Deposition of ruthenium (Ru) was done using chemical vapor deposition with bis(hexafluoroacetylacetonate)dicarbonyl ruthenium (Ru(hfac)(2)(CO)(2)) as the precursor, at temperatures, T, ranging from 548 <= T < 623 K. The initial growth behavior on Si (100) surfaces was investigated using atomic force microscopy and X-ray photoelectron spectroscopy. Three,,dimensional nucleus growth was observed. For T 573 K and a precursor partial pressure of 1.3x10(-1) Pa, after 5 min deposition, the nuclei density observed on an H-terminated Si surface of 4.7 x 10(9) cm(-2) was about three times that observed on an SiO2 surface. Kinetic analysis of nucleation showed a lower activation energy on an H-terminated surface (5 kcal/mole) than that on an oxide surface (11 kcal/mol). As predicted by quantum chemical calculations, the much larger dissociation energy of Ru-hfac (241 kcal/mol) than of Ru-CO (57 kcal/mol) suggests that the deposition is mainly controlled by the hfac dissociation step. Moreover, the existence of adsorbed H was demonstrated to facilitate Ru deposition by removing hfac ligands through the formation of volatile H(hfac). (c) 2004 Elsevier B.V All rights reserved.