Thermal dry etching of metallic Iron thin films using Cl₂ and acetylacetone (acacH) was investigated. Iron metal is commonly used in magnetic random-access memory (MRAM) technology. The etching pathway was followed by detecting expected desorbing fragments during a heating ramp via temperature-programmed desorption (TPD) technique. The chemical properties of the etched surfaces were then analyzed with ex situ X-ray photoelectron spectroscopy (XPS). The morphology of the surface was studied with microscopic techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). To desorb volatile etch products, having an oxidized or halogenated Iron surfaces is vital, since clean surfaces resulted in decomposition of the diketone ligands. The pre-chlorination of the surfaces was shown to lead to the formation of transition metals-containing products at lower temperature compared to those on oxidized surfaces. However, halogenation makes the mechanism more complicated, and the etch product can contain both Fe²⁺ and Fe³⁺. These products may have a combination of ligands, and their general formula can be expressed as Fe(acac)_xCl_y. To corroborate surface reaction mechanisms, density functional theory calculations with periodic slab geometries were performed using the Vienna Ab initio Simulation Package (VASP). Based on the computational investigations, it was concluded that the removal of iron is determined by the nature of the surface-bound acac ligand, favoring the process involving a monodentate configuration. Computational work also illustrated the effect of surface smoothing observed experimentally.

Atomic layer etching (ALE) is a promising etching technology based on sequential, self-limiting surface reactions. Recently, the ALE of conducting transition metal nitrides such as TiN, VN, and TaN has been widely researched. The thermal ALE process of TiN with O₃ and HF gas was reported.[1] However, the speed of process (etched thickness per cycle (EPC) = 0.02 nm/cycle) was too slow to be commercialized. The rapid ALE (EPC = 0.6 nm/cycle) process was reported by using CHF₃ and O₂ down-stream plasma[2], but the process was environmentally problematic because of the high global warming potential of CHF₃ (GWP₁₀₀ = 12,400).

In this work, plasma-based TiN ALE was demonstrated byusing O₂ plasma and non-global warming gas of CF₃I (GWP₁₀₀ = 0.3) plasma. All the processes were carried out at temperature of 35 ℃. In the ALE process, a cycle consists of two steps; oxidation and subsequent removal steps. In the first oxidation step, the O₂ plasma oxidizes the surface of the TiN film.Since the surface is oxidized with O₂ plasma, there are much oxidation state of Ti of +4. This Ti state easily makes the film volatile with the form of TiF₄ for the next steps, instead of non-volatile material of TiF₃. In the second etching step, the oxidized surface of TiN film reacts with F radical in CF₃I plasma. Thenthe oxidized TiN surficial layer is completely removed through a volatile byproduct of TiF₄.

The thickness change was analyzed by spectroscopic ellipsometry and transmission electron microscopy. Also, X-ray photoelectron spectroscopy was used to analyze the TiN surface step by step, after the first oxidation step with O₂ plasma and the second removal step of oxidized surface with CF₃I plasma. Saturation characteristics of EPC were observed in ALE process. The EPC of 0.3 nm/cycle was saturated after sufficient supply of both reactive plasma. Furthermore, the EPC was modulated from 0.15 nm/cycle to 0.3 nm/cycle, which is acceptable for the precise layer control at etching process, by only changing the O₂ plasma source power.

References

[1] Y. Lee, et. al., Chem. Mater., 29, 8202 (2017).

[2] K. Shinoda, et. al., J. Phys. D: Appl. Phys., 52, 475106 (2019)

As the device dimensions continue to shrink, the back end of line (BEOL) interconnects line must also shrink. [1] Copper was used as a material for interconnect lines, but as the line width narrows, resistance increases rapidly due to surface scattering of electrons. [2] In recent studies, cobalt has been used as a substitute for W in local interconnects. As an alternative metal, cobalt has a greater bulk resistance than copper, but does not require a thick barrier/liner and has less resistance than copper as the line critical dimension decreases. [3] Cobalt deposition based on atomic layer deposition requires an ALE process because the surface roughness increases as the thickness increases. [4] In this work, cyclic thermal atomic layer etching process was performed for cobalt in an inductively coupled plasma (ICP) reactor. The process consists of two steps: surface modification with chlorine plasma and removal with lamp heating. In the first step, the surface of cobalt is modified with a layer of CoCl₂ using chlorine plasma. In the second step, the modified surface is removed by thermal desorption with hexafluoroacetylacetone. Etch rate were compared at various conditions of flow rate, plasma power and plasma time. The thickness of the cobalt film was confirmed by cross-sectional SEM imaging, and the roughness of the surface was observed by AFM. The etch rate could be controlled below 1 nm/cycle. Surface roughness was compared according to chlorine plasma power and desorption temperature.

References

[1] M. He, X. Zhang, T. Nogami, X. Lin, J. Kelly, H. Kim, T. Spooner, D. Edelstein, and L. Zhao, J. Electrochem. Soc. 160, D3040 (2013)

[2] D. Bobb-Semple, K. L. Nardi, N. Draeger, D. M. Hausmann, and S. F. Bent, Chem. Mater 31, 1635 (2019)

[3] N. Bekiaris, Z. Wu, H. Ren, M. Naik, J. H. Park, M. Lee, T. H. Ha, W. Hou, J. R. Bakke, M. Gage, Y. Wang, and J. Tang, In 2017 IEEE International Interconnect Technology Conference (IITC), IEEE, 1 (2017)

[4] M. Konh, C. He, X. Lin, X. Guo, V. Pallem, R. Opila, A. Teplyakov, Z. Wang, and B. Yuan, J. Vac. Sci. Technol. 37, 021004 (2019)

Keywords: Thermal atomic layer etching, Metal etching, Surface roughness