| AVS 64th International Symposium & Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS+TF-ThA |
| Session: | Plasma Enhanced ALD |
| Presenter: | Harm Knoops, Oxford Instruments Plasma Technology, UK |
| Authors: | H.C.M. Knoops, Oxford Instruments Plasma Technology, UK M.F.J. Vos, Eindhoven University of Technology, The Netherlands W.M.M. Kessels, Eindhoven University of Technology, The Netherlands A.J.M. Mackus, Eindhoven University of Technology, The Netherlands |
| Correspondent: | Click to Email |
In this work we used TMA (AlMe3) and SF6 plasma for atomic layer deposition (ALD) of aluminum fluoride (AlF3) films. SF6 plasma is a novel co-reactant for ALD and we employed quadruple mass spectrometry (QMS) and optical emission spectroscopy (OES) to study the film growth. AlF3 as well as other metal fluorides such as MgF2 and CaF2 generally have a wide bandgap (>10eV) and low refractive index (1.3-1.6). Due to these interesting properties they find use in many applications, including passivation layers in Li-ion batteries, electron transport layers in photovoltaics and protective coatings for optical devices. Previously, ALD of fluorides has been demonstrated using TiF4 and TaF5 as the fluorine source for the deposition of MgF2, CaF2 and LaF3 and more recently using HF for AlF3, ZrF4, MnF2, HfF2, MgF2 and ZnF2.1,2 The novel approach of using SF6 plasma as a fluorine source is a promising alternative to HF, because of the ease of handling that SF6 offers. Furthermore, SF6 plasma provides increased reactivity at lower temperatures and allows for reduced purge times, similar to the benefits of using an O2 plasma instead of H2O for metal oxide ALD.
AlF3 films were prepared on Si samples over a temperature window of 50°C to 300°C. Since SF6 plasma etches Si and SiO2, a thin Al2O3 layer was deposited prior to AlF3 growth. Using in situ spectroscopic ellipsometry (SE) the growth per cycle (GPC) was determined to decrease from 1.5Å at 50°C to 0.5Å at 300°C. Interestingly, no significant impurity levels of S, C and O were detected in the bulk of the AlF3 films using X-ray photo-electron spectroscopy (XPS), even for low deposition temperatures. Furthermore, XPS measurements showed a F/Al ratio of 3.0±0.2. The low impurity content and the stoichiometric F/Al ratio are in line with a refractive index of 1.35 at 633nm as determined by SE.
The reaction mechanism of the ALD process was addressed based on a combination of OES and QMS. These measurements suggest that CH4 is released during the TMA dosing, and that CH4, C2H2, HF, and CHxFy–species are formed during the plasma exposure. Furthermore, the reaction products during the plasma exposure show different trends in their release (e.g., mostly directly after striking the plasma or peaking after a few seconds of plasma exposure), which will be used to suggest a possible reaction mechanism. In addition, consumption of F can be observed similar to what is found in etching using SF6 plasma. Overall this work shows that SF6 plasma is a promising co-reactant which can inspire the ALD of a wide range of metal fluorides.
1Pilvi et al., Chem. Mater. 20 (2008)
2Lee et al., Chem. Mater. 28 (2016)