“Being able to deposit these films with the same or better material quality as conventional approaches while being more cost-effective, less complicated, and with fewer safety concerns is a critical advantage of this technique.”Bharat Jalan
Researchers from the University of Minnesota Twin Cities have developed a new way to manufacture important metals and metal oxides more efficiently and in an atomically precise manner. The new technology for making “stubborn” metals, which reduces the cost of metal deposition while achieving the deposition of more complex materials, improves the production of components for thin films for semiconductors in many electronic devices, displays, fuel cells and catalytic applications.
This group of metals, which includes platinum, iridium, ruthenium and tungsten, are very difficult to convert into thin films as they need very high temperatures to evaporate. However, in their study published in PNAS [Nunn et al. Proc. Natl. Acad. Sci. U.S.A. (2021) DOI: 10.1073/pnas.2105713118], the team showed how to evaporate these metals at much lower temperatures – less than 100°C instead of several thousands.
These metal films are usually synthesized by sputtering or electron beam evaporation, with evaporation involving melting and evaporating at high temperatures before letting the film form on top of wafers. The problem is that this is expensive, requires a great deal of energy and can be dangerous because of the high voltages used. However, by designing and adding organic ligands – a combination of carbon, hydrogen and oxygen atoms – to the metals, the materials’ vapor pressures significantly increased, making them easier to evaporate at lower temperatures, an approach that is easily scalable.
Many of these “stubborn” metals are used in a variety of industrial applications, such as platinum in catalytic converters, fuel cells, temperature sensors, and in next-generation devices involving spintronics. As senior author Bharat Jalan told Materials Today, “Being able to deposit these films with the same or better material quality as conventional approaches while being more cost-effective, less complicated, and with fewer safety concerns is a critical advantage of this technique”.
Using platinum and ruthenium as examples, the researchers showed that both the low vapor pressure and the difficulty in oxidizing a “stubborn” element can be resolved using a solid metal-organic compound with significantly higher vapor pressure and with the added benefits of being in a pre-oxidized state and having excellent thermal and air stability.
Their synthesis technique approach of solid-source metal–organic molecular beam epitaxy (MBE) can be applied to any other elements and in physical vapor deposition (PVD) applications such as evaporation, sputtering, pulsed laser deposition, all of which have been modified and advanced over the years. Although this approach was developed with only MBE, the study could bring new synthesis paths for many of these techniques and other complex materials.
