Kinetic and mechanistic studies of the chemical vapor deposition of tungsten nitride from bis(tertbutylimido)bis(tertbutylamido)tungsten

The chemical vapor deposition (CVD) of tungsten nitride from a single source reagent, bis(tertbutylimido)-bis(tertbutylamido)tungsten ((f-BuN)₂W(NHBu-r)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 terf-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).