Hafnia and zirconia precursors
New thin film dielectrics based on hafnium and zirconium oxides are being developed to increase the performance 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 by-products. We use density functional theory (DFT) to study the thermal stability in the gas phase of Zr and Hf precursors that contain cyclopentadienyl (Cp = C5H5-xRx) and alkyl (Me=CH3) ligands. We probe the non-ALD decomposition pathway and find a mechanism via intramolecular beta-H transfer that produces an alkylidene complex. This model of the decomposition pathway can be very helpful in proposing chemical modifications to enhance thermal stability, illustrating how the ALD process window can be widened by rational molecular design based on mechanistic understanding.