Atomic Layer Deposition (ALD) Techniques Finding New Applications in Nanotech

[For background on ALD check out this How Stuff Works video: http://videos.howstuffworks.com/multi-media-productions/1159-the-future-of-semiconductors-video.htm

Overall, atomic layer deposition -- or ALD -- is a technique that could be used to develop literally hundreds of products or devices. Yet perhaps the most exciting application is in the development of electrical systems that use mechanical parts, rather than solid-state properties.

Electromechanical systems are all around us, from your standard wristwatch to a remote control car. In the modern world, we are virtually surrounded by machines that convert electrical energy into mechanical energy – the energy of motion.

But until the evolution of nanotechnology brought the world increasingly effective techniques for constructing materials on the nanoscale, all of these machines were relatively large and could not compete with other techniques that did not use moving parts, for certain applications – such as in computer processors.

Now, the evolution of ALD is allowing researchers to build mechanical parts so small that, in theory, they could one day be part of a mechanically-powered computer – a computer relying on minute levers, gears and switches, rather than unmoving, solid-state inductors, capacitors and resistors.

“Mechanical structures have less loss than solid-state materials,” said Yuan-Jen Chang, a doctoral candidate in the Department of Mechanical Engineering at the University of Colorado-Boulder.

Chang works in Professor Victor M. Bright’s lab at CU, and is using ALD to construct electromechanical devices on the scale of just a few nanometers. These devices are known as nanoelectromechanical systems, or NEMs.

“The idea of mechanical computers were suggested 100 years ago,” Chang said. “But the techniques are more mature now.”

Two of Chang’s projects involve depositing a layer – one atom or molecule thick – of material onto a substrate, and then shaping this material into certain mechanical structures.

After depositing layers of gold and then nickel to a silicon substrate, Chang applies an ALD layer of tungsten on top of these. Next, using a technique known as electron-beam discography, Chang carves a portion of the nickel layer – about 100 nanometers thick – out from in-between the ALD tungsten and the gold electrodes. The result is a single-atom-thick tungsten lever, held at one end by a bit of nickel, and free to move at the other end up and down on the gold electrode.

“This ALD tungsten works as a switch,” he said. “Since ALD tungsten actuates at a lower voltage than sold state computers, we could reduce the heat in these and still keep the performance.”

Chang’s ALD tungsten switch has been shown to maintain its properties at around 2,000 cycles. Although other groups are working on similar projects, no one has published their findings yet, Chang said.

“We suspect they have only achieved five to ten cycles before failure,” he said. “So this is a very big achievement.”

Using a similar approach, Chang has also managed to develop a nanoelectromechanical resonator that is capable of sensing masses of a bout one-quadrillionth of a gram – only a few times larger than the mass of a single DNA molecule.

“I believe we are the first group to use ALD for this purpose,” he said. “This could have many applications in the bio-field.”

The Nano Risk Framework

According to Environmental Defense – a national organization funded to conduct environmental research and aid in developing environmental policy – the possible hazards of nanotechnology are only magnified by the tiny amount of research that has been conducted to assess these hazards.

“The few data now available give cause for concern: Some nanomaterials appear to have the potential to damage skin, brain, and lung tissue, to be mobile or persistent in the environment, or to kill microorganisms (potentially including ones that constitute the base of the food web),” according to Environmental Defense.

The organization calls for the direct partnership between industry leaders in nanotechnology and public advocacy groups to aid in developing comprehensive strategies to maximize the benefits of nanotechnology while minimizing the risks.

In June of 2007, a major step forward was made in this aim with the partnership of Environmental Defense and DuPont. Building off of a co-authored article published in the Wall Street Journal in 2005, DuPont and Environmental Defense executives published a “Nano Risk Framework”: a “proposal for a comprehensive, practical, and flexible Nano Risk Framework – a systematic and disciplined process – to evaluate and address the potential risks of nanoscale materials.”

The proposal was not binding; nor did it establish any legislation regarding nanotechnology policy. Rather, it was a demonstration of what two organizations with seemingly opposing goals could agree upon and accomplish, according to Nigel Cameron, director of the Institute of Nanotechnology and Society at Chicago’s Kent College..

“Both organizations are still worried about what the federal government is going to think about it,” he said. “But, if it were me, if Environmental Defense and DuPont can agree on something, I’d just sign the check.”

The resulting proposal suggested a six-step process for bringing any nanomaterial to market: first, the material must be scientifically described and characterized; second, the material’s entire life-cycle must be analyzed and its reactivity with any other materials must be stated; third, the risks of these possible interactions must be evaluated; fourth, risk management procedures must be developed and assessed; fifth, a course of action must be decided upon and documented; and sixth, an evaluation procedure must be implemented for some point in the future.

According to the framework, its most effective application as a tool for policy would be in a setting in which its execution would be ensured – such as under federal law.

Yet, the 2003 Nano Act is the only federal legislation dealing directly with the matter of nanotechnology’s environmental, social and ethical impacts. And, according to the act, all that is required of any federally funded nanotechnology research and development project is that it includes “activities that address societal, ethical, and environmental concerns.”

The nature of these activities remains implicit, and it is not designed to be quantitatively assessed.