Ab initio calculations of group 4 metallocene reaction mechanisms: atomic layer deposition and bond activation catalysis

Thin film dielectrics based on titanium, zirconium or hafnium oxides are being introduced to increase the permittivity of insulating layers in transistors for micro/nanoelectronics and memory devices. Atomic layer deposition (ALD) is the process of choice for fabricating these films, as it allows for high control of composition and thickness in thin, conformal films which can be deposited on substrates with high aspect-ratio features. The success of this method depends crucially on the chemical properties of the precursor molecules. A successful ALD precursor should be volatile, stable in the gas-phase, but reactive on the substrate and growing surface, leading to inert by-products. In recent years, many different ALD precursors for metal oxides have been developed, but many of them suffer from low thermal stability. Much promise is shown by group 4 metal precursors that contain cyclopentadienyl (Cp = C5H5-xRx) ligands. One of the main advantages of Cp precursors is their thermal stability. In this work ab initio calculations were carried out at the level of density functional theory (DFT) on a range of heteroleptic metallocenes [M(Cp)4-n(L)n], M = Hf/Zr/Ti, L = Me and OMe, in order to find mechanistic reasons for their observed behaviour during ALD. Based on optimized monomer structures, reactivity is analyzed with respect to ligand elimination. The order in which different ligands are eliminated during ALD follows their energetics which was in agreement with experimental measurements. Titanocene-derived precursors, TiCp*(OMe)3, do not yield TiO2 films in atomic layer deposition (ALD) with water, while Ti(OMe)4 does. DFT was used to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* ligand of TiCp*(OMe)3 lowers the Lewis acidity of the Ti centre and prevents its coordination to surface O (densification) during both of the ALD pulses. Blocking this step hindered further ALD reactions and for that reason no ALD growth is observed from TiCp*(OMe)3 and water. The thermal stability in the gas phase of Ti, Zr and Hf precursors that contain cyclopentadienyl ligands was also considered. The reaction that was found using DFT is an intramolecular α-H transfer that produces an alkylidene complex. The analysis shows that thermal stabilities of complexes of the type MCp2(CH3)2 increase down group 4 (M = Ti, Zr and Hf) due to an increase in the HOMO-LUMO band gap of the reactants, which itself increases with the electrophilicity of the metal. The reverse reaction of α-hydrogen abstraction in ZrCp2Me2 is 1,2-addition reaction of a C-H bond to a Zr=C bond. The same mechanism is investigated to determine if it operates for 1,2 addition of the tBu C-H across Hf=N in a corresponding Hf dimer complex. The aim of this work is to understand orbital interactions, how bonds break and how new bonds form, and in what state hydrogen is transferred during the reaction. Calculations reveal two synchronous and concerted electron transfers within a four-membered cyclic transition state in the plane between the cyclopentadienyl rings, one π(M=X)-to-σ(M-C) involving metal d orbitals and the other σ(C-H)-to-σ(X-H) mediating the transfer of neutral H, where X = C or N. The reaction of the hafnium dimer complex with CO that was studied for the purpose of understanding C-H bond activation has another interesting application, namely the cleavage of an N-N bond and resulting N-C bond formation. Analysis of the orbital plots reveals repulsion between the occupied orbitals on CO and the N-N unit where CO approaches along the N-N axis. The repulsions along the N-N axis are minimized by instead forming an asymmetrical intermediate in which CO first coordinates to one Hf and then to N. This breaks the symmetry of the N-N unit and the resultant mixing of MOs allows σ(NN) to be polarized, localizing electrons on the more distant N. This allowed σ(CO) and π(CO) donation to N and back-donation of π*(Hf2N2) to CO. Improved understanding of the chemistry of metal complexes can be gained from atomic-scale modelling and this provides valuable information for the design of new ALD precursors. The information gained from the model decomposition pathway can be additionally used to understand the chemistry of molecules in the ALD process as well as in catalytic systems

Mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium

The mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium and H(2)O was examined using ab initio calculations of hydrolysis energies to predict the order of ligand loss. These predictions were tested using in situ mass spectrometric measurements which revealed that the methyl ligand, and 65% of the methylcyclopentadienyl ligands are lost during the zirconium precursor adsorption. The remaining 35% of the methylcyclopentadienyl ligands and the methoxy ligand are lost during the subsequent H(2)O exposure. These measurements agree very well with the predictions, demonstrating that thermodynamic calculations are a simple and accurate predictor for the reactivities of these compounds. (c) 2007 American Institute of Physics. (DOI: 10.1063/1.2824814

Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe₃ (NPN = PhP(CH₂SiMe₂NPh)₂) and [^MesNPN]TaMe₃ (^MesNPN = PhP(CH₂SiMe₂N(2,4,6-Me₃C₆H₂))₂)

The thermolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ leads to the elimination of methane and the formation of cyclometalated derivative [PhP­(CH₂SiMe₂NPh)­(CH₂SiMe₂N-<i>o-</i>C₆H₄)]­TaMe₂, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an <i>ortho</i>-<i>N</i>-phenyl-C–H bond and a Ta-methyl unit. The photolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP­(CH₂SiMe₂NPh)₂]­TaCH₂(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the <i>N</i>-phenyl substituent with <i>N</i>-Mesityl (Mesityl = 2,4,6-Me₃C₆H₂) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H₂ were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ with H₂ to generate dinuclear tetrahydride ([PhP­(CH₂SiMe₂NPh)₂]­Ta)₂(μ-H)₄

Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe₃ (NPN = PhP(CH₂SiMe₂NPh)₂) and [^MesNPN]TaMe₃ (^MesNPN = PhP(CH₂SiMe₂N(2,4,6-Me₃C₆H₂))₂)

The thermolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ leads to the elimination of methane and the formation of cyclometalated derivative [PhP­(CH₂SiMe₂NPh)­(CH₂SiMe₂N-<i>o-</i>C₆H₄)]­TaMe₂, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an <i>ortho</i>-<i>N</i>-phenyl-C–H bond and a Ta-methyl unit. The photolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP­(CH₂SiMe₂NPh)₂]­TaCH₂(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the <i>N</i>-phenyl substituent with <i>N</i>-Mesityl (Mesityl = 2,4,6-Me₃C₆H₂) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H₂ were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ with H₂ to generate dinuclear tetrahydride ([PhP­(CH₂SiMe₂NPh)₂]­Ta)₂(μ-H)₄