July 3, 1998

A matter of timing: Labs' micromachine prototype performs job of quartz crystals

By Chris Burroughs

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Jim Smith, Manager of Intelligent Micro- machine Dept. 1725, together with colleagues from Sandia and Trey Roessig, Al Pisano, and Roger Howe from the University of California at Berkeley, have built a MEMS prototype that functions as a clock source. The minuscule machines with moving parts the size of a pollen grain perform the same job as quartz crystals, the traditional technology used in timing devices in all digital electronics.

THE TIMING IS RIGHT - Jim Smith, Manager of Intelligent Micromachine Dept. 1725, looks through a high-power microscope at a MEMS prototype that functions as a clock source. (Photo by Randy Montoya)

Roessig, accompanied by Jim, made the first public announcement of the prototype in June at the Solid State Sensor and Actuator Workshop in Hilton Head, S.C.

"We have taken the same technology that is now being used in such devices as sensors in car airbags and applied it to a timing device," Jim says. "It looks extremely promising."

Micromachines are made from polysilicon, which is the same material used in manufacturing integrated circuits, the building blocks of digital electronics. Because of this, the micromachines and integrated circuits can be constructed on one chip.

Systems on a chip

The micromachined clock source, conventional integrated circuits, and other micromachined elements can be built simultaneously to form a complete "system on a chip," which if mass produced could yield dramatic reductions in price and increases in reliability, Jim says. Hundreds to thousands can be built on a single silicon wafer. In addition, the cost of manufacturing could be significantly reduced because the need for assembly would be eliminated. Under current production methods, quartz crystal timing devices and integrated circuits are manufactured separately and then assembled. Since the two systems would be on one unit, there would be no need to piece them together, saving a significant cost source.

The system-on-a-chip concept, only about three years old, embeds the micromachines in a shallow trench on a silicon wafer. These wafers with the microelectromechanical devices are then used as the starting material for the conventional complementary metal oxide semiconductor (CMOS) manufacturing process of integrated circuits. The integrated circuits are built on the surface of the wafer, while the MEMS are sealed in the trench.

This technology, which in 1996 won Sandia an R&D 100 award, has already been licensed to industry for use in applications such as computer game joy sticks, automotive stability systems, and airbag deployment sensors. With incorporation of a timing device, applications for this technology will continue to grow.

Currently, quartz crystals - precision-cut and polished single-crystal silicon dioxide (a main ingredient of sand and window glass) - serve as the clock source. A piezoelectric material, the crystals expand and change shape when an electric field is applied, storing up electric charge. When no current is administered, the crystals release the charge. Electrical energy sloshes back and forth at a fixed frequency between the crystal and the timing circuit in a feedback loop. This fixed frequency generates timing signals, which allow calculations in digital electronics to occur in synchronized steps. For example, a modern wristwatch contains a quartz oscillator and a circuit that counts the oscillations. Once the correct number of counts is recorded, the display is advanced one second.

MEMS replacement clock source

The MEMS prototype would serve as a replacement clock source. It is different from the quartz crystals because it is excited and sensed electrostatically instead of piezoelectrically. Unlike their quartz counterparts, which expand or change shape, these polysilicon resonators physically move in much the same way as a tuning fork vibrates.

The prototype also acts somewhat differently from other micromachines used in products currently on the market - such as sensors for pressure and acceleration - that are minute moving gears and pins.

Observed through a high-power microscope, the MEMS timing device prototype looks exactly like a tiny double-ended tuning fork. It consists of two very fine strings or tines - 10 would fit on a pinhead - anchored in parallel to actuator frames the size of red blood cells. Voltage, set up in a continuous feedback loop (the oscillating effect), is applied through the actuator frames, causing the strings to move back and forth. Because they are so very small, the MEMS vibrate extremely fast and generate frequencies of about 1 MHz. Although this is a relatively low frequency for a system clock, the prototype oscillator is the first integrated oscillator that operates above the audio range.

The process for building these devices at Sandia appears capable of fabricating integrated oscillators with frequencies above 10 MHz. Despite the high frequencies, these micromachines are producing very low noise - due primarily to the integration of the mechanical structure with electronics and the design of the electronic circuit.

The frequencies provide the constant timing signals necessary for the digital electronics device to operate. Because of the low noise, the signals are constant and not disrupted, resulting in more accuracy.

Micromachines in the shape of a tuning fork serving as oscillators are not new. The uniqueness is putting the MEMS oscillator on the same chip as the integrated circuits.

Collaboration with Berkeley

Jim says his efforts build on work done at the University of California at Berkeley by Howe and Clark Nguyen (now at the University of Michigan).

"They were able to get a few of the tuning fork oscillators to work," he recalls. "But when they saw the dramatic improvement in manufacturability of devices offered by Sandia's integrated MEMS process, we soon were collaborating with them. They have the expertise in design, and we have the expertise in manufacturing. It's a natural match."

He adds that having the clock source on the same chip as other electronic circuitry is one of the building blocks toward developing complete electromechanical systems in a single monolithic piece of silicon.

"Internally, we are working with Thom Fischer and Kurt Wessendorf from 1732 to adapt these devices to defense program applications," he says. "Industry has already shown great interest in Sandia's ability to use this technology to build accelerometers - sensors that measure acceleration used in airbags - and gyroscopes that sense the rotation of a vehicle. The new ability to use micromachines as timing devices will greatly expand the fields of application for these systems on a chip."

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Last Modified: July 1, 1998

September 26, 1997

Black church leaders solicit Sandia's help in protecting people and property from arson, vandalism, violence

John German

Mary returned recently from a conference of 600 black ministers and church leaders in South Carolina where she preached the principles of practicality and affordability to an audience concerned about the recent spate of racially motivated burnings and other hate crimes against black churches in eight Southern states since January 1995.

The Congress of National Black Churches' (CNBC) Arson Prevention Task Force invited Mary and colleague Basil Steele (5804) to CNBC's Aug. 5-6 annual conference following some nationwide publicity last spring about Sandia's work helping New Mexico schools design practical security systems.

Although Basil couldn't go, Mary participated in a two-day workshop for South Carolina church leaders focusing on common-sense approaches to facility security.

"The type of security systems we are recommending are not really cutting-edge technology, but they can seem complicated to church leaders," she says. "Typically they end up buying whatever system a vendor recommends, and that's often not the most practical or inexpensive solution."

Cost and avoidance

Mary's advice is intended primarily to help church leaders sort through some of the technology-related decisions they encounter in choosing a modern security system - what systems work in what situations, where to put cameras and heat and motion sensors, which entryways to monitor, and (most critical) how much money they need to spend.

"You probably don't need a $30,000 police-dispatch alarm system for a rural church that would take 30 minutes for the police to get to anyway," she says. "And you don't want to put motion sensors near a bulletin board where children's drawings are posted. Every time there's a draft, the papers blow around and set off the alarm."

And false alarms can annoy local police, who may not respond as quickly the next time your alarm goes off.

The principles Mary advocates not only preserve funds when a church buys a new security system, they also can help avoid expenses associated with vandalism.

Church leaders often aren't thinking about crime when they build a new church or design its grounds, she says. That can result in vandalism that is preventable if they insist that builders "do things smart" from the beginning, she says.

For instance, many churches have large stained glass windows facing the street, which may make a church more attractive to passers-by but, in many neighborhoods, invites vandals to throw rocks or shoot them out. Replacing a big ornamental window can be costly. If stained glass windows are used, she recommends they be positioned away from the street where they are still available for enjoyment by the congregation but are not as easily damaged by vandals.

Mary also presented some of the principles of "Crime Prevention through Environmental Design" that have arisen from community policing efforts nationwide. Bushes near windows, security cameras vulnerable to theft, and fence lines that encourage graffiti "artists" are examples of common mistakes that can drain church coffers, she says.

Burnings and bombings

"You have to make people's safety a priority," she says. "Again, it's important to plan for the worst, and to work with local law enforcement."

For a church that has been bombed or that receives bomb threats, for instance, it's dangerous to have a nursery or day care near a street or parking lot. When rebuilding following an arson, use flame-retardant building materials, she says.

"And if it's a target once a month, you might look at an aqueous foam system to put out the fire," she says.

She says Sandia's expertise in this area comes from years of serving as the lead lab in protecting DOE facilities and nuclear weapons in the US and abroad.

"We understand the principles of security and we've worked with the equipment," she says. "It seems like common sense, but it's more than that. We know what works, and that is valuable to school and church officials who can't afford to make mistakes."

Although Mary's participation in the workshop was informal, she says CNBC may decide to provide future funding that will allow Sandia to help on a more formal basis.

As the safety of America's school children has become a concern in recent years, Mary and colleagues have consulted teachers and school officials about common-sense security approaches as well.

Mary has visited with school officials in Hobbs, Silver City, and Las Vegas, N.M., and helped design a security regimen for Belen High School that has resulted in fewer acts of on-campus violence and vandalism (Lab News, March 14, 1997).

"Schools and churches are in the same boat," she says. "They don't have a lot of money, and they're often easy targets for vandalism and theft."

Dept. 5861 now is compiling a common-sense "how to" set of security manuals for schools, scheduled for distribution next summer. The project is funded by the National Institute of Justice.

"What I hear so often from schools is: 'We've lost a VCR, so we're going to spend $10,000 on a security system," she says. "But that doesn't really make sense from a fiscal standpoint."

She encourages teachers and school officials to deface equipment with paint or indelible markers, or to affix microdots (tiny pieces of microfilm that contain information about the owner) to equipment that is easily resaleable. That helps get the equipment returned to the school if a thief is caught.

"The best step schools can take is to make it a lot of work for someone to sell a stolen piece of equipment," she says.

A simple alarm system can help notify police or school officials of an off-hours break-in as well. "A few kids on a weekend could do hundreds of thousands of dollars worth of damage," she says.

"For both schools and churches, deterrence is the name of the game," she adds. "You've got to convince your adversary that this isn't a good target."

If you have questions or need further information, contact Rod Geer by e-mail here.
Last Modified: September 25, 1997