Thermal stability of precursors for atomic layer deposition of TiO2, ZrO2, and HfO2: an ab initio study of alpha-hydrogen abstraction in bis-cyclopentadienyl dimethyl complexes.

Thin film dielectrics based on hafnium and zirconium oxides are being introduced to increase the permittivity of insulating layers in nanoelectronic transistor and memory devices. Atomic layer deposition (ALD) is the process of choice for fabricating these films, and the success of this method depends crucially on the chemical properties of the precursor molecules. Designing new precursors requires molecular engineering and chemical tailoring to obtain specific physical properties and performance capabilities. A successful ALD precursor should be volatile, stable in the gas-phase, but reactive on the substrate and growing surface, leading to inert byproduct. This study is concerned with the thermal stability in the gas phase of Ti, Zr, and Hf precursors that contain cyclopentadienyl (Cp = C(5)H(5-x)R(x)) ligands. We use density functional theory (DFT) to probe the non-ALD decomposition pathway and find a mechanism via intramolecular alpha-H transfer that produces an alkylidene complex. The analysis shows that thermal stabilities of complexes of the type MCp(2)(CH(3))(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. Precursor decomposition via this pathway in the gas phase can therefore be avoided by replacing the alpha-H donor or acceptor ligands or by increasing the electrophilicity of the metal. This illustrates how the ALD process window can be widened by rational molecular design based on mechanistic understanding.