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 1013 to 1014 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 x 10-10 molecules-1 cm2 s-1 (or ~1 x 106 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 cm2 s-1 at 358 K. The nature of multicomponent chemical vapor deposition processes that operate under the kinetic control of atomic diffusion is discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry