Atomic layer deposition of tantalum, hafnium and gadolinium nitrides

Z. Fang
Abstract & Cover

This research describes the development of ALD processes for the deposition of nitride materials including tantalum, hafnium and gadolinium nitrides. Ta and Hf nitrides are of significant interests for sub-lOOnm silicon based electronic devices, while Gd nitride may be exploitable in future spintronic devices. ALD has been established a key manufacturing tool in microelectronics, the development of ALD processes for these nitrides are essential for future manufacturing of electronic devices and can benefit future manufacturing of spintronic devices. In the current research, these nitrides were deposited using ALD and the films were characterised using MEIS, AES, XRD, TEM, SEM, AFM, and a four point probe. Ta nitride films were grown at temperatures ranging from 200°C to 375°C using ALD with Pentakis(dimethylamino)tantalum, Ta(NMe2)5 as the metal source and either ammonia or monomethyl-hydrazine (MMH) as a nitrogen co-reactant. Self-limiting behaviour was observed for both ammonia and MMH processes, with growth rates of 0.6 and 0.4 A/cycle respectively at 300°C. Films deposited using ammonia were found to have a mono-nitride stoichiometry with a cubic microstructure and resistivities as low as 70 In contrast, films deposited using MMH were found to be nitrogen rich TasNs with an amorphous microstructure and high resistivities (>4 A QCM was used to measure mass gain and loss during the cyclic ALD processes and the data was used in combination with MEIS to elucidate the Ta(NMe2)5 absorption mechanisms. For Hf nitride, films were firstly deposited using thermal ALD with tetrakis(dimethylamino)hafnium, Hf(NMe2)4 and ammonia between 100 and 400°C. Selflimiting behaviour was observed, however, the films exhibit a low density and were prone to oxidation during post-deposition exposure to air. A comparison between thermal and PE ALD was then made at 300°C with tetrakis(ethylmethylamino)hafnium, Hf(NEtMe)4 as the metal source and either molecular or plasma-cracked ammonia as a nitrogen source. PEALD allows shorter purge time, which significantly reduces the cycle length; PEALD also results in higher film density. The densities of the films deposited by PEALD and thermal ALD were found to be 11.6 and 9.7 g/cm3 respectively. Mass spectroscopy indicates that the process characteristics in PEALD are attributed to the nature of the co-reactants, namely, radicals of hydrogen and nitrogen. Their high reactivity and short life time are responsible for the resulted high density and the short required purge time. All films deposited were found to be insulators and with an amorphous microstructure. The films deposited by PEALD remain amorphous and stable with no interactions between Hf and Si after vacuum annealing up to 800°C. Gd nitride films were successfully deposited using a cyclic PEALD based process. The deposition was carried out with tris(methylcyclopentadienyl)gadolinium, Gd(MeCp)3, and remote nitrogen plasma exposure, separated by argon pulses. Films were deposited at temperatures between 150 and 300°C and capped with Ta nitride to prevent post deposition oxidation. Gd nitride with a 1:1 Gd:N ratio, low oxygen incorporation (5%), good thickness uniformity (95%), an amoiphous micro structure and smooth surface (Ra.=~0.7nm) have been deposited. Deposition with tris(silylamide)gadolinium, Gd{N(SiMe3)2}3, and either ammonia or MMH was also investigated. Although the process using ammonia was unsuccessful due to the insufficient reactivity of ammonia, the results show that a reaction between Gd{N(SiMe3)2}3 and MMh does take place. Gd{N(SiMe3)2}3 was found to be a self-limiting precursor, however, the as deposited films were found to be GdSixOy. The silicon incorporation was attributed to partial breakdown of silylamine groups, where the oxygen incorporation was attributed to the possible tetrahydrofuran (THF) contamination in the precursor. 

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University of Liverpool
(Liverpool, United Kingdom)
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