First principles mechanistic study of self-limiting oxidative adsorption of remote oxygen plasma during the atomic layer deposition of alumina

Abstract

Plasma-enhanced atomic layer deposition (ALD) of metal oxides is 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 intermediates 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.

Reaction Pathways for Atomic Layer Deposition with Lithium Hexamethyl Disilazide, Trimethyl Phosphate, and Oxygen Plasma

“…Horizontal slices of spectra during the oxygen plasma pulse are shown in Figure a. We can clearly distinguish between signals that are intrinsic to the oxygen plasma ( m / z 16 and 32) and signals from reaction products: peaks for H 2 O ( m / z 18), CO ( m / z 28), and CO 2 ( m / z 44), as well as traces of CH 2 O ( m / z 30) (as proposed in refs and ). The peaks at m / z 34 and 33 could be interpreted as SiH 4 peaks, but they are not transient and have a similar shape to the peaks at m / z 16 and 32, so we interpret them as isotopes of O.…”

“…When an oxygen plasma pulse reacts with the physisorbed LiHMDS, all methyl groups are combusted, leading to very little contamination compared to the thermal process. The result of the plasma is that the surface methyl groups are replaced with OH groups, so after a plasma pulse we have a mixed Si–OH/O–Li surface.…”

Atomic Layer Deposition of Localized Boron- and Hydrogen-Doped Aluminum Oxide Using Trimethyl Borate as a Dopant Precursor

“…Previous computational studies of O 2 − plasma-based ALD of Al 2 O 3 have led to the general observation that each combusted organic ligand leaves behind a surface OH group. 63,64 We therefore suggest that residual methoxide ligands bound to B are oxidized by O 2 −plasma to B−OH surface groups. This is further corroborated by the addition of B−OH surface groups in the FTIR spectrum in Figure 3, at 3705 cm −1 , just below those of the Al−OH surface groups.…”

Self-limiting nitrogen/hydrogen plasma radical chemistry in plasma-enhanced atomic layer deposition of cobalt

“…If we consider the next ALD cycle, at the post-plasma stage, the metal surface is supposed to be terminated with NH x species before the next metal precursor half-reaction, analogous to the regeneration of hydroxyl groups in thermal- or plasma-assisted metal oxide ALD. 48,49 To explore how this surface can form, we have performed MD calculations at 600 K for the interaction of ˙NH radicals with the Co(001) surface, where Co atoms were deposited at a coverage of 3.03 Co nm −2 and the original surface NH species were eliminated as ammonia. Two coverages of ˙NH radicals are explored, i.e.…”