MEMS oscillator maker aims to oust quartz

EDN Europe, 17 Oct 2007

Replacing quartz as the frequency reference in clock and timing oscillators is the ambition of SiTime, a company that is currently bringing its MEMS-based oscillators into volume production. It says that its technology has the potential to take over from quartz in virtually all timing oscillator sockets. The company builds a silicon MEMS structure that is a very small square, in which the four side “beams” are suspended in free space: a cruciform shape spans the diagonals of the square, and the structure is suspended by a fixing point in the centre. Under electrostatic excitation, the sides of the square flex, in unison, in the plane of the square – which is only microns in size. The oscillation is a fundamental frequency which for the products SiTime has so far released is around 5 MHz, although the company has built structures that resonate from 1 MHz up 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 – in the commercial version, of over 70,000. The underlying technology originates with Bosch, and SiTime holds the rights to exploit it as a timing reference: developed as a sensor, related technology forms the basis of developments by Bosch SensorTech.
This resonant structure, is mechanically very robust, has stability in the PPM range that is sufficient for the timing requirements of fast serial data standards, and exhibits aging – frequency drift over time – that is small compared to all but the highest-quality, selected and pre-aged quartz crystals. It does not exhibit the typical frequency/temperature curve of a crystal, but has a linear relationship which in its oscillator products, SiTime compensates with an on-chip temperature sensor.
The company pairs its MEMS resonator with a CMOS programmable frequency synthesiser chip. In its current product line-up it assembles both dice in a multi-chip single package to yield a fixed-frequency oscillator at any frequency from 1 to 200 MHz. The company sets the programmed frequency by fuse programming, before shipping. It could, says marketing VP John McDonald, sell a user-programmable part: but, to conform to the buying patterns that prevail in the existing supply chains for crystal oscillators, a pre-programmed offering works better. SiTime can program parts from stock to provide samples in days. SiTime 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 building its MEM structures first, then carrying out the complete CMOS fabrication as a separate operation. The company anticipates single-chip products in 2009: meantime, it has just announced ultra-thin and low-litter versions of its oscillators.
The low-jitter part, SiT8102 has sub-1psec rms phase jitter in most measurement bands, comes in a 2.5 x 2.0 x 0.8 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, Fibrechannel, SATA, PCI-Express, Gigabit Ethernet, and other high-speed serial communication protocols, SiTime says. In common with its existing products, the construction uses a stacked-die arrangement: a temperature sensor on the CMOS synthesiser chip is in close contact with the silicon of the resonator, sitting on top of it, allowing digital temperature compensation. Although the chip applies compensation by changing division ratios, it is glitch-free, SiTime says. Total error is within +/- 50 ppm including temperature and aging effects. Periodic jitter is under 5 psec rms for “most frequencies”.
By taking the dice out of the stacked layout and placing them side-by-side, the company is also able to release a very thin oscillator that still ahs all of the mechanical robustness attributes. The SiT8002UT is available with 1 to 125 MHz output, running form 1.8, 2.5 or 3.3V, and occupies a 4-pin QFN package that measures 3.0 x 3.5 x 0.37 mm. As the thermal contact between the dice is reduced, the stability over temperature (-40 to +85C) is relaxed to +/-100 ppm.
Prices for SiTime’s volume chips are “under $1”: McDonald says that the manufacturing costs are inherently lower than those of quartz, which will be reflected in volume prices.
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 – 25 db in phase noise away from that,” says McDonald. However he adds that the company has achieved better than 15 dB improvement per year while it has been developing the technology, and hints that he may have that high-performance quartz business in his sights as well.