MECHANISTIC STUDIES OF PALLADIUM THIN-FILM GROWTH FROM PALLADIUM(II) BETA-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 pal ladium on copper from the metalorganic precursor Pd(hfac)(2) have been established by means of reactive molecular beam-surface scattering wh ere 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 sele ctivity of the deposition process is a consequence of a ''redox transm etalation'' reaction, which is best described by the stoichiometric eq uation Pd(hfac)(2) + Cu --> Pd + Cu(hfac)(2). On polycrystalline coppe r foils, the production and subsequent desorption of Cu(hfac)a from th e 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 decompositio n of the hfac ligands on the surface becomes kinetically competitive. We have devised a steady-stare kinetic model of the adsorption of Pd(h fac)(2), desorption of Cu(hfac)(2), and thermolytic decomposition of h fac molecules that quantitatively fits the decrease in Cu(hfac)z yield seen at higher temperatures. The transmetalation reaction follows an apparent power rate law that is first order in Cu and first order in h fac coverage; the preexponential factor and the activation energy for the transmetalation reaction are A' = 2 x 10(-10) molecules(-1) cm(2) s(-1) (or similar to 1 x 10(6) s(-1) when normalized to the surface at om density of Cu) and E(a)' = 13 kcal mol(-1). The steady-state kineti c model accurately predicts the deposition rate so long as diffusion o f the Pd atoms into the Cu bulk is relatively fast; for the precursor fluxes used in the present study, this situation holds on polycrystall ine copper foils because the grain boundaries present provide a mechan ism for the rapid interdiffusion of Pd and Cu. On a single crystal cop per substrate, where the high diffusivity pathway due to grain boundar ies is absent, the transmetalation reaction is self-limiting at our pr ecursor fluxes owing to the slower rate of atomic diffusion. The diffu sion coefficient (D) for the interdiffusion of palladium and copper on single crystal substrates has been calculated from a kinetic model ex plicitly incorporating the transport processes and is estimated to be similar to 10(-18) cm(2) s(-1) at 358 K. The nature of multicomponent chemical vapor deposition processes that operate under the kinetic con trol of atomic diffusion is discussed.