By Candace Stuart
Small Times Senior Writer
Diamonds are becoming the crown jewel of small tech.
Researchers at two national labs have developed separate techniques to make diamond films that far outshine many materials
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| The ultrananocrystalline diamond tube made at Argonne is as small as 5 microns across. Courtesy Argonne National Laboratory. |
That will change soon, proponents say. In a year, these diamond films could be commercially available. And when that happens, the potential uses for MEMS and nanomachines could explode.
“We believe the material will prevail,” said Dieter Gruen, a senior scientist at Argonne National Laboratory near Chicago who invented a way to grow extremely small diamond crystals for use in MEMS and nanomachines.
“We have a lot of interest in the material,” agreed John Sullivan, a staff scientist at Sandia National Laboratories in Albuquerque, N.M.
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Others are more cautious.
“It could turn out to be the material of choice,” predicted Christian Zorman, a researcher and interim director of MEMS Resource at Case Western Reserve University in Cleveland. But these diamond films are more likely to find a niche among existing materials and not replace them, he said.
THE MANY FACETS OF DIAMONDS
Diamond has a number of properties that make it a good candidate for small technologies. For starters, it lasts forever — or nearly so.
Silicon, the most common material in MEMS, wears out too quickly when it is used for fast-moving and interlocking parts such as pumps and gears. Diamond, which is the hardest known mineral in the world, has been shown to last up to 10,000 times longer than silicon. That durability is especially critical in aerospace applications, where a technical failure can be catastrophic.
Other materials break down when they are exposed to certain chemicals or extreme temperatures. But diamond’s all-carbon structure makes it chemically and thermally stable. It can survive in corrosive environments — blood, for instance — or in intense hot or cold — inside an engine or in outer space. Unlike most materials, it’s not prone to expansion when heated. In a tiny device like MEMS, even a small amount of expansion can lock parts together.
“The diamond that we make is absolutely chemically inert; it cannot be attacked by acids or bases,” Gruen said.
With his diamond film, MEMS engineers could design workable microfluidic pumps and micromotor turbines, he said. “It’s a breakthrough for the creation of micromachines.”
Diamond films grown in the Argonne and Sandia labs are so smooth that friction is nonexistent. Friction, like expansion, can lock up a MEMS device.
Water, too, is a hazard for machines in the micron range. In a tiny device, a molecule or two of water can play havoc with the mechanics. Silicon’s chemical bonds attract water. Diamond, on the other hand, is hydrophobic; it gives water the chemical equivalent of the cold shoulder.
Because it is composed solely of carbon, diamond is suitable for biomedical and electronic applications. The body readily will accept a carbon-based MEMS device or nanomachine because carbon is present in every organic molecule. Its composition also makes diamond a good conductor of electricity.
“The physicists and electrical engineers know the fundamental electronic properties are better in diamonds,” said J. L. Davidson, an electrical engineer at Vanderbilt University in Nashville, Tenn. Davidson worked on silicon MEMS at Harris Semiconductor until the mid-1980s. He switched to diamonds when he started teaching because he believed diamond was a superior material, he said.
BUILDING A BETTER DIAMOND
For decades scientists have been able to grow diamond film through a chemical vapor deposition method. Typically they mix 99 parts of hydrogen with one part of methane, which contains one carbon atom and four hydrogen atoms. The mixture is heated, usually to about 800 or 1,000 degrees centigrade, which exposes the carbon atoms. Those atoms begin to bond and form diamond crystals.
But those crystals are far from perfect. They can be peppered with impurities that diminish the properties that make diamond attractive. They also are large — up to 10 microns — which makes them rough. Their surface is so bumpy that for some MEMS applications they must be polished, which increases cost.
Gruen at Argonne and Sullivan and Friedmann at Sandia independently developed approaches that eliminate those flaws. Both techniques create a smooth diamond film for small tech applications. With help from their research teams, both have demonstrated how the material can be incorporated into MEMS and nanomachines.
Gruen’s team at Argonne designed a microwave discharge chamber in which they mixed a gaseous form of fullerenes, pure carbon molecules that contain 60 atoms, with argon and hydrogen. By applying microwave energy, they broke the argon into a plasma of ions and electrons that would bang into the fullerenes, busting them into two-atom carbon molecules.
Called dimers, these molecules settled into diamond crystals as small as 3 to 5 nanometers in diameter — about the distance across five atoms — for a smooth surface.
Argonne’s process uses far less hydrogen (a mere one part, and that can be eliminated completely) and much less energy (350 degrees centigrade). The team has tweaked the process to include methane, which lowers the cost.
“You can make a film very similar using methane,” Gruen said.
A DIFFERENT APPROACH
The Sandia team’s contribution is not in the making of amorphous diamond, but in making it useful. The key ingredient in amorphous diamond film is graphite, the most common and least expensive all-carbon form. Amorphous diamond is among the hardest materials on Earth, second only to crystalline diamond.
But it’s also useless. Its extremely high internal stress forces amorphous diamond film to bow and peel off a substrate.
The stress is created by the round-shaped carbon molecules all trying to squeeze onto the substrate, Sullivan said. He discovered in the late 1990s that he could change the molecules’ shape into an ellipsoid by heating the film to 600 degrees centigrade. That process breaks the bonds, allowing the molecules to realign themselves.
“Imagine you have a dozen eggs and you want to add a thirteenth,” Sullivan said. “They’d like to expand but they can’t because of the carton. The film would like to expand, forcing the substrate to bow. But you don’t have a problem if the eggs can change shape. Then they’re not pushing on their neighbors so much.”
Using that process, Sullivan and Friedmann can create amorphous diamond film that is as smooth as its substrate. The film can be as thick as 7 microns or can be made into membranes approaching the nanometer range. It is at least 90 percent as strong as crystalline diamond.
In the past year, both the Argonne and Sandia teams designed potential MEMS and nanomachine parts with their materials. The Argonne group built diamond tubes that can be used in motor shafts or gears. Each tube is 5 microns wide with walls as thin as 300 nanometers.
Sandia unveiled a millimeter-square device with a primitive drive mechanism. Its key parts are two diamond combs — one stationary, one on a spring — with interlocking teeth. An electric charge makes the hinged comb move toward or away from the stationary comb, creating mechanical energy.
OTHER FACTORS TO CONSIDER
Zorman, who designs MEMS devices using silicon carbide, said silicon still has many advantages over diamond. It is inexpensive, familiar and is already established in the foundries and fabs. Silicon carbide, which is harder than silicon but easier to fabricate than diamond, is still a relative newcomer in MEMS.
“A new material has to show something that silicon doesn’t,” he said. “I wouldn’t project diamond replacing silicon altogether.”
He likes many of the properties of diamond but says it is not appropriate for everything. For instance, diamond exposed to oxygen and high heat dissolves into carbon dioxide and carbon monoxide.
Vanderbilt’s Davidson also likes diamond’s prospects — in all its synthetic forms. But drawing on his experience, he warns that the word “diamond” will prompt concerns about cost, even when those concerns are unfounded.
“There’s nothing fundamental in these processes that make them expensive, although cognitively diamonds are expensive,” he said.
DIAMONDS FOR EVERYONE
But diamond’s biggest drawback is the lack of commercial tools to manufacture it for industry, the researchers said. They lack the desire, experience and resources to design a manufacturing tool suitable for fabrication facilities.
“That’s the limit,” Sullivan said. “That’s the one thing that’s holding it back from being implemented.”
Gruen is optimistic a manufacturer will fill the niche. He’s negotiating with a company to license his ultrananocrystalline diamond technique to allow production in industrial quantities. He wouldn’t name the company until negotiations were final, he said, but if talks go smoothly a facility could be in place in 12 months.
“This is a totally new technology that requires some development to make it commercial,” Gruen said. “But it requires some investment.”
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CONTACT THE AUTHOR:
Candace Stuart at [email protected] or call 734-994-1106, ext. 233.
Cover photo: This all-diamond comb is part of a drive mechanism in a micromachine. Courtesy Sandia National Laboratories.