The hexafluoroacetylacetonate complexes Rh(hfac)(C 2 H 4 ) 2 and Pt(hfac) 2 are known to serve as chemical vapor deposition precursors to Rh and Pt thin films. In the absence of a reducing carrier gas, the depositions are surface-selective and occur preferentially on copper, but under these conditions, the metallization processes are unexpectedly inefficient relative to the rapid deposition of Pd on Cu seen for the palladium analogue Pd(hfac) 2 . Mechanistic studies of the reactions of Rh(hfac)(C 2 H 4 ) 2 and Pt(hfac) 2 on copper surfaces under ultrahigh vacuum conditions have now been performed in order to elucidate the factors responsible for the differences among these surface-selective metallization processes. The studies demonstrate that adsorption of the rhodium complex Rh(hfac)(C 2 H 4 ) 2 on copper surfaces is accompanied by the loss of the coordinated ethylene groups, even at 100 K. At these low temperatures, the adsorbed Rh(hfac) fragments are oriented in several ways with respect to the surface. Heating the substrate above ∼150 K causes the hfac ligands to realign to a perpendicular orientation relative to the surface. The platinum precursor Pt(hfac) 2 adsorbs molecularly at 100 K with the molecular planes of the hfac ligands oriented parallel to the copper surface. Heating the substrate to temperatures above 150 K again results in a realignment of the hfac ligands to a perpendicular orientation. This reorientation is accompanied by a partial reduction of the Pt centers (as judged from shifts seen in X-ray photoelectron spectroscopy core level data), a result suggesting that the hfac ligands begin to dissociate from the platinum centers near 150 K. At temperatures above 220 K, the transfer of the hfac ligands from both complexes to the copper surface is complete, as signaled by the reduction of the metal centers to the zero-valent state. The copper-bound hfac ligands are further transformed upon heating, either reacting with copper surface atoms to yield Cu(hfac) 2 (which desorbs at temperatures above 250 K) or decomposing (with fragments desorbing above 350 K). The presence of platinum on the copper surface promotes the former reaction as judged by the appearance of a new reaction-limited desorption process for Cu(hfac) 2 . The presence of rhodium on the copper surface does not promote the formation of Cu(hfac) 2 , although autocatalysis is noted in the steady-state reactive scattering data. The inability of the Rh and Pt precursors to engage in a sustained transmetalation reaction with the copper surface is attributed to the slow interdiffusion of copper through the Rh/Cu and Pt/Cu alloys that are produced in the near-surface region.

The chemical vapor deposition (CVD) of tungsten nitride from a single source reagent, bis(tertbutylimido)bis(tertbutylamido)tungsten ((t-BuN)2W(NHBu-t)2), is examined with particular focus placed on the mechanisms and energetics involved in the activation and thermal decomposition of this CVD precursor. The main reactions that take place are (1) activated adsorption of the precursor, (2) hydrogen addition/exchange, leading to the evolution of tert-butylamine, (3) ligand activation via both γ-hydride activation and β-methyl elimination processes, and (4) ligand decomposition via C−N bond rupture. The activation energies for each of these processes were examined and found to be ∼30 kcal/mol for the process(es) leading to the evolution of tert-butylamine and ∼40 kcal/mol for the various reactions which lead to the fragmentation of the precursor ligands (pathways which appear to involve both C−H and C−C bond activation as well as the rupture of the ligand C−N bonds). The growth surface of the deposited film contained extensive quantities of carbon in addition to tungsten and nitrogen. The data also suggest that the growth in UHV does not yield a stable bulk nitride phase. Rather, it was found that the nitrogen appears to be present at levels consistent with the formation of a solid solution and that annealing to 700 K results in the loss of the nitrogen from the bulk film (as N2).

We report a study of the thermal decomposition and reactions of isobutyl iodide on Al(111). Using temperature-programmed reaction and Auger electron spectroscopies, it was found that more than one product-forming pathway involving the alkyl moiety exists on this surface. A first-order, β-hydride elimination reaction converts surface-bound isobutyl groups derived from the dissociation of the C−I bond to gas phase isobutene and dihydrogen at temperatures above ∼420 K. Competing with this unimolecular process is a collection of complex associative reactions which effect the etching of the aluminum surface via the formation of volatile organometallic species. This includes formation and subsequent desorption of diisobutylaluminum iodide (desorption peak maximum at ∼490 K), diisobutylaluminum hydride (∼515 K), methylaluminum dihydride (∼725 K), and AlI x , x = 1−3 (∼620 K). The kinetics of the processes yielding the various aluminum hydrides are coupled to that of the β-hydride elimination pathway (which serves as the hydrogen atom source) and are strongly coverage dependent. The formation of MeAlH2 reveals the occurrence of a kinetically competitive β-methyl elimination reaction of the surface alkyl groups.

In this Account, we present several representative studies of thin-film growth by chemical vapor deposition, with particular emphasis given to elucidating the mechanistic, energetic, and structural aspects of nucleation and growth. These understandings have allowed us to develop new methods to deposit patterned, as opposed to blanket, thin films. We show how such procedures can be exploited to effect the directed assembly (i.e., the additive fabrication) of a device architecture.

The collision-induced activation of the endothermic surface reaction of isobutyl iodide chemisorbed on an Al(111) surface is demonstrated using inert-gas, hyperthermal atomic beams. The collision-induced reaction (CIR) is highly selective towards promoting the β-hydride elimination pathway of the chemisorbed isobutyl fragments. The cross section for the collision-induced reaction was measured over a wide range of energies (14–92 kcal/mol) at normal incidence for Ar, Kr, and Xe atom beams. The CIR cross section exhibits scaling as a function of the normal kinetic energy of the incident atoms. The threshold energy for the β-hydride elimination reaction calculated from the experimental results using a classical energy transfer model is ∼1.1 eV (∼25 kcal/mol). This value is in excellent agreement with that obtained from an analysis of the thermally activated kinetics of the reaction. The measured cross section shows a complex dependence on both the incident energy of the colliding atom and the thermal energy provided by the surface where the two energy modes are interchangeable. The dynamics are explained on the basis of an impulsive, bimolecular collision event where the β-hydride elimination proceeds via a possible tunneling mechanism. The threshold energy calculated in this manner is an upper limit given that it is derived from an analysis which ignores excitations of the internal modes of the chemisorbed alkyl groups.

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