Ritala & Leskelä – Masters of the atom

Chemists at the University of Helsinki have been developing a technology loved by jewellery makers, solar-cell manufacturers and microelectronics businesses alike.

Talking about the new Xeon processor in 2008, Gordon Moore, founder of the semiconductor giant Intel, said its materials represented the biggest leap forward in microelectronics since the 1960s. One of these major groundbreaking innovations was ALD, or atomic layer deposition, which has become a particularly precious technology to Finns and the University of Helsinki.

The idea behind ALD – the ability to control the growth of material layer by layer – was originally devised by Tuomo Suntola, DSc (Tech.).

“After one of my first public presentations on ALD in the early 1980s, I received a letter from a German professor, who said he had experimentally proved that atomic layer deposition was unfeasible,” recalls Suntola, now enjoying retirement. “I didn’t take the trouble to answer, since we already had our first ALD screens in test production.”

ALD allows industry to shrink microelectronics nearly endlessly, which, in turn, paves the way for increasing the power of the equipment.

The first commercial ALD application was a large electroluminescent display screen set up at the Helsinki-Vantaa airport in 1983. The electroluminescence phenomenon as such – the way in which a material emits light when passing through an electric field – has been known to researchers for ages.

Suitable chemicals

To develop the technology, Suntola sought partners from universities around Finland. The essential refinements made to the method two decades ago, which also benefited Intel’s Xeon processor, were made at the University of Helsinki. Mikko Ritala, who back then was working on his dissertation, identified the chemical compounds suitable for the process. He now works as a professor in inorganic chemistry at the University of Helsinki Science Campus and is one of the most cited researchers in the field.

The applications also continue to evolve. Today, ALD provides employment for four Finnish companies, all of which cooperate with the University of Helsinki.

“We didn’t patent anything back in the days, since we didn’t have a clue what our research might lead to,” explains Professor Markku Leskelä, one of the key developers of ALD and the supervisor of Ritala’s dissertation. “It was basic research, which proved to be extremely significant.”

Solar panels and jewellery

The beauty of the ALD technology lies in its simplicity. Once the right chemical compounds have been found, the gaseous molecules in an ALD reaction chamber adhere to the surface of, say, a glass or silicon sheet one atomic layer at a time. At its thinnest, the resulting film can measure less than one nanometre, meaning that one million films make a layer one millimetre thick.

In the late 1980s, flat screens and the recently introduced solar panels were the main applications of ALD. In Japan, ALD was trialled on the basis of Suntola’s work in the production of compound semiconductor materials, but the results were meagre.

At the turn of the century, the electronics industry finally stopped grumbling about the slowness of ALD technology and adopted it. It really had no other choice. Electronics components continued to shrink, and ALD was practically the only technology around that could manufacture materials with a thickness of only a few atomic layers in a controlled and reliable manner.

Until 2000, only a few dozen ALD-related patents were filed annually worldwide, but by 2010, the number had jumped to over 800.

In ALD, gaseous molecules find their way into the deepest recesses of integrated circuits. This ability to follow three-dimensional surfaces is also appreciated in the jewellery industry. Kalevala Koru, a big name in the Finnish jewellery tradition, uses ALD to coat all its silver products with a ten-nanometre aluminium oxide layer to prevent tarnishing.

The list of current and potential applications for ALD goes on and on: anti-corrosion coating, X-ray optics, lithium-ion batteries, fuel cells, and so on. We are talking about cutting-edge technology that can be used to prevent climate change as well as to construct increasingly advanced home entertainment electronics.

The ABC of ALD

Thin film is made in an ALD reaction chamber using cyclical gaseous pulses. Consisting of four phases, the ALD cycle can be repeated indefinitely – depending on the required thickness of the film.  

1. The silicon wafer or other surface is exposed to the required precursor. The choice of the precursor determines the properties of the film, making it, for example, insulating or electrically conductive.
2. The excess molecules are removed from the reaction chamber, leaving a single molecule layer on the surface.
3. The chamber is exposed to the second precursor, which reacts with the first precursor layer.
4. The chamber is again purged of excess molecules to reveal a thin film of the required type.

A top placing among the most frequently cited researchers is not the only credit that the ALD findings have brought Markku Leskelä and his colleagues. Professor Leskelä heads the Centre of Excellence for Atomic Layer Deposition at the University of Helsinki.

Atoms as building blocks

Text: Tuomo Tamminen
Photo: Veikko Somerpuro
Translation: Language Services/Language Centre, University of Helsinki