MEMS-based oscillator threatens quartz; resonator could move on-chip
By Maury Wright, Editor in Chief -- EDN, April 10, 2006
Few electronic components have stood the test of time better than the quartz-crystal oscillator or resonator. But researchers continue to search for a quartz replacement because the components don’t scale downward sizewise—at least relative to ICs—and manufacturing and packaging are problematic, even though the industry has developed ways to turn out millions of discrete oscillators at low prices. MEMS (microelectrome-chanical-system) technology is one silicon technique that designers can use to build an oscillator. Start-up SiTime believes that it has unlocked the secret to MEMS-based oscillators that, as discrete components, can undercut quartz prices and that designers might ultimately integrate directly into large digital chips. SiTime claims that vendors this year will manufacture 10 billion quartz crystals with complex popular products, such as handsets, hosting a half-dozen or more such components. Despite challenges from ceramic and silicon approaches, quartz has remained dominant due to excellent temperature-stability and phase-noise performance. Early silicon-based MEMS oscillators failed to offer such stability due to both the temperature properties of silicon and contamination that gathered on the vibrating beams in MEMS oscillators. SiTime claims to have solved the MEMS-oscillator problem with a packaging technology. The company has addressed problems with earlier MEMS oscillator packages using the MEMS First silicon-process technology, which it based on SOI (silicon-on-insulator) wafers. In MEMS First, a silicon etch creates 10-micron trenches that form the resonator bean. The process then relies on glass that fills in the trenches so that an epitaxial reactor can grow a layer of silicon and polysilicon on top. The process then relies on vents that are etched in the silicon so that hydrofluoric acid can remove the glass, freeing the beams to oscillate. The reactor again seals the cavities, leaving a clean vacuum underneath. SiTime claims that the process eliminates stability problems and offers better stability than quartz technology. Further, the semiconductor process can churn out oscillators that are a fraction of the size of quartz. Initially, SiTime will package its resonators as direct replacements of quartz resonators, simply offering a lower price and presumably superior performance. Among the offerings is the SiT8002 programmable oscillator that can output a signal ranging from 1 to 125 MHz. The product will sell for 69 cents (1 million). Meanwhile, the SiT11xx family of fixed-frequency oscillators will sell for 49 cents (1 million). The company may later use the MEMS First process to embed oscillators in the substrate below CMOS digital ICs.
