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1. Thermodynamic Considerations in the Design of Electrochemical Atomic Layer Etching of Copper

   https://doi.org/10.1149/1945-7111/ac030e  

   Gong, Yukun; Akolkar, Rohan (June 2021, Journal of The Electrochemical Society)

   Electrochemical atomic layer etching (e-ALE) is a unique approach for etching metals one atomic layer at a time. If practiced under optimal conditions, e-ALE ensures minimal evolution of surface roughness due to the atomic layer-by-layer etching characteristics. During e-ALE of copper (Cu), the crucial first step is the formation of a cuprous sulfide (Cu₂S) monolayer via the surface-limited sulfidization reaction. In this paper, we investigate the surface coverage of this sulfide layer as a function of the sulfidization potential, and show that the equilibrium coverage attained can be modeled using the Frumkin adsorption isotherm. At a potential of –0.74 V vs SHE, sulfidization provides near-complete monolayer coverage of Cu by Cu₂S, which then facilitates e-ALE in a layer-by-layer etching mode thereby maintaining a smooth post-etch surface. Operation at potentials negative with respect to –0.74 V provides sub-monolayer coverage, which manifests in roughness amplification during etching. This work provides a thermodynamics-guided foundation for the selection of operating conditions during Cu e-ALE. 

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2. Isotropic plasma-thermal atomic layer etching of superconducting titanium nitride films using sequential exposures of molecular oxygen and SF6/H2 plasma

   https://doi.org/10.1116/6.0002965  

   Hossain, Azmain A.; Wang, Haozhe; Catherall, David S.; Leung, Martin; Knoops, Harm C.; Renzas, James R.; Minnich, Austin J. (September 2023, Journal of Vacuum Science & Technology A)

   Microwave loss in superconducting TiN films is attributed to two-level systems in various interfaces arising in part from oxidation and microfabrication-induced damage. Atomic layer etching (ALE) is an emerging subtractive fabrication method which is capable of etching with angstrom-scale etch depth control and potentially less damage. However, while ALE processes for TiN have been reported, they either employ HF vapor, incurring practical complications, or the etch rate lacks the desired control. Furthermore, the superconducting characteristics of the etched films have not been characterized. Here, we report an isotropic plasma-thermal TiN ALE process consisting of sequential exposures to molecular oxygen and an SF6/H2 plasma. For certain ratios of SF6:H2 flow rates, we observe selective etching of TiO2 over TiN, enabling self-limiting etching within a cycle. Etch rates were measured to vary from 1.1 Å/cycle at 150°C to 3.2 Å/cycle at 350°C using ex situ ellipsometry. We demonstrate that the superconducting critical temperature of the etched film does not decrease beyond that expected from the decrease in film thickness, highlighting the low-damage nature of the process. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits. 

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3. A cellular automata simulation of atomic layer etching

   https://doi.org/10.1117/12.2297435  

   Bonnecaze, Roger T.; Chopra, Meghali J.; Strotmann, Jan (March 2018, Proc. SPIE 10589, Advanced Etch Technology for Nanopatterning VII)

   A two-dimensional, cellular automata model for atomic layer etching (ALE) is presented and used to predict the etch rate and the evolution of the roughness of various surfaces as a function of the efficiencies or probabilities of the adsorption and removal steps in the ALE process. The atoms of the material to be etched are initially placed in a two-dimensional array several layers thick. The etch follows the two step process of ALE. First, the initial reaction step (e.g., Cl reacting with Si) is assumed to occur at 100% efficiency activating the exposed, surface atoms; that is, all exposed atoms react with the etching gas. The second reaction step (e.g., Ar ion bombardment or sputtering) occurs with efficiencies that are assumed to vary depending on the exposure of the surface atoms relative to their neighbors and on the strength of bombardment. For sufficiently high bombardment or sputtering, atoms below the activated surface atoms can also be removed, which gives etch rates greater than one layer per ALE cycle. The bounds on the efficiencies of the second removal step are extracted from experimental measurements and fully detailed molecular dynamics simulations from the literature. A trade-off is observed between etch rate and surface roughness as the Ar ion bombardment is increased. 

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4. Intrinsic and atomic layer etching enhanced area-selective atomic layer deposition of molybdenum disulfide thin films

   https://doi.org/10.1116/6.0002811  

   Soares, Jake; Jen, Wesley; Hues, John D.; Lysne, Drew; Wensel, Jesse; Hues, Steven M.; Graugnard, Elton (September 2023, Journal of Vacuum Science & Technology A)

   For continual scaling in microelectronics, new processes for precise high volume fabrication are required. Area-selective atomic layer deposition (ASALD) can provide an avenue for self-aligned material patterning and offers an approach to correct edge placement errors commonly found in top-down patterning processes. Two-dimensional transition metal dichalcogenides also offer great potential in scaled microelectronic devices due to their high mobilities and few-atom thickness. In this work, we report ASALD of MoS2 thin films by deposition with MoF6 and H2S precursor reactants. The inherent selectivity of the MoS2 atomic layer deposition (ALD) process is demonstrated by growth on common dielectric materials in contrast to thermal oxide/ nitride substrates. The selective deposition produced few layer MoS2 films on patterned growth regions as measured by Raman spectroscopy and time-of-flight secondary ion mass spectrometry. We additionally demonstrate that the selectivity can be enhanced by implementing atomic layer etching (ALE) steps at regular intervals during MoS2 growth. This area-selective ALD process provides an approach for integrating 2D films into next-generation devices by leveraging the inherent differences in surface chemistries and providing insight into the effectiveness of a supercycle ALD and ALE process. 

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5. Isotropic plasma-thermal atomic layer etching of aluminum nitride using SF6 plasma and Al(CH3)3

   https://doi.org/10.1116/6.0002476  

   Wang, Haozhe; Hossain, Azmain; Catherall, David; Minnich, Austin J. (May 2023, Journal of Vacuum Science & Technology A)

   We report the isotropic plasma atomic layer etching (ALE) of aluminum nitride using sequential exposures of SF6 plasma and trimethylaluminum [Al(CH3)3]. ALE was observed at temperatures greater than 200 °C, with a maximum etch rate of 1.9 Å/cycle observed at 300 °C as measured using ex situ ellipsometry. After ALE, the etched surface was found to contain a lower concentration of oxygen compared to the original surface and exhibited a ∼35% decrease in surface roughness. These findings have relevance for applications of AlN in nonlinear photonics and wide bandgap semiconductor devices. 

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