ALD/ALE 2026 Session ALE-MoA: Plasma and/Energy-Enhanced ALE I

Abstract

Ferroelectric tunnel junctions (FTJs) based on Hf₀.₅Zr₀.₅O₂ (HZO) are promising candidates for next-generation non-volatile memory and neuromorphic computing owing to their CMOS compatibility, low power consumption, and fast switching speed. Scaling HZO to ultrathin dimensions (<4 nm) is critical to enhance FTJ performance in neuromorphic computing while maintaining robust ferroelectricity and energy efficiency. In this work, we present a systematic investigation of plasma-enhanced atomic layer etching (PE-ALE) of HZO thin films grown by molecular beam epitaxy (MBE), or by plasma-enhance atomic layer deposition (PEALD). The PEALE process employs a Cl₂/BCl₃/Ar plasma chemistry at a substrate temperature of 50 °C, targeting controlled, layer-by-layer material removal. To support process development, density functional theory (DFT) and molecular dynamics (MD) simulations are used to establish a macroscopic fluid-dynamics-based framework for atomic layer etching, enabling identification of the energy window favorable for monolayer-scale removal of HZO. By tuning key process parameters such as RF power, plasma exposure time, and gas composition, an ALE window for HZO is identified. Furthermore, a comparative study between MBE- and ALD-grown HZO films highlights differences in etching behavior, including process window, surface morphology evolution, and implications for achieving ultrathin ferroelectric layers. These results provide important insights into thickness scaling strategies for ferroelectric HZO and offer a pathway to improve the FTJs device performance.

Reference

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2. Martemucci, M.; Rummens, F.; Malot, Y.; Hirtzlin, T.; Guille, O.; Martin, S.; Carabasse, C.; Vincent, A. F.; Saïghi, S.; Grenouillet, L.; Querlioz, D. A Ferroelectric–Memristor Memory for Both Training and Inference. Nat. Electron. 2025, 8, 921–933.
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4. Hoffmann, M.; Murdzek, J. A.; George, S. M.; Slesazeck, S.; Schroeder, U.; Mikolajick, T. Atomic Layer Etching of Ferroelectric Hafnium Zirconium Oxide Thin Films Enables Giant Tunneling Electroresistance. Appl. Phys. Lett.2022, 120, 122901.

Atomic layer etch (ALE) processes were developed for Nb and Ta on Si using a CF₄/H₂ plasma for the surface modification step and Ar⁺ irradiation for the removal step. These materials are widely used in superconducting quantum device fabrication. The processes were investigated with respect to RF bias, CF₄/H₂ dose time, and Ar⁺ etch time in order to identify the ALE window and saturation points. Ta and Nb yielded identical 0.23 +/- 0.01 nm/cycle etch rates for a soft-saturation process. The total cycle time was 16 sec with synergies of > 99% and 87% for Ta and Nb, respectively, and surface roughnesses were significantly reduced as compared to both the as-deposited films and an RIE process with similar chemistry. Over-saturated and under-saturated process were also investigated. A significant difference in EPC between different phases of Tantalum was also observed, suggesting crystal structure plays an important role in etch dynamics. Tantalum Nitride was also investigated due to its thin native oxide, which could help improve superconducting device performance.

To demonstrate the usability of these processes in industry, the effect of reduced purge times on ALE process performance was studied. For all processes the etch per cycle, selectivity, synergy, and surface roughness before and after were reported. A full process for Si was not studied but relevant parameters were reported. These processes are promising for real world manufacturing of devices that are sensitive to damage and require precise etch control.

Plasma-enhanced atomic layer etching (PEALE) enables anisotropic etching with atomic-scale precision and low roughness, yet its detailed mechanism remains unclear. Here, we present a molecular dynamics framework with machine-learning force field (MLFF) to study SiNₓ PEALE driven by sequential CF₄ adsorption and Ar⁺ bombardment. Multi-cycle simulations capture the evolution of the chemically modified layer and reveal descriptors that govern etch behavior.

At low Ar energy (30 eV), the modified layer gradually evolves toward a pseudo-steady state in composition and thickness. Fluorine accumulation saturates at F/Si ≈ 1.0, while carbon remains at consistently low levels due to preferential removal as volatile C–N-containing species (e.g., NCF). Silicon desorbs mainly as SiFₓ (SiF₄ dominant with substantial SiF₂), and nitrogen is removed primarily as N₂. The per-cycle N/Si removal ratio converges to ~1.3, indicating that near-stoichiometric SiNₓ etching is obtained.

At higher Ar energies (50–70 eV), however, deeper fluorine penetration combined with increased nitrogen sputtering produces an under-coordinated, Si-rich surface. CF₄-derived carbon readily binds to the surface, forming a rigid SiC network within the modified layer. This suppresses SiF_x formation and increases surface roughness, ultimately leading to etch stop.

These results reveal an Ar–energy–dependent transition in carbon fate—from volatile removal to SiC formation—that determines whether SiN_x remains in a steady-etch regime or reaches etch stop. Building on this mechanistic picture, we will also discuss practical strategies to suppress SiC buildup.

Atomic Layer Etching (ALE) has emerged as a critical technology for achieving atomic-scale precision in next-generation semiconductor fabrication. However, the high Global Warming Potential (GWP) of conventional perfluorocarbon gases widely used in the process, such as C₄F₈, necessitates the urgent development of eco-friendly alternative processes. In this study, we investigate the ALE characteristics of C₃F₆, a promising low-GWP candidate, in comparison with conventional C₄F₈ on silicon oxide (SiO₂) and silicon nitride (Si₃N₄) films to evaluate its feasibility for sustainable manufacturing, targeting high-selectivity applications such as the Self-Aligned Contact (SAC) process. The etching process was performed in an Inductively Coupled Plasma (ICP) reactor, where key parameters including bias power and step times were varied to verify the self-limiting behavior essential for ALE. We primarily focused on analyzing the process windows, etch rates, and etch selectivity derived from both C₄F₈ and C₃F₆ plasmas. Furthermore, to elucidate the reaction mechanisms and difference in dissociation pathways between the two gas systems, Residual Gas Analysis (RGA) was employed to analyze the gas-phase chemistry and monitor the evolution of neutral species and reaction by-products. In this presentation, we will discuss the potential of C₃F₆ to replace C₄F₈ by presenting the comparative analysis of process feasibility and investigating the correlation between plasma species and etch characteristics, thereby providing guidelines for eco-friendly semiconductor processing.