Electronic Design

Programmable CMOS MEMS Timing IC Targets Direct Plug-in Replacement of Traditional Quartz Crystal Oscillators

A strategic Alliance between Discera Inc. and Vectron International has led to the introduction of the first MEMS-based CMOS timing circuit that can directly replace traditional quartz crystal oscillators. Other oscillator technologies the MOS1 can replace are ceramic, film bulk acoustic resonator (FBAR), and surface acoustic-wave (SAW) timing devices.

A Fabless Design

The MOS1 family of MEMS oscillators delivers high performance and low cost in a small form-factor to address a wide variety of consumer and some military applications (Figure 1). It includes a MEMS resonator, and is a fabless design wire-bonded to a signal-conditioning ASIC using wafer-level processing. “The MOS1 is a drop-in replacement for existing timing devices on the market,” said Discera Vice President of Marketing Venkat Bahl. The MOS1 is ideal for camcorders, still cameras, MP3 players, DVD players, mini disk drives, PDAs, TV players, set-top boxes, etc.

Compared with a 33-MHz Epson SG-8002 quartz crystal oscillator, a MOS1 MEMS oscillator at the same frequency exhibits comparable jitter performance (RMS periodic jitter of 25.38 ps vs. 29.22 ps), 1/3 lower power consumption (6.8 mA vs. 4.5 mA at 3 V), and smaller frequency variation across the temperature, as shown in Figure 2.

The four versions in the MOS1 family generate frequencies ranging from 1 MHz to 125 MHz with a choice of frequency tolerance and stability figures of ±100 ppm, ±50 and, ±20 ppm over an operating-temperature range of –40°C to 85°C. They all consume less than 1 μA of standby current, an extremely important parameter for consumer electronics items like mobile phones and other portable products. All of Discera’s oscillators are specified to age at a rate of ±5 ppm/year or less, and are available in either plastic quad no-lead (QFN) or ceramic packages, with dimensions of 3.2 by 5 by 0.85 mm (QFN) or 1 mm (ceramic).

The first version in the MOS1 family, the S1, operates over the frequency range of 1 MHz to 10 MHz. The S2 version operates from 10 MHz to 40 MHz; the S3 version is specified for the 40 MHz to 80 MHz range; and the S4 version covers the range of 80 MHz to 125 MHz. The S1 and S2 dissipate less than 6 mA of current, while the S3 and S4 versions dissipate less than 7.5 and 9.5 mA, respectively. All four operate from a nominal supply voltage of 2.7 to 3.3 V, and produce a 3-V output with a maximum output load of 15 pF.

Discera is homing in on three key features for its CMOS MEMS oscillators (when compared with quartz-based oscillators): lower costs, shorter lead times, and better reliability. “The cost to enter the MEMS oscillator market is less than one-half of that for a quartz oscillator,” said Bahl. The company sees a 15% per year cost reduction for CMOS MEMS devices—a rate manufacturers of quartz oscillators would be hard pressed to keep up with. MEMS oscillators also provide a unique manner in defining an operating frequency over a wide range with high resolutions up to 2 ppm.

Reliability and ruggedness are key attributes of these MEMS oscillators. And, they’re some 100 times lower in cost for high-performance special applications in extreme environments. Testing has shown that the MOS1’s resonator can survive 30,000 gs of shock with no degradation in performance and without the need for specialized packaging. Discera plans to market this ruggedness feature for certain military applications.

A Large Market Looms

The market for silicon- and quartz-crystal-based timing devices looms large and continues its growth on a steady pace. That’s the assessment of market analyst firm ABI Research, which predicts that total revenues will grow from $4.2 billion in 2006 to $5.7 billion in 2011. They conclude that silicon-based solutions are getting better and will eventually replace quartz crystals, a sentiment Discera and Vectron agree on.

“We believe MEMS oscillators are an important part of the future of the frequency control market,” says Vectron Vice President of North America Products and Operations Ed Grant. “While the promise of MEMS oscillators has been around a long time, no vendor has been able to prove their reliability and manufacturability. We believe Discera is uniquely positioned to deliver on this promise. We look forward to working together with Discera, utilizing our complementary skills to create industry defining products.”

Amplifying on Discera’s market role is Vectron’s Director of Products Mario Saucedo who points out that although Vectron’s largest market is in high-end applications, “Discera’s CMOS MEMS oscillator with its small form factor and low cost fits in nicely with our smaller market of commodity timing devices.”

A Sophisticated Resonator Technology

The key to Discera’s MEMS resonator structure is its patented PureSilicon micromechanical resonator technology. The technology is the culmination of over 10 years of product R&D conducted by Discera founder Dr. Clark Nguyen and a host of employees, including Discera’s Chief Technology Officer, Dr. Wan-Thai Hsu. Work on low-frequency MEMS resonators has its origins at the University of California at Berkeley. Later on, work on higher-frequency and higher-stability high-Q resonators was undertaken (and is still ongoing) at the University of Michigan.

The fabrication of PureSilicon resonator-based products are made with sub-μm design rules using well-established CMOS manufacturing techniques with 2 polysilicon layers and a single metal-layer (Figure 3). The PureSilicon resonator technology exhibits more linear characteristics than quartz-crystal-based products across a much wider operating-temperature range. Future products will improve temperature range than and target much tougher environments.

The resonator is designed for optimal performance that’s balanced between the need for a low-voltage dc bias potential, a high Q value, and good power handling. The resonator has sufficient mechanical stiffness to allow for the use of less-stringent vacuum-packaging and bonding techniques, and thus lower-cost production (Figure 4). This approach allows for the production of oscillators that meet or exceed critical crystal-oscillator specifications for jitter, power consumption, voltage variation, and temperature stability.

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