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CMOS MEMS oscillator enters realm of reality

Posted: 01 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS oscillators? MEMS? oscillator? quartz crystal? timing device?

Hsu: MEMS resonators have an opportunity to make a big impact on the market, and will grow and evolve over time.

Almost all electronic systems need a clock source for synchronization between sub-systems as well as between systems. Over the past few decades, quartz crystals have been providing us very accurate frequency references. There is almost no doubt when a designer designs a chip that needs frequency reference, he or she will naturally reserve pins for quartz crystals. Timing components are the heartbeat of consumer devices. They help coordinate the release memory in video and still cameras to take pictures and help set top boxes turn the volume up and flip channels at the same time.

With more and more consumer devices having more and more features in smaller and smaller form functions, there is a change afoot in clock systems that will transform the way devices are manufactured while also spurring the creation of new devices. How big a change? Think of the television. Once you had to reserve a corner of your house or buy a special piece of furniture to house your unit. When I built my house a few years ago, my parents complained that I did not reserve enough space in the living room for a TV. They were surprised that plasma TVs do not have CRT, freeing them to put a TV unit anywhere there is a wall in the house.

So if it is, in fact, time for a change in timing devices, what devices can provide the same function?

Quartz crystal is a resonator, normally a mechanical device that provides good selection and good stability of frequency for electronic systems. In the past 30 years, resonators such as quartz crystals, SAW and ceramic resonators are used for most of the applications. However, they are large in size, hard to be batch processed, and hard to integrate on chip. On the other hand, CMOS MEMS resonator is a mechanical resonator fabricated on top of silicon wafer with CMOS compatible processes and materials on silicon wafersthinner, cheaper and better integration.

The research of MEMS resonator can be traced back to the 1960's where Westinghouse Research Lab utilized a vibrating metal beam as the gate of a MOS transistor such that the transistor shows a function of filters. As for using MEMS resonators for frequency reference applications, the work started in the early 1990's at UC Berkeley and then blossomed in early 2000's in Michigan. There have been start-up companies from various Universities taking advantage of very solid and very broad IP's from both Universities. Starting from there, techniques and IP's had to be developed not only on the resonator itself, but also include process, temperature compensation, packaging, circuitry, as well as system applications.

From research to product
People started thinking about commercializing this technology after a series of breakthroughs in frequency stability and manufacturability of MEMS resonators.

Small resonators normally required at least a hermetic seal to ensure the performance, a wafer level vacuum package process needed to be added on top of the resonator process. Therefore, the first task for a company was to identify an overall process not only manufacturable and CMOS compatible, but could also be globally outsourced. Then the company needs to transfer the process know-how to volume foundries, making sure the wafers are yielded well.

One of the major hurdles of commercializing CMOS MEMS resonator was frequency accuracy and temperature compensation. Resonators fresh out of the fab showed 1 percent of frequency variation across the wafer. Typical MEMS resonator exhibits TCf of -20ppm/C, which is far worse than that of the quartz crystal. However the temperature characteristic of MEMS is linear from liquid nitrogen temperature to 150C so the compensation is a lot easier. Currently, with mechanical compensation, MEMS resonators can be compensated to -0.24ppm/C. If circuit compensation technique is used, 20ppm frequency variation from -40C to 85C in volume production has been demonstrated. On the other hand, the initially frequency trimming could be within 2.5ppm with laser trimming on the resonators and less than 1ppm with electronic trimming.

A product contains not just MEMS and IC, but more importantly the package. Unlike quartz crystals that need to be hermetically packaged individually, MEMS resonators were packaged on wafer level processing. Therefore, low cost plastic package can be used, which poses a great cost advantage over quartz crystals in ceramic package.

CMOS MEMS oscillators are real
Questions people always asked about MEMS are typically around the stability and reliability. Aging is normally the first question. Imagine a mechanical beam vibrates ten million times per second with an amplitude of 10nm. The beam actually travels a distance around the earth in about 3 years. In this period of time, the error of this journey has to be 200 meters! Imagine this performance needs to be guaranteed from all process corners with different types of manufacturing environments. Fortunately, based on the data from volume production wafers, the resonator aging is less than 5ppm over 10 years at 85C.

The second question, then, is usually about shock resistance and vibration operationvery typical question for traditional MEMS. Since the mass of the resonator is so small. The deviation of the resonator beam under 30,000G shock is only 6.6nm. Furthermore, while three MEMS oscillators are shot by an air gun, which generates 30,000G shock on the MEMS resonators, 100 percent survival rate was reported.

In addition to these two extreme reliability tests, MEMS oscillators have also passed other tests such as HTSL, autoclave, vibration operation and temperature cycling. These reliability tests have to be done with qualified volume manufacturing parts. It takes time and needs resources from all technical disciplines.

So far, MEMS oscillators have met all the requirements in XO category. It is a market mainly for digital consumer electronic applications. This market is large and mature, but is moving at an extremely slow pace in terms of new product introduction. Another reason, not to be ignored, is the lack of disruptive technology changes in this area.

When will it be seen in the market?
The promise of CMOS MEMS resonators has been anticipated for a long time. Finally, the technology is commercially viable and is being rolled out as we speak. While quartz crystal technology is very important, relevant and will continue to have its share of the market, that market share will undoubtedly decline over time. MEMS resonators have an opportunity to make a big impact on the market, and will grow and evolve over time. With the ability to put multiple CMOS MEMS resonators into footprints the size of quartz crystal resonator, manufacturers will bring new products to market as revolutionary as the plasma TV.

Where is this technology heading?
A few companies are following the promise of the disruptive technology of CMOS MEMS resonators with significant investments. While it is very hard to go from initial samples to commercial volumes, Discera has done tests that show that the commercialization of MEMS resonators is a reality with the last hurdle of reliability being accomplished. Now the technology is ready for digital applications. In the future, the natural progression of MEMS oscillator is from digital to mixed signal to RF. Interestingly enough, because of the cost-reduction trend, RF system designers are seeking ways to lower the stringent requirements of highly-accurate quartz crystals. We believe in the near future, RF and MEMS will merge together on one chip.

About the author
Dr. Wan-Thai Hsu
is the chief technology officer of Discera Inc. He may be reached at whsu@discera.com.




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