Mechanistic Studies of Palladium Thin Film Growth from Palladium(II) β-Diketonates. 2. Kinetic Analysis of the Transmetalation Reaction of Bis(hexafluoroacetylacetonato)palladium(II) on Copper Surfaces

The reaction pathways and kinetics for the selective deposition of palladium on copper from the metal−organic precursor Pd(hfac) 2 have been established by means of reactive molecular beam−surface scattering where a flux of Pd(hfac) 2 (ranging from 10 13 to 10 14 molecules cm -2 s -1 ) impinges continuously on the copper surface. The surface selectivity of the deposition process is a consequence of a “redox transmetalation” reaction, which is best described by the stoichiometric equation Pd(hfac) 2 + Cu → Pd + Cu(hfac) 2 . On polycrystalline copper foils, the production and subsequent desorption of Cu(hfac) 2 from the surface occurs with unit efficiency at temperatures between 400 and 600 K. At temperatures above 600 K, the yield of Cu(hfac) 2 decreases and eventually falls to zero at 800 K as the thermolytic decomposition of the hfac ligands on the surface becomes kinetically competitive. We have devised a steady-state kinetic model of the adsorption of Pd(hfac) 2, desorption of Cu(hfac) 2, and thermolytic decomposition of hfac molecules that quantitatively fits the decrease in Cu(hfac) 2 yield seen at higher temperatures. The transmetalation reaction follows an apparent power rate law that is first order in Cu and first order in hfac coverage; the preexponential factor and the activation energy for the transmetalation reaction are A‘ = 2 × 10 -10 molecules -1 cm 2 s -1 (or ∼1 × 10 6 s -1 when normalized to the surface atom density of Cu) and E a ‘ = 13 kcal mol -1 . The steady-state kinetic model accurately predicts the deposition rate so long as diffusion of the Pd atoms into the Cu bulk is relatively fast; for the precursor fluxes used in the present study, this situation holds on polycrystalline copper foils because the grain boundaries present provide a mechanism for the rapid interdiffusion of Pd and Cu. On a single crystal copper substrate, where the high diffusivity pathway due to grain boundaries is absent, the transmetalation reaction is self-limiting at our precursor fluxes owing to the slower rate of atomic diffusion. The diffusion coefficient ( D ) for the interdiffusion of palladium and copper on single crystal substrates has been calculated from a kinetic model explicitly incorporating the transport processes and is estimated to be ∼10 -18 cm 2 s -1 at 358 K. The nature of multicomponent chemical vapor deposition processes that operate under the kinetic control of atomic diffusion is discussed.