MEMS oscillators, has their time come?

by Jérémie Bouchaud and Bernardo Knoblich, Wicht Technologie Consulting

WTC has recently seen an increasing amount of  interest in MEMS oscillators with several startups making important announcements.  WTC believes that MEMS oscillators have a promising future, but notes the warning signs of confusion and inflated expectations.   This article summarizes our view on this exciting topic.

First of all, MEMS oscillators are not new. The first publications mentioning RF MEMS resonators for oscillator applications begin in the 1980s. Until recently however, the low temperature stability of silicon and the need for expensive ceramic or metal vacuum packaging had hindered MEMS from being a serious alternative to quartz. But tremendous engineering work on processes, packaging and integration of the circuitry over the last 5 years has gone some way to addressing these issues.

This work has been driven by US start-ups like Discera, Silicon Clocks and SiTime and as a result, samples are now available from Discera and SiTime and serial production is planned for the last quarter of 2006 by SiTime.

MEMS oscillators: what are we talking about?

Before going further let’s define the MEMS oscillator, since we have noticed some confusion between resonators and oscillators, especially when talking about price. The micro mechanical resonator is a micro machined device that vibrates at a specific frequency due to an external excitation (mainly electrostatic to ensure high-Q). The resonator is part of a MEMS oscillator device. The MEMS oscillator chip consists typically of 1) the resonator, 2) the circuits for oscillator function, frequency compensation and temperature compensation, and finally 3) the package.

The resonator is inexpensive in volume – estimated less than US$0.10 – and the typical price of an oscillator will start at US$0.40 and will reach a few dollars in some instances.
The schematic view and cost breakdown of MEMS oscillators is shown below. Our market figures concern MEMS oscillators and not only MEMS resonators, since we expect most suppliers to provide a full oscillator.

The market today

Early suppliers of MEMS oscillators claim they can address 50%-90% of the clock and oscillators market. This is a mass market estimated in the range of 10 billion units and amounting to US$3.2 billion. We differentiate mainly 3 types of products, as featured in the table above.

1. Clock Oscillator: A circuit that creates a series of pulses that pace the electronic system. According to the different manners of implementation, clock oscillators can be divided into XO (crystal oscillators), VCXO (voltage controlled crystal oscillators), TCXO (temperature compensated), OCXO (oven controlled), VCSO (voltage controlled SAW oscillators).

2. Clock Generator: A clock oscillator with more complex arrangements. The basic parts that are shared by clock generators are a resonant circuit and amplifier, and they may have additional sections to modify the basic signal. Clock generators in this article refer to oscillators with multi-PLLs (Phase Lock Loop) or jitter cleaners.

3. Real time clock: A clock that keeps track of the time even when the system is turned off.

Today, the main technologies for clock products are quartz, ceramic and CMOS silicon clocks. The features are as follows:

• The market today is mostly serviced by quartz (70%-80% of the value). Japanese companies such as Kyocera, NDK, Toyocom, and Rakon dominate this market. Quartz technology has been established for decades and is well understood. Quartz exhibits a very good temperature and long term stability, which makes it ideal for high precision oscillators. The frequency accuracy, including all the environmental effects and 1 year of aging, is of the order of few ppm to tens of ppm.
• Silicon (non-MEMS, just electronic) timing or CMOS clocks are available from Maxim or Linear Technologies and gradually replace ceramic devices in applications with relaxed specifications. Worth noticing are also new kinds of inductive (LC) CMOS clocks such as those from Mobius Microsystems. These devices target new architectures with a frequency reference in the GHz range instead of the MHz range. Their frequency accuracy is now better than 500 ppm.
• Somewhere between CMOS clocks and quartz oscillators, MEMS clocks are emerging and are looking for their piece of this multi-billion dollar pie…

Why MEMS? Theory and praxis…

The advantages of MEMS are well known and have been repeated in number of articles over the last year. On close examination there are some inconsistencies in the arguments. Here we provide some clarifications of the situation today and the conditions needed to fully leverage the advantages of MEMS. In fact, it appears that the first generation of RF MEMS oscillators will not initially fully capitalize on the MEMS advantages of smaller size or better integration.

• Size IS important: The MEMS resonating element is much smaller than its quartz counterpart. This is a key selling argument in the mobile communication industry. A tiny MEMS oscillator has often been pictured on top of a 100 times bigger quartz oscillators to settle this argument. However, one should also acknowledge the latest shrinkage in some quartz products. A new generation of quartz TCXO is emerging with similar dimensions to the first MEMS oscillators, i.e. 2x2.5x0.8 mm³ for a quartz TCXO from NDK compared to 2x2.5x0.85 mm³ for the MEMS XO from SiTime. Indeed, the first generation of MEMS oscillators will be pin-to-pin compatible with quartz products to allow an easy one-on-one replacement. Since the size of the package will be the same, the main advantage of MEMS oscillators on the “small oscillator segment” will then be price since manufacturing and encapsulating of small quartz devices is expensive compared to MEMS. PCB space can also be saved with MEMS oscillators which do not require additional bypass capacitors and resistors.
• Multi-frequency oscillator: Several RF MEMS resonators with different frequencies can be manufactured on the same die allowing for multi frequency devices, while retaining size and low cost. Crystals are single frequency devices. If multiple frequencies are needed such as in a cell phone, e.g. 32 kHz oscillator for the real time clock and several MHz oscillator for transmit, receive and processing functions, several discrete-packaged crystals must be used.
• High Q: MEMS devices exhibit a higher Q than CMOS clocks, however, MEMS has not yet reached the quality factor of quartz (100,000 to 200,000 with quartz compared to 75,000 for MEMS). The related phase noise of MEMS is also an issue that must be overcome to reach the performance required for TCXOs.
• Low cost: The MEMS resonator can be manufactured at a lower cost than the quartz since they leverage semiconductor batch manufacturing techniques. However, contrary to what early players have announced, we do not expect MEMS oscillators to compete on price in mainstream markets. MEMS clocks should remain focused on markets where they can obtain a premium for example for small packages, or low power consumption, higher frequencies and in harsh environment applications.
• Integration: Contrary to quartz, RF MEMS oscillators are manufactured with CMOS compatible processes and several announcements said this would pave the way to the true single-chip solution with built-in frequency references. Advantages in terms of power consumption (lower parasitics from the wires) and size are obvious. However, although early suppliers actively promote the CMOS compatibility of their process - above or under IC - the first MEMS oscillators will actually be hybrid parts. For example, in SiTime’s initial offerings, the MEMS resonators are wire bonded to the ASIC. Monolithic integration of the MEMS resonator with the IC, e.g. for a single-chip solution actually does not make sense economically when the MEMS die is much smaller than the IC.
• Low power: The low power consumption of MEMS resonators is also a key advantage in portable applications. However, MEMS solutions only reduce power consumption when monolithically integrated, because all of the parasitics associated with an off-chip component (i.e. wire bonding quartz or discrete MEMS) are eliminated.

Key players

Developments in this field have long been driven by three US start-ups: Discera, SiTime, and Silicon Clocks. The first commercial serial products are expected from SiTime and Discera. These companies are fabless model and will sell devices since this model allows for higher revenues than licensing. Large semiconductor companies are following including Philips, STM and maybe Freescale.

The MEMS sensor company VTI Technologies recently announced its cooperation on RF MEMS with VTT. The first products are anticipated in 2008. Patent research also reveals that large Japanese quartz oscillator companies like Epson are also carrying out RF MEMS oscillator development, and some of these may enter the market by 2010. Some of the quartz vendors are also currently exploring acquisition of MEMS technology.

Applications and product roadmap

Early on, the product with the most potential was not always clear. A TCXO meeting tough temperature stability and phase noise requirements was first announced around 2002, but did not come. In 2004, people were saying real time clocks used in stand-by functions at 32 kHz would be first to market, which also did not emerge. Now, a consensus has formed around the following roadmap:

• XO MEMS clocks targeting consumer electronics products will emerge first. The specifications are relaxed, e.g. temperature stability typically 20 to 100 ppm from -40o to 85oC, but where small packages matter. Typical end products are digital cameras and camcorders, in addition to USB sticks and portable music players. SiTime announced it will deliver one million oscillators to consumer electronics applications as early as December 2006 and Discera is expected to follow in 2007, both with pin-to-pin MEMS devices compatible with quartz crystal based products, allowing for easy implementation and rapid market penetration.
• The second generation of MEMS oscillators will target a larger market for compact oscillators meeting TCXO performance requirements. Phase noise requirements are a challenge however and further progress is needed on temperature compensation and frequency accuracy. MEMS-based TCXOs could hit the market by 2008 or 2009. New players such as Silicon Clocks, Philips and VTI are targeting this market directly.
• In parallel, the IC value of MEMS oscillators should increase, allowing multi-PLL and first devices with multiple frequencies on one die (including a real time clock at 32 kHz) are anticipated in the next 3 years. Progress is also expected toward integration and the first monolithically-integrated MEMS oscillators could appear at Silicon Clocks, for example.

With so many nice things going on, we must interject two words of caution: first, specifications and second, reliability. With specifications, you need to make sure the MEMS oscillator works at high temperatures. Reliability testing requires effort and time with production devices rather than research wafers.

So how will it all shape up? Overall, we expect the market for MEMS oscillators to in the range of US$200 million in 2012. Taking a longer term perspective, there is a strong R&D effort on integration and new architectures that use resonators as filters or mixers. Among these, STMicroelectronics’ vision of embedded NEMS leveraging new manufacturing processes such as the Silicon-On-Nothing (SON) is interesting. Clark Nguyen, the MEMS resonator pioneer at the university of Michigan, also pushes its development of the “all MEMS receiver front-end” resonator, allowing for full tunability from 600-2500 MHz. Also worth noticing are carbon nanotube resonators explored at EPFL in Switzerland.

Conclusion: hype or hit?

During the period 2002 and 2004, we asked potential users in the cell phone and consumer electronics industries about RF MEMS and they did not see why they should consider abandoning the established quartz technologies. Today, the same people are talking with a lot of enthusiasm about MEMS oscillators after testing the latest generations of samples. This proves that several years of engineering have paid off and solved the associated growth pains.

Like every exciting new technology targeting mass markets and driven by start-ups, confusion or exaggeration are present, but all in all, we believe that MEMS oscillators will follow the successful bulk acoustic wave devices as the second RF MEMS mass product. If SiTime keeps to schedule and ships its first million devices by years-end, a major step will have been taken towards a bright future!

Comments

[reza salehi]

I need more data about MEMS and it,s aplication on aeronative.

[Ian Chen]

This is a great article and analysis, but since Mobius Microsystems is mentioned I want to make a clarification. The accuracy of the Mobius all-CMOS frequency generator is significantly better than 0.1%. An article by Dr. Michael S. McCorquodale that will soon be published in JSSC features one of our earlier designs that outperform USB 2.0 requirement with better than 500 ppm (0.05%) accuracy. We are currently making progress in our first frequency generator products with targets for further improvements, so please stay tuned.

Ian Chen
VP Marketing, Mobius Microsystems

[Gary R. Johnson]

Very informative article.

I think you'll find that Rakon is a successful New Zealand company that does excellent work, but is neither located in Japan nor of the scale of the other Japanese producers you mentioned.

There is another MEMS resonator technology out there that would be interesting for you to include: F-BAR or film bulk acoustic resonator. Typically, these are produced on silicon wafers with AlN (a piezoelectric) films to drive the resonator structure.

Gary R. Johnson
co-vice chair IEEE International Frequency Control Symposium

[Nobuo Ito]

We are interest of MEMS oscillators
and resonater and with the IC.