersen has been working with the crew at SiTime Corp., a company he founded to commercialize the technology after another industry veteran, Joe Brown, introduced him to some innovative processing and packaging techniques. Today SiTime announced it is sampling its SiT1xxx-series and SiT8002 MEMS oscillator families. The tiny oscillators generate frequencies between 1 to 125 MHz.
 |
The initial product families use QFN-type packages within which are a CMOS driver IC and a MEMS resonator. The complete oscillator product measures 2 x 2.5 x 0.85 mm.
The company says they are are intended to compete with quartz oscillators in electronic products like digital still cameras, portable media players and notebook computers. Although they offer distinct advantages over quartz — they are significantly smaller, the devices are tunable, and the company claims a variety of performance improvements, such as less susceptibility to shock and vibration — what ultimately makes them competitive with quartz is price.
This was achieved not so much by MEMS engineering as by an innovative approach to packaging in what SiTime calls its “MEMS First” process.
“One of the things you worry about is if you have MEMS on top (of a wafer) you can’t seal it at a high enough temperature to keep the moisture out,” Petersen said. “We encapsulate and anneal at 1,000 degrees Celsius.”
Rather than build the MEMS structures on top of a fully processed silicon wafer and then cover them with caps or some other type of package, SiTime builds its resonators on top of an SOI (silicon on insulator) wafer and covers them with an epitaxial layer of silicon — the net result being that the MEMS structures are within the wafer. And the wafer itself is the de facto package.
The ability to seal them at high heat creates a more effective barrier to moisture but it also offers other advantages. For example, the resulting wafer, with its MEMS structured buried inside, can then go through CMOS processing for a fully integrated MEMS and CMOS on a single chip for applications where a very low profile would be required. (The current samples are two-chip solutions because the amount of wafer real estate required for the CMOS and the MEMS are not equal, hence it’s cheaper to process two wafers.)
The company says its innovative packaging solution is one of the reasons it can compete with quartz on price. Another reason is that the MEMS chips are digital components and don’t require the signal conditioning chips required by traditional quartz oscillators, which eliminates additional cost. The company says using its oscillators is merely a matter of swapping out quartz for MEMS in a design since it has made the oscillators to fit within existing specs for quartz.
Finally, say company executives, the ability to provide smaller oscillators could be a boon to makers of cell phones and other portable electronic devices like digital music players or even such mundane things as the key fobs used to lock and unlock cars.
The MEMS First processing technology is licensed from Bosch, where it was originally developed. SiTime holds a license for using the process to make oscillator products, while Bosch retains rights for other uses. Bosch is a leading maker of MEMS products such as gyros, accelerometers and other types of sensors.
The company claims it can currently compete with about 80 percent of the traditional quartz market, though some applications aren’t yet viable. For example, Petersen acknowledged that the company’s technology cannot yet compete with the highest end of the quartz oscillator market, specifically those that are used as signal references in cellular telephones (though there are plenty of other uses in phones). But, he said, it’s only a matter of time.