The Chemistry of ALD Precursors

ALD (Atomic Layer Deposition) is a gas-phase thin-film deposition process involving the sequential addition of a -complex or metal precursor and a reagent (H ₂ O, N ₂ O...) to a heated substrate. Each step is associated with a chemical reactivity: that of the precursor on the surface, then that of the reagent on the precursor, and purge sequences are provided for the elimination of coproducts.

This process requires precursors that are volatile, thermally stable (for storage and transport) and reactive with a given surface (in particular with the surface species present). The combination of all these properties within a single molecule is often linked to its three-dimensional structure and the strength of intra- and intermolecular interactions. This structure/properties linkage of molecular precursors for ALD will therefore be addressed by detailing the impact of different atomic parameters of the metal and structural parameters of the ligand such as its steric hindrance, the presence of fluorine atoms, its asymmetry and/or its electronic properties (donor or acceptor) on the volatility, stability and reactivity of these molecules. The aim of this article is to give users of ALD processes, whether chemists or not, a practical guide to understanding the performance of molecules as ALD precursors, and even to selecting the right precursor family for a process.