The reaction pathways and kineti
cs for the selective deposition ofpalladium on copper from the metal-organic precursor Pd(hfac)
2 have been established bymeans of reactive molecular beam-surface scatteringwherea flux of Pd(hfac)
2 (ranging from 10
13 to10
14 molecules cm
-2 s
-1)impinges continuously on the copper surface.The surface selectivity of the deposition process is a consequenceof a "redox transmetalation" reaction, which isbest described by the stoichiometric equationPd(hfac)
2 + Cu
Pd +Cu(hfac)
2. On polycrystalline copperfoils,the production and subsequent desorption of Cu(hfac)
2from the surface occurs with unit efficiency attemperaturesbetween 400 and 600 K. At temperatures above 600 K, the yield ofCu(hfac)
2 decreases and eventually falls tozeroat 800 K as the thermolytic decomposition of the hfac ligands on thesurface becomes kinetically competitive. Wehave devised a steady-state kinetic model of the adsorption ofPd(hfac)
2, desorption of Cu(hfac)
2,and thermolyticdecomposition of hfac molecules that quantitatively fits the decreasein Cu(hfac)
2 yield seen at highertemperatures.The transmetalation reaction follows an apparent power rate lawthat is first order in Cu and first order in hfaccoverage; the preexponential factor and the activation energy for thetransmetalation reaction are
A' = 2 ×10
-10molecules
-1 cm
2 s
-1 (or ~1× 10
6 s
-1 when normalized to the surfaceatom density of Cu) and
Ea' = 13 kcalmol
-1.The steady-state kinetic model accurately predicts the depositionrate so long as diffusion of the Pd atoms into theCu bulk is relatively fast; for the precursor fluxes used in thepresent study, this situation holds on polycrystallinecopper foils because the grain boundaries present provide a mechanismfor the rapid interdiffusion of Pd and Cu.On a single crystal copper substrate, where the high diffusivitypathway due to grain boundaries is absent, thetransmetalation reaction is self-limiting at our precursor fluxes owingto the slower rate of atomic diffusion. Thediffusion coefficient (
D) for the interdiffusion ofpalladium and copper on single crystal substrates has beencalculatedfrom a kinetic model explicitly incorporating the transport processesand is estimated to be ~10
-18 cm
2s
-1 at 358K. The nature of multicomponent chemical vapor depositionprocesses that operate under the kinetic control ofatomic diffusion is discussed.