Metal ALD and pulsed CVD: Fundamental reactions and links with solution chemistry

Atomic layer deposition (ALD) is a thin film deposition technique which operates via repeated alternating and self-terminating surface-based reactions between a precursor and a co-reactant, separated in time by purge steps. This technique is particularly well-suited to the deposition of highly uniform and conformal thin films, even on surfaces with nano-scale high aspect ratio features. Furthermore, use of a metal precursor and a co-reactant in ALD and the related technique of pulsed-CVD (pulsed-chemical vapour deposition), provides the potential for deposition of materials that may be inaccessible using CVD methods that rely upon the thermal decomposition of a single metal precursor. This review surveys the different classes of co-reactant used for thermal metal ALD/pulsed-CVD with a focus on the reaction chemistries known or proposed to be involved. Parallels are drawn between surface-based metal ALD/pulsed-CVD reactivity and solution-based reactivity including electroless deposition, solution-based nanoparticle synthesis, and the synthesis of zero-valent complexes bearing labile ligands. Also described are applications of solution screening and solution mechanistic studies to the identification of promising new ALD/pulsed-CVD reactivities, and the generation of initial mechanistic hypotheses as to the fundamental reaction steps involved in metal ALD/pulsed-CVD. A primary goal of this review is to provide a unique reactivity-based perspective of metal ALD/pulsed-CVD. In addition, we have endeavoured to illustrate commonalities between solution-based and surface-based reactions relevant to metal deposition, and to highlight beneficial applications of the former to the development of the latter.