ALD/ALE 2026 Session ALE2-TuM: Thermal and Gas-Phase ALE

As semiconductor devices continue to scale down, precise dimensional control of individual device components has become increasingly critical. In this context, atomic-scale processing techniques have emerged as key enablers for next-generation semiconductor manufacturing. Among them, thermal atomic layer etching (ALE) has attracted attention as a surface-reaction-driven process capable of removing materials with atomic-layer precision through sequential surface modification and material removal steps. Compared to conventional plasma-based etching, thermal ALE minimizes sputtering-induced damage and effectively suppresses surface roughening. High-k dielectric materials are widely employed as replacements for Si-based insulators in advanced semiconductor devices, enabling sufficient capacitance retention even at deeply scaled technology nodes. However, the dielectric properties of high-k oxides are strongly correlated with their crystalline phases, and achieving ultrathin crystalline films with well-controlled thickness remains challenging. An etch-back approach, in which sufficiently thick crystalline films are first deposited and subsequently thinned while preserving crystallinity, is therefore required. Thermal ALE is well suited for this purpose due to its inherently low-damage characteristics. Most reported thermal ALE processes for high-k materials normally use fluorine-based sources. From this point of view, we propose a novel thermal ALE process for zirconium dioxide (ZrO₂) that does not use the fluorination agent. ZrO₂ films were etched using alternating exposures of H₂O (water) and SOCl₂ (thionyl chloride), achieving a self-limiting etch rate of approximately 0.07 Å per cycle. chlorine residues were not detected within the ZrO₂ layer after etching. Surface hydroxylation induced by H₂O exposure forms a reactive termination that facilitates subsequent reactions with SOCl₂, enabling continuous and controlled etching. The proposed reaction mechanism is further supported by density functional theory (DFT) calculations. This fluorine-free thermal ALE approach provides precise, damage-free thickness control while preserving the crystalline properties of high-k dielectrics, offering a promising alternative to conventional fluorine-based etching processes for advanced semiconductor manufacturing.

N-heterocyclic carbenes (NHCs) are strong electron pair donors and can bind to metal centers by ligand-addition.In this study, etching of Al₂O₃ was demonstrated using halogenation with hydrofluoric acid (HF) followed by ligand-addition using NHCs.The NHC sources were 1,3-dimethylimidazolium-2-carboxylate (IMeCO₂) or 1,3-di-tert-butylimidazol-2-ylidene (ItBu) (Figure 1).The thermal ALE of Au and Co was also demonstrated using hydrochloric acid (HCl) together with IMeCO₂ or ItBu.The studies were conducted using in situ quartz crystal microbalance (QCM) measurements.

Al₂O₃ thermal ALE was achieved using sequential HF and IMeCO₂ or ItBu exposures at temperatures from 230 to 290 °C.QCM profiles showed a self-limiting mass gain during fluorination and a self-limiting mass loss during the NHC ligand-addition step (Figure 2a).The mass change per cycle yielded an etch rate of 3.3 Å per cycle at 290 °C. The spontaneous etching of Al₂O₃ films was also observed during exposure to 2,2-difluoro-1,3-dimethylimidazolidine (DFI) (Figure 1).DFI provides both fluorination and ligand-addition by the NHC remaining after fluorination.DFI leads to continuous Al₂O₃ spontaneous etching at 200 to 270 °C.

The NHCs were also effective for Au and Co thermal ALE.Sequential exposures of HCl and IMeCO₂ or ItBu led to a linear mass decrease with the number of ALE cycles. The maximum Au mass change yielded an etch rate of 0.83 Å per cycle at 250 °C using HCl and IMeCO₂ (Figure 2b). For Co, the maximum etch rate was 3.0 Å per cycle at 290 °C using HCl and ItBu. These ALE processes are believed to proceed through the formation of volatile metal chloride–carbene adducts.The cycle times during Au and Co ALE were much shorter than earlier cycle times for Au and Co ALE measured using phosphines for ligand-addition.