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Posted: Jun 19, 2008
Atomic Layer Deposition - a true and tested nanotechnology
(Nanowerk Spotlight) One of the true nanotechnologies that pre-dates the explosion of the popular use of the word during the past few years is Atomic Layer Deposition (ALD). This gas phase chemical process is used to create extremely thin coatings only a few nanometers thick which can be deposited in a precisely controlled way.
Initially used as a technique for making a specific type of light display (electroluminescent display) smaller and more efficient, the ALD process was invented and patented by Tuomo Suntola and his co-workers in Finland in 1974 (co-incidentally, this is the year that the term ‘nanotechnology’ was first defined by Norio Taniguchi).
Suntola’s advancement built upon existing technologies at the time; the general idea of electroluminescence had been around since 1907, when Captain Henry Joseph Round found that silicon carbide emitted light when sandwiched between two materials carrying electricity. Yet it was not until after ALD was developed that this electro-physical property could be made small and efficient enough to market electroluminescent products – many such systems are used in streetlights, and as the backlight in liquid crystal televisions and other technologies.
Thanks to ALD, electroluminescent displays could be made smaller, and required less electricity to function. Products using this nanotechnology suddenly became marketable in the 1980s – such as neon lights – once their electricity consumption became comparable to other options.
Dynamics of Atomic Layer Deposition
The fundamental notion behind Atomic Layer Deposition is rather simple: It is a process by which an atomic layer of material can be affixed to a surface material one layer at a time.
By depositing one layer per cycle, ALD offers extreme precision in ultra-thin film growth since the number of cycles determines the number of atomic layers and therefore the precise thickness of deposited film.
The process involves pushing a pulse of a heated chemical gas (or plasma) into a chamber, which contains a chemical substrate or some other material waiting to be coated. What makes ALD so perfectly uniform – meaning it doesn’t allow these gaseous atoms to pile-up on top of each other – is that it takes advantage of a limiting chemical reaction. This reaction only allows one atom at a time to bind to the surface material. And once all of the possible bonding-sites have been taken-up, the reaction stops, leaving behind a pinhole-free layer of molecules – like the thinnest paint coat physically possible.
Naturally, the ability to achieve this level of control for a layer of material has found an easy home in electroluminescence displays; since all that was needed to make the simple sandwiching technology more efficient was to make the sandwiched material thinner.
Not surprisingly, even more applications for ALD have been imagined since 1974 – including nanomaterials.
Atomic Layer Deposition for Nanoparticles
Altering the surface properties of ultra-fine powders is one of the many potential applications of ALD, according to Dr. Alan Weimer, professor of chemical and biological engineering at the University of Colorado-Boulder. The possible uses of these fine powders can be found in virtually any area of materials science, he said.
“Basically, they’re particles that might be a micron in size, that might be used to fabricate parts,” Weimer said. "A lot of things that could be imprecisely considered nanoparticles have been around for a long time, simply because the processes used to make them have not required extremely high levels of control. Some of these particles are used to make shampoos thick or to give different properties to toothpaste, for example. Self-cleaning windows are another example. The particles are so small they’re transparent."
Although there may be nothing really new about the particles in many fine powder-type materials, there could be thousands of new applications for these materials if their surface chemistry could be altered in some way. Enter Atomic Layer Deposition.
Using ALD to increase biocompatibility of nanoparticles
“There’s a concern that certain particles will break down the skin, if they come into contact with it,” Weimer said. “The way to get around this is to put a coating on the particle so that it doesn’t make contact with the skin.”
According to Weimer, there are certain materials – such as some lithium compounds – that could be very effective sunscreens. However, there is a fear that these compounds might react with the skin to cause adverse side effects.
ALD offers an ideal way to change the surface properties of such chemicals, so that vastly more effective products might be brought to consumers. “We’ve basically invented a method for putting ALD on these particles,” he said.
Weimer’s method of ALD makes the application of the molecule-thick layer line-of-sight independent. This means that it is unimportant which way the material is facing or from where the coating is injected into the system – the ALD process will generate a perfect coating on every side, regardless.
Coating sunscreen particles using ALD would effectively block the type of ultraviolet radiation that has been lined to skin cancer, Weimer said. In general terms, there are two types of ultraviolet light: UVA and UVB. However, sunscreens on the market today only effectively block UVB light, Weimer said. This is unfortunate since UVA has been more strongly connected to the development of skin cancer.
“Coating lithium nanoparticles with Aluminum Oxide would make them unreactive with the skin,” he said. “And they’d block out the sun better.”
Weimer and Professor of Chemistry and Biochemistry Dr. Steven George at CU, have started ALD NanoSolutions, Inc. to aid the development of products produced by their ALD technique.
“A lot can be done with ALD,” George said. “But its biggest application is really in semiconductor applications.”
Atomic Layer Deposition for Electromechanical Systems
Perhaps the most exciting application of ALD 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, such as 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.”
Future of Atomic Layer Deposition
Despite being invented more than three decades ago, the technique of atomic layer deposition is continuing to advance and it promises to hold the key to perhaps hundreds of future advancements. From the creation of new or more effective chemicals to the development of mechanically driven computers, everything suggests that ALD’s role in nanotechnology and nanoscience will only continue to grow.