Thermal atomic layer etching can be utilized for the surface preparation of aluminum in order to improve its optical performance at ultraviolet wavelengths. In this work we report on the use of trimethyaluminum and anhydrous hydrogen flyoride to remove the native oxide of aluminum prior to encapsulation with fluoride dielectric materials. This ALE/ALD process is used for the fabrication of reflective coatings and bandpass filters operating at wavelengths shorter than 200 nm. The etch rate of aluminum oxide is observed to be dependent on chamber conditioning with a significant enhancement in etch rate observed when the cyclic etching is performed in the presence of alkali halide materials. This enhancement can reduce the temperature threshold where etching dominates the reaction cycle over deposition.

The reduction of the overall processing temperature can enhance the compatibility of the full coating process with some temperature-sensitive substrates, and limit the amount of etch damage experienced by aluminum surfaces. Etching into the aluminum surface is generally observed to result in non-conformal etching which greatly increases the surface roughness of films and degrades the optical performance of resulting structures. Reducing the etch temperature can mitigate this effect by increasing the selectivity of the native oxide removal over the underlying metal. Optimization of these processes may provide insight into achieving conformal ALE of aluminum surfaces. The extension of these atomic layer processing methods towards the fabrication of meter-class mirror coatings is also discussed in the context of future large UV space observatories for NASA astrophysics applications.

Etching metal oxides with halogen-free methods is important during processing to avoid corrosion.Acetylacetone (Hacac) is an organic hydrocarbon.Hacac can supply acac ligands that can form volatile metal complexes with most transition metals.Consequently, Hacac can spontaneously etch metal oxides to form M(acac)_x and H₂O.One difficulty is that Hacac can also decompose on the metal oxide surface and block the spontaneous etching.However, this surface poisoning also leads to a self-limiting reaction. The O₃ exposure can then remove the carbonaceous decomposition species and produce a pathway for atomic layer etching.

Thermal etching of first row metal oxides was demonstrated using Hacac and O₃ at pressures of 2.5 Torr at 250 °C.A quadrupole mass spectrometer (QMS) reactor with molecular beam expansion and line-of-sight to the ionizer was employed to detect etch species with high sensitivity.Metal oxide nanopowders were used to maximize the surface area and signal intensity of the etch products. The reactant sequence used five sequential Hacac exposures, one O₃ exposure, and one final Hacac exposure to check for etch product enhancement after O₃ exposure.Etching was monitored by the production of M(acac)_x etch products.

M(acac)_x etch products were observed for Sc₂O₃, V₂O₅ and VO₂, Cr₂O₃, Mn₂O₃ and MnO, Fe₂O₃ and Fe₃O₄, Co₃O₄ and CoO, CuO and Cu₂O, and ZnO. No etching was observed for TiO₂, MnO₂, and NiO. The metal oxides that etched either displayed (1) spontaneous etching by Hacac with no self-limiting behavior or (2) etching that limited itself versus Hacac exposure.The metal oxides that were spontaneously etched by Hacac were Mn₂O₃ and MnO, Co₃O₄ and CoO, and ZnO.The metal oxides that displayed self-limiting behavior were Sc₂O₃, V₂O₅ and VO₂, Cr₂O₃, Fe₂O₃ and Fe₃O₄, and CuO and Cu₂O.ALE processes for these metal oxides that displayed self-limiting reactions are possible using Hacac and O₃.

A comparison between the M(acac)_x etch products and the metal oxide also provided information about oxidation state changes during etching.The x in M(acac)_x is both the number of acac ligands and the oxidation state of the M metal center.Sc₂O₃, Cr₂O₃, MnO, Fe₂O₃, CoO, CuO, and ZnO all formed M(acac)_x etch products with the same oxidation state as the metal oxide.In contrast, the other metal oxides all displayed evidence for reduction during etching.This reduction may occur by oxygen loss during the combustion of Hacac.

Sub-10-nm technology nodes must overcome the limits of photolithography. This requires selectivity between various Si-based materials for thermal atomic layer etching (ALE) and spontaneous etching. This work examined selectivity between silicon dioxide (SiO₂) and silicon nitride (SiN_x) for thermal ALE using trimethylaluminum (TMA) and hydrogen fluoride (HF), as well as for spontaneous etching using HF alone, at 275℃. Distinct etch rates between SiO₂ and SiN_x achieved inherent selectivity.

Experiments were conducted in a hot-wall, viscous-flow vacuum reactor with good control over the pressure during static reactant dosing to ensure reproducibility. In situ spectroscopic ellipsometry (iSE) was utilized to study etch-per-cycle (EPC), synergy, and selectivity characteristics. Sodium bifluoride (NaHF₂) was tested as an alternative HF source. NaHF₂ is a solid salt with negligible HF vapor pressure at room temperature, making NaHF₂ safer to handle than HF-pyridine. NaHF₂ delivered HF pressures up to 15 Torr when heated to 150℃ without releasing sodium. During thermal ALE of alumina (Al₂O₃), NaHF₂ exhibited diffusion-limited fluorination and EPC characteristics comparable with HF-pyridine.

For thermal ALE of SiO₂ alternating TMA and HF, the EPC and synergy were –0.2 Å and 88%, indicating minor spontaneous etching by HF alone. This moderate synergy for SiO₂ thermal ALE improved to 95% by ensuring water-free conditions during fluorination. On the other hand, the EPC for SiN_x thermal ALE was –1.1 Å. The EPC for SiN_x was expected to be much lower than for SiO₂ because no oxygen reactant was employed to oxidize SiN_x. However, iSE experiments revealed that repeated exposures of HF alone spontaneously etched SiN_x. Anhydrous HF vapor might form F⁻ species at the surface that have been attributed to dominate SiN_x etching. Spontaneous etching using static exposures of 45 s at 3 Torr HF alone obtained a high selectivity of ~50:1 for SiN_x removal over SiO₂ retention.

For thermal ALE alternating TMA and HF in co-dose with ammonia (NH₃), the selectivity inverted to ~9,000:1 for SiO₂ over SiN_x. HF+NH₃ co-dosing led to rapid spontaneous etching of SiO₂. NH₃, similar to water, might facilitate the dissociation of HF into H⁺ and F⁻, where the increased F⁻ concentration immediately produces HF₂⁻ species. HF₂⁻ species have been attributed to dominate SiO₂ etching.

In conclusion, this work demonstrated conditions for inherently selective gas-phase etching of either SiO₂ or SiN_x.

Thermal atomic layer etching (ALE) can be achieved with sequential, self-limiting surface reactions. One mechanism for thermal ALE is based on fluorination and ligand-exchange reactions. For metal oxide ALE, fluorination converts the metal oxide to a metal fluoride. The ligand-exchange reaction then removes the metal fluoride by forming volatile products. Previous studies have successfully applied this thermal ALE strategy for Al₂O₃, HfO₂, and ZrO₂ ALE. However, no previous investigations have explored the thermal ALE of SnO₂ films.

This study demonstrated the thermal ALE of SnO₂ thin films using sequential, self-limiting thermal reactions with hydrogen fluoride (HF) and trimethylaluminum (Al(CH₃)₃, TMA) as the reactants. The initial SnO₂ films were grown by atomic layer deposition (ALD) using tetrakis(dimethylamino) tin and H₂O₂. The thermal SnO₂ ALE process was then studied using various techniques including quartz crystal microbalance (QCM), spectroscopic ellipsometry (SE), and quadrupole mass spectrometry (QMS).

In situ QCM experiments monitored SnO₂ ALE at temperatures from 250 to 300 ℃. The SnO₂ etching was linear versus the number of HF and TMA reaction cycles. The QCM studies also showed that the sequential HF and TMA reactions were self-limiting versus reactant exposures. The SnO₂ etching rates increased at higher temperatures. The QCM analysis measured mass change per cycle (MCPC) values that varied from −44.32 ng/(cm² cycle) at 250 °C to −123.5 ng/(cm² cycle) at 300 °C. These MCPCs correspond to SnO₂ etch rates from 0.64 Å/cycle at 250 °C to 1.78 Å/cycle at 300 °C.

SE measurements confirmed the linear removal of SnO₂ and the etching rates. QMS analysis also revealed the volatile etching products during the sequential HF and TMA exposures on SnO₂ at 300 ℃. These QMS investigations observed Sn(CH₃)₃⁺, indicating Sn(CH₃)₄ as the etch product during TMA exposures. Al_xF_y(CH₃)_z dimer and trimer species were identified as the ligand-exchange products. QMS analysis during multiple sequential TMA doses before HF/TMA cycling also revealed that fluorination was necessary for Sn(CH₃)₄ etch product evolution. This observation indicated that TMA does not convert SnO₂ to Al₂O₃. The results indicate that thermal SnO₂ ALE using sequential HF and TMA exposures occurs by fluorination and ligand-exchange reactions.

The etching of silicon nitride (SiN_x) was explored using vapor-phase HF exposures at various temperatures. The investigations were performed using in situ quadrupole mass spectrometry (QMS) and ex situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to detect the volatile and non-volatile etch products, respectively. These QMS and ATR-FTIR studies provide valuable understanding of the SiN_x atomic layer etching (ALE) process employing hydrofluorocarbon plasma to form (NH₄)₂SiF₆ salt at low temperatures and then thermal annealing at higher temperatures to desorb the salt [N. Miyoshi et al., Jpn. J. Appl. Phys. 56, 06HB01 (2017)].

At low temperatures, T≤60^oC, QMS detected the evolution of SiF₄ from HF exposure at 0.5 Torr on SiN_x. SiF₄ formed concurrently with the formation of a (NH₄)₂SiF₆ salt layer on the SiN_x surface according to: Si₃N₄ + 16HF(g) —> 2(NH₄)₂SiF₆ + SiF₄(g). To verify the presence of the salt, the temperature could be ramped up to 200^oC in the absence of HF exposure. During this temperature ramp, QMS detected SiF₄ at higher temperatures T≥80^oC corresponding to the thermal decomposition of the (NH₄)₂SiF₆ salt according to: (NH₄)₂SiF₆ —> 2NH₃(g) + 2HF(g) + SiF₄(g).

When the HF exposure was performed at higher temperatures T≥120^oC, SiF₄ was again observed as an etch product. However, no secondary rise of SiF₄ was detected by QMS during the temperature ramp to 200^oC in the absence of HF exposure. This behavior indicated that the (NH₄)₂SiF₆ salt did not form on the surface at temperatures T≥120^oC. The spontaneous etching of SiN_x with no salt on the SiN_x surface is possible at these higher temperatures. ATR-FTIR studies corroborated the salt formation at lower temperatures and the salt decomposition at higher temperatures.

Thermal atomic layer etching (ALE) of Hf_0.5Zr_0.5O₂ (HZO) was previously demonstrated using hydrogen fluoride (HF) and dimethylaluminum chloride (DMAC) [1]. This current work focused on crystallographic transformations of HZO during ALE. Grazing incidence x-ray diffraction (GIXRD) analysis of initial 10 nm thick HZO film on 20 nm thick TiN on Si revealed orthorhombic (o-phase), tetragonal (t-phase), and monoclinic phases (m-phase). Ex situ spectroscopic ellipsometry and X-ray reflectivity (XRR) measurements showed that sequential exposures of HF and DMAC at 250 ^oC resulted in a linear decrease in film thickness with an HZO etch rate of ~0.45 Å/cycle.

GIXRD studies observed that the peaks associated with the o- and t-phases decreased faster in intensity than the m-phase peaks. As the number of ALE cycles increased, only the m-phase remained before the majority of the HZO film was removed by etching. Interestingly, as o- and t-phases were removed, the grain size of the m-phase crystallites increased in size according to the Scherrer equation. XRR investigations also monitored a decrease in the film density with ALE. In addition, atomic force microscopy (AFM) measurements observed that the density decrease was accompanied by an increase in film roughness.

Powder diffraction (PXRD) studies were also conducted to investigate the phase transformation of crystalline ZrO₂ powder at 250 ^oC. ZrO₂ powder was used as a model system since the chemical properties of HfO₂ and ZrO₂ are very similar. PXRD analysis of as-received ZrO₂ powder showed crystallographic planes of mostly m-phase with some cubic (c-phase) and t-phase. As expected, the etching of ZrO₂ powder resulted in a mass loss. PXRD also observed the loss of c- and t-phases and an increase in grain size of m-phase crystallites. The results for the HZO films and ZrO₂ powder are similar. There are crystal phase transformations that occur with loss of o- and t-phases and growth of m-phase during thermal ALE.

[1] J. A. Murdzek and S. M. George, J. Vac. Sci. Technol. A 38, 022608 (2020)