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.”