Chemistry of Ruthenium Diketonate Atomic Layer Deposition (ALD) Precursors on Metal Surfaces

The thermal chemistry of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)ruthenium(III) (Ru(tmhd)(3)), a potential precursor for the chemical deposition of ruthenium-containing films, on Ni(110) single-crystal surfaces was characterized by using a combination of temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and reflection absorption infrared spectroscopy (RAIRS). Additional characterization of the surface chemistry of the protonated ligand, Htmhd, was evaluated as well for reference. It was found that the molecularly adsorbed ruthenium compound reacts readily by approximately 310 K, loosing its ligands to both the gas phase and the surface as the central ion is reduced to its Ru metallic state. The diketonate ligand, now bonded to the nickel surface, starts to decompose at around 400 K, and generates gas-phase carbon monoxide and molecular hydrogen in TPD peaks at 435 K. More extensive decomposition is seen at 535 K, yielding 2,2-dimethyl-3-oxopentanal, isobutene, ketene, and carbon monoxide, and also carbon dioxide and molecular hydrogen at slightly higher temperatures. The XPS data corroborate the early reduction of the metal center and the losses of carbon- and oxygen-containing adsorbates to the gas phase, and the RAIRS traces show similar chemistry followed by the Ru complex and the free ligand, both converting via an initial decarbonylation step and a subsequent loss of the terminal tert-butyl groups. The early decomposition of the ligand on the metal surface points to potential problems with the clean deposition of metal films using diketonate complexes, but the ease with which those ligands are displaced from the central ion suggests that there is a potential for low-temperature film deposition chemistry under specific circumstances.