ALD2020 Session ALE2-MoA: ALE of Metal Oxides

Thermal ALE is a novel approach for the isotropic etching of materials with atomic-level precision [1]. This technology will be important for the fabrication of advanced semiconductor devices, such as GAAFET and 3D NAND-Flash. The goal of this work was to understand the chemical mechanisms of thermal ALE using ab initio calculations. We chose Al₂O₃ ALE using HF and Al(CH₃)₃ as the case study. In the first half-cycle of Al₂O₃ ALE, it has been known that the fluorine precursor HF reacts with the Al₂O₃ surface to form an intermediate AlF₃ layer and H₂O molecules [1, 2]. The present work specifically focused on the second half-cycle reaction, where the metal precursor Al(CH₃)₃ etches the intermediate AlF₃ layer releasing volatile by-products.

Ab initio thermodynamic calculations were performed to predict the preferred etch products. Several possible Al-monomers and -dimers have been considered as the gaseous products. The calculation results show that the Al-dimers are more stable than their corresponding monomers. The most favorable etch product is the Al₂F₂(CH₃)₄ dimer, where two F atoms bridge the adjacent Al centers.

The surface reactions between the AlF₃ surface and Al(CH₃)₃ were investigated using dispersion-corrected density functional theory (DFT-D). The etch of the pristine surface by Al(CH₃)₃ is unfavorable due to a large reaction barrier (> 3 eV). Stability of the AlF₃ surface can be reduced by surface modification where some surface F atoms are replaced by the CH₃ groups of Al(CH₃)₃. This step is accomplished by a ligand-exchange reaction, as shown in Fig.1. The transition state calculated by DFT agrees well with that proposed by George et al [1].

The subsequent etch of the modified surface proceeds via two possible pathways: (1) direct etching by Al(CH₃)₃ with formation of Al-dimers; and (2) desorption of etch products caused by surface reconstruction. In both cases, the etch kinetics is mainly determined by the number of surface CH₃ groups. The surface CH₃ has two positive roles in etch reactions. First, the strength of the Al−CH₃ bond is much weaker than that of the Al−F bond. Second, the F atom in AlF₃ has a coordination number of two, whereas the CH₃ group typically favors a coordination number of one. Therefore, some chemical bonds of AlF₃ have been broken after surface modification.

Lastly, based on a detailed understanding of the reaction mechanisms, we have discussed the strategies for the design and screening of reactive precursors for thermal ALE.

[1] George, S.M. and Lee, Y. ACS Nano, 2016, 10(5), 4889-4894. [2] Natarajan, S.K. and Elliott, S.D. Chem. Mater., 2018, 30(17), 5912-5922.

Identification of gas products from thermal atomic layer etching (ALE) is critical to understand the underlying surface chemistry. Previous quadrupole mass spectrometry (QMS) experiments identified the etch products from Al₂O₃ ALE using HF and trimethylaluminum (TMA) as the reactants [1]. These experiments sampled gas products with no line-of-sight between the substrate surface and the QMS ionizer. These QMS experiments observed dimers of the etch product, dimethylaluminum fluoride (DMAF), with itself (DMAF + DMAF) or with TMA (DMAF + TMA) [1].

For higher sensitivity and no wall-effects, a new apparatus (Figure 1) was built to provide line-of-sight between the substrate and the QMS ionizer. In this apparatus, a small reactor containing a powder sample is positioned in a larger vacuum chamber. The etch products emerge from an aperture in the reactor and are supersonically expanded into a lower vacuum region. The etch products then pass through a skimmer into the even lower pressure region containing the QMS ionizer. With this arrangement, the reactor can be maintained at ~1 Torr. The etch products can also have line-of-sight to the QMS ionizer with the ionization region at ~10^-8 Torr.

Using this new apparatus, the ligand-exchange reaction of DMAC with AlF₃ was examined to study thermal Al₂O₃ ALE using HF and DMAC as the reactants [2]. Various monomer and dimer species were observed versus temperature (Figure 2). DMAC dimers and DMAC dimers that have undergone Cl/F exchange with AlF₃ are detected at lower temperatures. The main etch product, AlCl₂(CH₃), is observed with increasing intensity at higher temperatures >230 °C. The ligand-exchange reaction of SiCl₄ with ZrF₄ was also examined to study thermal ZrO₂ ALE using HF and SiCl₄ as the reactants [3]. SiF_xCl_y species are observed from the halogen ligand-exchange reaction at >240 °C (Figure 3). ZrCl₄ is observed as the main Zr-containing etch product at >240 °C. ZrFCl₃ is detected as an additional product at higher temperatures >320 °C.

[1] J.W. Clancey, A.S. Cavanagh, J.E.T. Smith, S. Sharma and S.M. George, “Volatile Etch Species Produced During Thermal Al₂O₃ Atomic Layer Etching”, J. Phys. Chem. C 124, 287 (2020).

[2] Y. Lee and S.M. George, “Thermal Atomic Layer Etching of Al₂O₃, HfO₂, and ZrO₂ Using Sequential Hydrogen Fluoride and Dimethylaluminum Chloride Exposures”, J. Phys. Chem. C123, 18455 (2019).

[3] Y. Lee, C. Huffman and S.M. George, “Selectivity in Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions”, Chem. Mater.28, 7657 (2016).

Thermal atomic layer etching (ALE) can be performed using sequential fluorination and ligand-exchange reactions. For example, thermal Al₂O₃ ALE can be achieved using HF for fluorination and Al(CH₃)₃ (trimethylaluminum (TMA)) as the metal precursor for ligand-exchange [1]. Sequential exposures of HF and TMA lead to Al₂O₃ etch rates of 0.47 Å/cycle at 285°C. The ability to block thermal ALE selectively will be useful for advanced nanofabrication. This study demonstrates how thermal Al₂O₃ ALE can be blocked with removable ZrF₄ etch stop layers. In situ quartz crystal microbalance (QCM) measurements were utilized to monitor the etching and the effect of the etch stop layers.

The ZrF₄ etch stop layers could be deposited on Al₂O₃ using tetrakis(ethylmethylamido) zirconium and H₂O at 285°C. These reactants deposit ZrO₂ layers that are then converted to ZrF₄ during the subsequent HF exposure. Because Al(CH₃)₃ does not undergo ligand-exchange with ZrF₄ [2], the ZrF₄ layer serves as an etch stop layer. QCM measurements revealed that an initial ZrO₂ thickness of just one monolayer prior to fluorination was able to completely inhibit thermal Al₂O₃ ALE. Prior to reaching a ZrO₂ thickness of one monolayer, the etching inhibition was proportional to the ZrO₂ fractional coverage. The ZrF₄ etch stop layer was observed to arrest the thermal Al₂O₃ ALE for >100 ALE cycles.

The ZrF₄ etch stop layer could then be easily removed by a ligand-exchange reaction with AlCl(CH₃)₂ (dimethylaluminum chloride (DMAC)) [3]. The ZrF₄ etch stop layer could be applied and removed repeatedly without changing the Al₂O₃ etch rate. X-ray photoelectron spectroscopy (XPS) studies observed no trace of Zr on the Al₂O₃ surface after 7 cycles of DMAC and HF sequential exposures. Area selective deposition of the ZrF₄ etch stop would lead to area selective etching using HF and TMA as the reactants. Area selective deposition could be achieved based on selective reactant adsorption or substrate-dependent nucleation delays.

[1] Younghee Lee, Jaime W. DuMont and Steven M. George, “Trimethylaluminum as the Metal Precursor for the Atomic Layer Etching of Al₂O₃ Using Sequential, Self-Limiting Thermal Reactions”, Chem. Mater. 28, 2994 (2016).

[2] Younghee Lee, Craig Huffman and Steven M. George, “Selectivity in Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions”, Chem. Mater.28, 7657 (2016).

HfO₂ and ZrO₂ are two high-k materials that are crucial in semiconductor devices. Atomic level control of material processing is required for fabrication of thin films of these materials at nanoscale device sizes. Thermal Atomic Layer Etch (ALE) of metal oxides, in which up to one monolayer of the material can be removed per cycle, can be achieved by sequential self-limiting fluorination and ligand-exchange reactions at elevated temperatures. However, to date a detailed atomistic understanding of the mechanism of thermal ALE of these technologically important oxides is lacking. In this contribution, we investigate the hydrogen fluoride pulse in the first step in the thermal ALE process of HfO₂ and ZrO₂ using first principles simulations. We also present a thermodynamic analysis approach to compare reaction models representing the self-limiting (SL) and continuous spontaneous etch (SE) processes taking place during an ALE pulse. Applying this to the first HF pulse on HfO₂ and ZrO₂ we find that thermodynamic barriers impeding continuous etch are present at ALE relevant temperatures. Explicit calculations of HF adsorption on the oxide surfaces allow us to investigate the mechanistic details of the HF pulse. A HF molecule adsorbs dissociatively on both oxides by forming metal-F and O-H bonds. HF coverages ranging from 1.0 ± 0.3 to 17.0 ± 0.3 HF/nm² are investigated and a mixture of molecularly and dissociatively adsorbed HF molecules is present at higher coverages. Theoretical etch rates of -0.61 ± 0.02 Å /cycle for HfO₂ and -0.57 ± 0.02 Å /cycle ZrO₂ were calculated using maximum coverages of 7.0 ± 0.3 and 6.5 ± 0.3 M-F bonds/nm² respectively (M = Hf, Zr).

Ta₂O₅ ALE was demonstrated using BCl₃ and HF as the reactants. The Ta₂O₅ ALE surface chemistry could proceed via a fluorination and ligand-exchange mechanism or a conversion-etch mechanism. The experimental results support a conversion-etch mechanism. BCl₃ is believed to convert the surface of Ta₂O₅ to a B₂O₃ surface layer via the favorable conversion reaction Ta₂O₅ + 10/3 BCl₃(g) → 5/3 B₂O₃ + 2TaCl₅(g) [ΔG°(250°C = -38 kcal]. HF can then spontaneously etch the B₂O₃ surface layer via the favorable reaction B₂O₃ + 6HF(g) → 2BF₃(g) + 3H₂O(g) [ΔG°(250°C = -17 kcal] [1]. In contrast, HF fluorination of Ta₂O₅ to TaF₅ or TaOF₃ is thermochemically unfavorable.

In situ spectroscopic ellipsometry and ex situ x-ray reflectivity measurements were employed to study Ta₂O₅ ALE. Evidence for the conversion of Ta₂O₅ to B₂O₃ was provided by the dependence of the Ta₂O₅ etch rate on BCl₃ exposure and BCl₃ pressure. The Ta₂O₅ etch rate increased progressively with both longer BCl₃ exposures and higher BCl₃ pressures. The longer BCl₃ exposures and higher BCl₃ pressures convert more Ta₂O₅ to B₂O₃. Using three BCl₃ exposures (130 mTorr for 30 s) and one HF exposure, the Ta₂O₅ etch rate was 1.05 Å/cycle at 250°C (Figure 1). Under these conditions, the Ta₂O₅ etch rates increased with temperature ranging from 0.36 Å/cycle at 200°C to 1.96 Å/cycle at 295°C. Ta₂O₅ etch rates also increased at higher BCl₃ pressure. The etch rates varied from 0.48 to 1.46 Å/cycle using one BCl₃ exposure (30 s) at BCl₃ pressures from 130 to 1000 mTorr, respectively (Figure 2).

TaN ALE was also demonstrated using an O₃ oxidation step to oxidize TaN to Ta₂O₅. The Ta₂O₅ was then etched using BCl₃ and HF as the reactants. TaN ALE was performed using supercycles defined by an O₃ exposure followed by 60 Ta₂O₅ ALE cycles using BCl₃ and HF as the reactants. The etch rate was 38 Å/supercycle at 250°C (Figure 3). This high TaN etch rate was attributed to the large Ta₂O₅ thickness produced by O₃ oxidation. The oxidation of TaN using O₃ was studied using x-ray reflectivity measurements. TaN oxidation was observed to be fairly self-limiting at Ta₂O₅ thicknesses of 60-70 Å after longer O₃ exposures at 250°C.

[1] N.R. Johnson and S.M. George, “WO₃ and W Thermal Atomic Layer Etching Using “Conversion-Fluorination” and “Oxidation-Conversion-Fluorination” Mechanisms” ACS Appl. Mater. Interfaces9, 34435 (2017).