What Is Ruthenium?

Ruthenium is a hard, silvery-white transition metal with atomic number 44, occupying group 8 of the periodic table alongside iron and osmium. It is one of the six platinum-group metals, which share chemical similarities but differ sharply in abundance and economic role. Ruthenium has a hexagonal close-packed crystal structure, a melting point near 2334 degrees Celsius, and a density of about 12.4 grams per cubic centimeter. It exhibits oxidation states from minus two to plus eight, a range unusual among metals, and forms a volatile tetroxide that is useful as a polishing etchant in electron microscopy. Elemental properties and isotopic data are cataloged by the Royal Society of Chemistry Periodic Table.

Named for Ruthenia, the Latin term for Russia, the element was isolated in 1844 by Karl Ernst Claus from platinum ore residues. It is among the rarest of the platinum-group metals, with an average crustal abundance of roughly one part per billion, and is recovered chiefly as a byproduct of nickel, copper, and platinum refining.

Chemical and Physical Properties

Ruthenium resists corrosion by most acids at room temperature, including aqua regia, and oxidizes only slowly in air. It alloys readily with other platinum-group metals, increasing their hardness and wear resistance. Small additions of ruthenium harden platinum and palladium for use in electrical contacts and fountain-pen nib tips. The metal is paramagnetic and has an electrical resistivity of about 7.1 microhm-centimeters at room temperature, low enough to compete with copper at nanoscale dimensions. Its high work function and stability in contact with oxide dielectrics make it attractive for gate and electrode applications in microelectronics.

Catalysis

Ruthenium is a versatile catalyst in both homogeneous and heterogeneous chemistry. Ruthenium-based complexes underpin the Grubbs catalysts for olefin metathesis, work recognized by the 2005 Nobel Prize in Chemistry, the history of which is documented by the Nobel Foundation. Heterogeneous ruthenium catalysts support ammonia synthesis, Fischer-Tropsch conversion of syngas into hydrocarbons, and hydrogenation of aromatic rings. In electrocatalysis, ruthenium oxide and mixed Ru-Ir oxides are standard materials for the oxygen evolution reaction in proton-exchange-membrane water electrolyzers, a key technology for low-carbon hydrogen production.

Semiconductor Interconnects

As copper interconnects approach fundamental scaling limits below 10 nanometers, ruthenium has emerged as a leading replacement candidate. At narrow line widths, copper resistivity rises sharply because the mean free path of electrons becomes comparable to the conductor dimensions, and the necessary barrier and liner layers consume an ever larger fraction of the cross section. Ruthenium has a shorter electron mean free path, roughly 6.7 nanometers compared with about 40 nanometers for copper, so its effective resistivity penalty is smaller at advanced nodes. It can also be deposited directly without a separate diffusion barrier. Industry reporting tracks these developments, including coverage from Semiconductor Engineering on adoption at leading-edge foundries.

Applications

Ruthenium finds use across industrial chemistry, electronics, and energy technology, including:

• Back-end-of-line interconnects and electrodes in advanced CMOS logic
• Thick-film chip resistors, where ruthenium dioxide sets the resistance value
• Olefin metathesis and asymmetric hydrogenation catalysts in pharmaceutical synthesis
• Oxygen-evolution catalysts for proton-exchange-membrane water electrolysis
• Hard disk drive recording layers in the form of cobalt-chromium-platinum-ruthenium alloys
• Dye-sensitized solar cells using ruthenium polypyridyl complexes as sensitizers
• Catalysts for ammonia synthesis and Fischer-Tropsch conversion of syngas