| AVS 66th International Symposium & Exhibition | |
| Atomic Scale Processing Focus Topic | Thursday Sessions |
| Session AP+PS+TF-ThM |
| Session: | Thermal Atomic Layer Etching |
| Presenter: | Abdulrahman Basher, Osaka University, Japan |
| Authors: | A.H. Basher, Osaka University, Japan I. Hamada, Osaka University, Japan M. Krstic, Karlsruhe Institute of Technology (KIT), Germany M. Isobe, Osaka University, Japan T. Ito, Osaka University, Japan K. Fink, Karlsruhe Institute of Technology (KIT), Germany K. Karahashi, Osaka University, Japan Y. Morikawa, Osaka University, Japan W. Wenzel, Karlsruhe Institute of Technology (KIT), Germany S. Hamaguchi, Osaka University, Japan |
| Correspondent: | Click to Email |
Thermal atomic layer etching (ALE) may be used for precise and damageless etching of difficult-to-etch materials such as Ni, Co, NiFe, MgO, and CoFeB, which can be used as materials for magnetic tunnel junction (MTJ) stacks of magnetic random access memory (MRAM) devices. The goal of this study is to understand the mechanisms of surface chemical reactions during thermal ALE of metal in general with oxidation and exposure to organic molecules. As a model case, we consider a two-step thermal ALE process of nickel (Ni) with an oxidation step and a gas exposure step at an elevated substrate temperature [1]. In the latter step, hexafluoroacetylacetone (hfacH) CF3C(OH)=CHC(O)CF3 is used as a reactive gas. In the oxidation step, a thin layer of NiO is formed on the Ni film surface and, in the gas exposure step, only (part of) this NiO layer is removed and thus self-limiting etching of Ni is achieved. Our main question is why NiO is etched but Ni is not etched by hfacH. This mechanism is studied with first-principle simulation of interaction of hfacH with Ni and NiO surfaces.
First, we examined interaction of hfacH with a metallic Ni surface, using a simulation code STATE [2,3], which is based on density functional theory (DFT) with pseudo-potentials and a plane wave basis set. Computationally, a metal surface is better represented by a plane wave basis set in general. It has been found in our simulation that, as an hfacH molecule approaches a metallic Ni surface with thermal velocity, it is more likely to be decompose and fragmented, rather than forming a hexafluoroacetylacetonate anion (hfac-) by deprotonation. This is consistent with earlier experimental observations [1,4]. The simulation clearly shows an energy threshold for deprotonation of hfacH with a metallic Ni surface.
Second, we examined interaction of enol hfacH with a NiO surface using a simulation code Turbomole [5], which is based on DFT but with Gaussian type orbitals. To better represent a NiO surface, we used the embedded cluster method (ECM) with Turbomole. It has been found that, as an hfacH molecule approaches a NiO surface, it is likely to deprotonate by transferring its hydrogen ion (H+) to an O atom of the NiO surface and the resulting hfac- tends to bond with a Ni atom of the surface because of the highly ionic nature of NiO, where Ni and O atoms are positively and negatively charged, respectively. In this way, volatile Ni(hfac)2 and H2O can be formed when hfacH molecules interact with a NiO surface. Reaction energies of such interactions have been evaluated from the simulations.
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[1] T. Ito, et al., AVS 65th International Symposium & Exhibition (2018).
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[3] I. Hamada, Physical Rev. B 89, 121103 (2014).
[4] H. L. Nigg and R. I. Masel, J. Vac. Sci. Techol. A 17,3477 (1999).
[5] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 162, 165 (1989).