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submitted on 29.05.2018, 15:35 and posted on 30.05.2018, 12:21by Glen N. Fomengia, Michael Nolan, Simon D. Elliott
Plasma-enhanced atomic layer deposition (ALD) of metal oxides is a
rapidly gaining interest especially in the electronics industry because of its
numerous advantages over the thermal process. However, the underlying reaction
mechanism is not sufficiently understood, particularly regarding saturation of
the reaction and densification of the film. In this work, we employ first
principles density functional theory (DFT) to determine the predominant reaction pathways, surface intermediates
and by-products formed when constituents of O2-plasma or O3
adsorb onto a methylated surface typical of TMA-based alumina ALD. The main outcomes are that a wide variety of barrierless and highly
exothermic reactions can take place. This leads to the spontaneous production
of various by-products with low desorption energies and also of surface
intermediates from the incomplete combustion of –CH3 ligands.
Surface hydroxyl groups are the most frequently observed intermediate and are formed
as a consequence of the conservation of atoms and charge when methyl ligands
are initially oxidized (rather than from subsequent re-adsorption of molecular
water). Anionic intermediates such as formates are also commonly observed at the
surface in the simulations. Formaldehyde, CH2O, is the most
frequently observed gaseous by-product. Desorption of this by-product leads to
saturation of the redox reaction at the level of two singlet oxygen atoms per
CH3 group, where the oxidation state of C is zero, rather than
further reaction with oxygen to higher oxidation states. We conclude that the
self-limiting chemistry that defines ALD comes about in this case through the
desorption by-products with partially-oxidised carbon. The simulations also show that densification occurs when ligands are
removed or oxidised to intermediates, indicating that there may be an inverse relationship
between Al/O coordination numbers in the final film and the concentration of
chemically-bound ligands or intermediate fragments covering the surface during
each ALD pulse. Therefore reactions that generate a bare surface
Al will produce denser films in metal oxide ALD.