Energy-enhanced atomic layer deposition : offering more processing freedom

Atomic layer deposition (ALD) is a popular deposition technique comprising two or more sequential, self-limiting surface reactions, which make up an ALD cycle. Energy-enhanced ALD is an evolution of traditional thermal ALD methods, whereby energy is supplied to a gas in situ in order to convert a traditional thermal ALD co-reactant to a highly reactive species with short-term stability. Therefore, energy-enhanced ALD encompasses plasma-enhanced ALD and ozone-based ALD techniques. In this article, we aim to provide insight into precursor considerations, such that the advantages of energy-enhanced ALD can be exploited. The examples of such advantages are that a wider variety of precursors can be used, and that deposition temperatures down to room temperature with a high growth-per-cycle can be employed. The precursor freedom is demonstrated here by Ti compounds of the general formula [Ti(Cpx)L3] (Cpx = alkyl-substituted ¿5-cyclopentadienyl, L = OMe, OiPr or NMe2). Such heteroleptic cyclopentadienyl complexes allow for improved volatility by preventing oligomerisation and offering improved thermal stability, thereby leading to a longer storage life in bubblers and allowing for ALD at higher deposition temperatures than with analogous homoleptic precursors. However, experimental data show that [Ti(Cpx)L3] compounds are not reactive with water during ALD but do react with plasmas and ozone. Density Functional Theory calculations suggest that this is because chemisorption is prevented by the steric hindrance of the cyclopentadienyl ligand. Further processing versatility afforded by energy-enhanced ALD is also observed with depositions at low temperatures