Computational Study of the Gas Phase Reactions of Isopropylimido and Allylimido Tungsten Precursors for Chemical Vapor Deposition of Tungsten Carbonitride Films: Implications for the Choice of Carrier Gas

Computational chemistry was employed to investigate possible gas phase decomposition pathways of the precursors Cl-4(CH3CN)W=NR (2: R = Pr-i; 3: R = C3H5) for MOCVD of tungsten carbonitride (WNxCy) thin films. The preferred pathways were demonstrated to be dependent on the presence of reactive species as a model for the selection of carrier gas in deposition experiments. For all gases, rapid dissociation of the acetonitrile ligand to generate Cl4W=NR (2a: R = Pr-i; 3a: R = C3H5) was assumed to be the first step. Calculations on a unimolecular pathway to generate WCl4 from 2a or 3a via a-shift of hydrogen and subsequent W-N(imido) bond cleavage are consistent with mass spectrometric data on 2 and 3. The presence of H-2 facilitated cleavage of the metal-chloride bonds via alpha-bond metathesis, resulting in an increase in the strength of the W-N(imido) bond compared to the N(imido)-C bond. The presence of NH3 led to exchange of the alkylimido ligand (NR) with NH via transamination and also to addition-elimination pathways that generated derivatives with an additional N-bound ligand (amido or imido) and concomitant loss of HCl. These reaction pathways had similar activation energies, suggesting that film growth and film properties could be altered by the choice of carrier gas.