«Mechanism-Based Design of Precursors for MOCVD Lisa McElwee-White,a,* Jürgen Koller,a Dojun Kim,b and Timothy J. Andersonb a Department of ...»
The compositions of WNxCy films grown from 4-6 show an interesting correlation to the W-N bond lengths (Table 2) and by inference, the relative contributions of the minor resonance structure B to bonding. Complex 5, in which the N lone pair is also conjugated into one of the phenyl rings, shows the shortest W-N bond (largest contribution of limiting structure B) because the N lone pair is less available for the N-N multiple bonding seen in limiting structure A. By examination of the structural data, the W-N bond strengths increase 4 5, but the N content of the film decreases concomitantly.
These results provide a contrast to the behavior of imido complexes 1-3, for which strong W-N bonding and weak N-C bonding in the imido moiety are associated with higher N contents in the films (12). In films grown from hydrazido complexes 4-6, a greater contribution of N-N multiple bonding (limiting structure A) hinders N-N cleavage.
For the hydrazido complexes, this stronger N-N bonding can facilitate pathways that result in incorporation of both N atoms into the film instead of release of one nitrogen through N-N cleavage. Thus the weaker N-N bond of 5 (more B character) may cleave more readily than that of 4. This N-N cleavage would release one of the nitrogens as an NPh2 fragment. For complex 4, the greater extent of multiple bonding in the W-N -N moiety (more A character) would minimize N-N cleavage and facilitate other pathways in which the intact hydrazido ligand is incorporated into the film, raising the N content.
This interpretation is also consistent with the positive ion CI mass spectra of 4 and 5, in which no fragments corresponding to N-N cleavage are found in the MS of 4 while the base peak of 5 is [Ph2NH2]+, a product of dissociation of the N-N bond (29).
Unprecedented Reaction Pathways. Although prediction of decomposition mechanisms using well-precedented solution chemistry can be quite successful, the higher temperature and different conditions (gas phase and gas-solid reactions) encountered in CVD can result in new reaction pathways that have not been reported for solution phase chemistry. An interesting example can be seen in the CVD of WNxCy films from 1-3 in the presence of H2 carrier gas. In these cases, no chlorine could be detected in the resulting films. RGA (Residual Gas Analyzer) data indicated the presence of HCl (but not Cl2 or alkyl/aryl chlorides) in the reactor effluent, suggesting involvement of the H2 carrier gas in removing the chloride ligands from the precursors. Since known reaction pathways of metal chlorides in the presence of H2 would not lead to HCl, a DFT computational study was undertaken (9).
Downloaded 26 Oct 2009 to 184.108.40.206. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp ECS Transactions, 25 (8) 161-171 (2009) Assuming that the dissociative loss of CH3CN from 1-3 under CVD conditions would be rapid (vide supra), we examined possible reactions of the coordinatively unsaturated complexes WCl4(NR) [1a: R = Ph; 2a: R = iPr; 3a: R = allyl] with H2 in the gas phase to model reactions occurring in the heated zone above the substrate surface. Oxidative addition of H2 is not possible due to the d0 electron count of 1a-3a and as expected, local minima associated with formation of the dihydride could not be found. Although precoordination of H2 as a -complex has been reported to lie in a shallow well in prior DFT studies of electron poor d0 species (31), we could not locate such a minimum for approach of H2 to 1a-3a in preparation for proton transfer to chloride from an acidic H2 ligand. We then explored pathways for -bond metathesis, in which ligand exchange occurs via a 4-center transition state. Such pathways are common for the reaction of d0 metal alkyls with H2, but have not been reported for the reaction of metal chlorides with H2 in solution under standard laboratory conditions. However, we could computationally locate transition states for -bond metathesis of 1c-3c with H2 to generate the metal hydride products WCl3H(NR) (1b-3b) and HCl. Although the reactions are endothermic with calculated activation energies of approximately 37 kcal/mol, under the high temperature (450 to 750 oC) and H2 flux in the CVD reactor, -bond metathesis appears to be a viable route to the experimentally observed chloride-free films and HCl byproduct, providing an example of an unexpected reaction that becomes accessible during CVD.
Gas Phase Variants of Solution Reactions. The observation that addition of NH3 during film deposition from 2-4 and 6 resulted in higher N content in the resulting films (20, 30, 32, 33), led us to carry out a DFT computational study of the reactions of 2 and 3 with NH3 (18). Loss of CH3CN was once again assumed to be rapid under CVD conditions and calculations began with the coordinatively unsaturated species WCl4(NR) (2a and 3a). Exothermic coordination of NH3 to the empty coordination site of 2a (Figure 6a) was followed by conversion to the bis-amide complex TI-2. A second proton transfer completed the transamination process to afford the parent imido complex Cl4W=NH and isopropylamine. Comparison of the energetics in Figure 6a with calculated results for unimolecular reactions of 2a suggests that transamination will be the dominant reaction of the imido moiety in the presence of NH3, consistent with calculations previously reported for the reaction of NH3 with (tBuN)2W(NHtBu)2 under ALD conditions (34) and the observation of transamination reactions of imido compounds in solution (35-37).
Computational results also elucidate an additional pathway for removal of the chloride ligands of 1-6 as HCl. In the presence of H2 carrier gas, the W-Cl bonds were cleaved by -bond metathesis. When NH3 is added to the system, the chlorides could also be removed either in the gas phase by addition of NH3 followed by elimination of HCl (38-40) or via surface reaction with species generated from NH3 (41, 42).
Calculations on elimination of HCl from the NH3 adduct TI-1 reveal a low energy pathway that results in formation of the amido complex EI-1 (Figure 6b). These pathways not only provide routes for loss of chloride during deposition, they also afford intermediates with additional N-bound ligands in the coordination sphere, facilitating incorporation of higher levels of N in the resulting films as experimentally observed for depositions in the presence of NH3.
Downloaded 26 Oct 2009 to 220.127.116.11. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp ECS Transactions, 25 (8) 161-171 (2009) Figure 6. Energetics for (a) transamination of 2a with NH3 and (b) 1,2-elimination of HCl from TI-1 and TI-2. Enthalpy values ( H°298) are reported in kcal/mol. Hydrogens on the isopropyl groups are omitted for clarity. Figure reproduced with permission from reference (18). Copyright 2008, American Chemical Society.
Design of organometallic CVD precursors can utilize data obtained from several standard spectroscopic, computational and structural techniques to provide insight into possible decomposition pathways during deposition of thin films. Although care must be taken in extrapolating results obtained under typical laboratory conditions to the temperatures and heterogeneous reactions found in CVD, it is still possible to correlate predicted reaction pathways to film properties. Continued work along these lines will Downloaded 26 Oct 2009 to 18.104.22.168. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp ECS Transactions, 25 (8) 161-171 (2009) surely result in increasing sophistication of precursor designs and improved control over film composition and microstructure.
Our work on CVD precursors has been generously supported over the years by the Army Research Office, the Office of Naval Research and the National Science Foundation.
Downloaded 26 Oct 2009 to 22.214.171.124. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp ECS Transactions, 25 (8) 161-171 (2009)