MEMS-oscillator maker aims to oust quartz

SiTime, which is bringing MEMS (microelectromechanical-system)-based oscillators into volume production, aims to replace quartz as the frequency reference in clock and timing oscillators. The company claims that its technology could potentially take over quartz’s role in virtually all timing-oscillator sockets.

The company builds a small, square silicon-MEMS structure whose four side “beams” are suspended in free space; a cruciform shape spans the diagonals of the square, and a fixing point in the center suspends the structure. Under electrostatic excitation, the sides of the square flex in unison in the plane of the square, which measures only microns in size. The oscillation is at a fundamental frequency: In the products SiTime has so far released, this frequency is approximately 5 MHz, although the company has built structures that resonate from 1 to 40 MHz and above. The MEMS element exhibits high Q factor; the company says its design rules allow it to design for a given Q—of more than 70,000 in the commercial version.

The underlying technology originates with Bosch, and SiTime holds the rights to exploit it as a timing reference; related technology from Bosch Sensortec forms the basis of developments of the technology for use as a sensor.

This resonant structure features high mechanical robustness, has a stability in the parts-per-million range that is sufficient for the timing requirements of fast serial-data standards, and exhibits aging—frequency drift over time—that is smaller than that of all but the highest quality selected and preaged quartz crystals. It does not exhibit the typical frequency/temperature curve of a crystal but has a linear relationship that, in its oscillator products, SiTime compensates with an on-chip temperature sensor.

The company pairs its MEMS resonator with a CMOS programmable-frequency synthesizer chip. In its current product lineup, it assembles both dice in a multichip package to yield a fixed-frequency oscillator at 1 to 200 MHz. The company sets the programmed frequency by fuse programming before shipping. It could, says Marketing Vice President John McDonald, sell a user-programmable part, but a preprogrammed offering better conforms to the buying patterns in the supply chains for crystal oscillators. SiTime can program parts from stock to provide samples in days. The company aims to combine the MEMS element and the CMOS oscillator on a single die. It proposes to avoid difficulties inherent in a combined process flow by first building its MEMS structures and then carrying out the complete CMOS fabrication as a separate operation. The company anticipates the emergence of single-chip products in 2009; meanwhile, it has announced ultrathin and low-jitter versions of its oscillators.

The low-jitter part, SiT8102, has less-than-1-psec-rms phase jitter in most measurement bands, comes in a 2.5×2×8-mm QFN package, and will cost less than a comparable quartz device. You can use it as a clock reference for USB 2.0 HS, FireWire, Fibre Channel, SATA, PCI Express, Gigabit Ethernet, and other high-speed serial-communication protocols. In common with the company’s other products, the construction uses a stacked-die arrangement; a temperature sensor on the CMOS-synthesizer chip is in close contact with the silicon of the resonator, sitting on top of it and allowing digital-temperature compensation. Although the chip applies compensation by changing division ratios, it is glitch-free, McDonald says. Total error is within ±50 ppm, including temperature and aging effects. Periodic jitter is less than 5 psec rms for most frequencies.

Removing the dice from the stacked layout and placing them side by side yields a thin oscillator that still has all of the mechanical-robustness attributes. The SiT8002UT is available with a 1- to 125-MHz output; requires a 1.8, 2.5, or 3.3V supply; and occupies a four-pin QFN package that measures 3×3.5×0.37 mm. The reduced thermal contact between the dice relaxes the device’s stability over temperature of –40 to +85°C to ±100 ppm. Prices for the chips are less than $1 (high volumes). McDonald says that the manufacturing costs are inherently lower than those of quartz, which volume prices reflect.

SiTime’s resonator technology does not achieve the close-in low-phase-noise specifications that digital-communications protocols demand. “We don’t claim to be able to do the sort of high-RF-stability performance that is needed there; we are still perhaps 20 to 25 dB in phase noise away from that scenario,” says McDonald. However, the company has achieved a better-than-15-dB improvement per year while developing the technology. McDonald also hints that the company may have the high-performance-quartz business in its sights.