Energy-enhanced atomic layer deposition for more process and precursor versatility

Abstract

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 η5-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 (<200°C) and even room temperature. Such low temperatures are often not considered viable for thermal ALD as there is insufficient thermal energy to drive reactions, resulting in low growth per cycle values and relatively high film contamination. The high reactivity of plasmas and ozone allows the deposition of good to fair quality films at low temperatures with a high growth-per-cycle, exemplified by [Al(CH3)3]2 (TMA) and [Al(CH3)2(OiPr)]2 (DMAI) aluminium precursors. The considerations required for energy-enhanced room-temperature ALD (RT-ALD) are also presented here, using Al2O3, SiO2 and TiO2, from TMA, [SiH2(NEt2)2] (BDEAS) and [Ti(OiPr)4] (TTIP), as examples. The essential criterion is that incoming metalorganic precursors and co-reactant gases must be reactive with the surface groups at room temperature. As long as this is met, then further desirable conditions for RT-ALD are a high precursor vapour pressure and short purge times. Using these examples, the advantages of energy-enhanced ALD with respect to precursor choices are highlighted.