AVS 71 Session AP-ThP: Atomic Scale Processing Poster Session

Thermal atomic layer etching (ALE) of lanthanum oxide (La₂O₃) was demonstrated using sequential exposures of acetylacetone (Hacac) and ozone (O₃). Hacac reacts with La₂O₃ by a ligand addition and hydrogen transfer reaction to form volatile La(acac)₃ and H₂O according to: La₂O₃ + 6Hacac → 2La(acac)₃ + 3H₂O. Ozone was then used to remove carbon residue resulting from Hacac exposure on the surface.

In situ spectroscopic ellipsometry (SE) was used to monitor the film thickness change with number of ALE cycles. SE observed the linear decrease of La₂O₃ film thicknesses versus number of Hacac and O₃ cycles. Semicrystalline La₂O₃ thin films displayed etch rates of 0.2, 0.4 and 0.69 Å/cycle at 230, 250 and 270 ^oC, respectively. The SE studies also showed that the Hacac and O₃ surface reactions were self-limiting.

Atomic force microscopy (AFM) analysis of semicrystalline La₂O₃ on Si with a thickness of 20 nm displayed surface smoothing versus ALE cycles.The RMS surface roughness was 3.3 Å prior to ALE and 0.9 Å after ALE. Quadrupole mass spectrometry (QMS) was also utilized to study the Hacac-O₃ etch process on crystalline La₂O₃ powder at 250 ^oC. La(acac)₃ organic fragments were detected during Hacac exposure.During O₃ exposure, combustion products were observed from the oxidation of organic residuals left from Hacac exposures.

Hexafluoroacetylacetone (hfacH) has also been utilized instead of Hacac to etch La₂O₃. One advantage of Hhfac over Hacac is that Hhfac has a lower pKa value and hfac-metal complexes are generally more volatile. However, La₂O₃ ALE using hfacH and O₃ displayed a substantially lower etch rate of 0.06 Å/cycle at 250 ^oC.This result was attributed to significant film fluorination by Hhfac as revealed by XPS analysis. Etching lanthanum fluoride using the Hhfac-O₃ chemistry is more challenging.

Spontaneous etching is characterized by a physicochemical reaction of a thin film surface with a reactant vapor that releases volatile products with a continuous etch rate. Spontaneous etching provides the benefit of a single processing step with simply one etchant exposure, as well as typically high inherent selectivity.

Previous work has demonstrated that anhydrous HF vapor does not spontaneously etch SiO₂. However, co-adsorbing ammonia (NH₃) with HF has led to rapid SiO₂ spontaneous etching. These results have suggested that the nature of the active etch species changes in the presence of NH₃. Without co-adsorbed NH₃, the active etch species is believed to be F^–. With the polar NH₃ co-adsorbate, the active etch species is thought to switch to HF₂^–. [Junige, George: Chem. Mater. 36, 6950 (2024)]

Co-adsorbing polar molecules with HF has been proposed to form HF₂⁻ species to enable SiO₂ etching. Examples of suitable polar molecules include dimethylamine ((CH₃)₂NH: 1.0 D), NH₃ (1.4 D), methanol (CH₃OH: 1.7 D), water (H₂O: 1.85 D), or ethylene glycol ((CH₂OH)₂: 2.28 D); where the number in parentheses refers to the dipole moment of the respective molecule in the gas phase. In theory, these polar co-adsorbates solvate HF and stabilize the dissociation products H⁺ and F^–. As a result of this more extensive HF dissociation, F^– species at increased concentration react further with HF to produce HF₂^– species.

In situ spectroscopic ellipsometry (iSE) experiments were performed to test the idea that other polar molecules co-adsorbed with HF may enable SiO₂ spontaneous etching. These investigations revealed that co-adsorbing H₂O or CH₃OH with HF did not spontaneously etch SiO₂ at 200 or 275℃. The adsorption and desorption kinetics of H₂O or CH₃OH molecules at SiO₂ surfaces might not yield an adequate solvation layer at these elevated temperatures. In contrast, co-adsorbing DMA+HF enabled SiO₂ spontaneous etching with a substantial etch rate of 34.70 Å/min at 200℃. Similar results have been observed previously for NH₃+HF co-dosing at 275℃. These results suggested that co-adsorbing polar molecules with HF to form HF₂^– species can etch SiO₂ if there is sufficient solvation. Co-adsorbing (CH₂OH)₂+HF, as well as (CH₂OH)₂ adsorbed layers on SiO₂ surfaces, may be tested in future experiments.

Abstract—The United States Department of Energy (DOE) Advanced Materials and Manufacturing Technology Office (AMMTO) is leading a multi-organization effort to counter alarming trends in U.S. computing energy use (e.g. LBNL 2024 forecasts - lbnl-2024-united-states-data-center-energy-usage-report.pdf [https://eta-publications.lbl.gov/sites/default/files/2024-12/lbnl-2024-united-states-data-center-energy-usage-report.pdf] - that data centers will account for 26% of US electricity use by 2028 when cyrptomining is included) with its initiative in energy efficiency scaling for two decades (EES2) for microelectronics.Under this initiative, DOE/AMMTO has funded a portfolio of EES2 device technology R&D projects that promise >10X energy efficiency increase by 2030. This [talk] will highlight the first of these projects with Sandia National Laboratories to build on atomically precise manufacturing techniques to create a vertical tunnel field effect transistor (vTFET). Updates will be provided on the successful integration of front end of line (FEOL), back end of line (BEOL) and mid-end of line (MEOL) manufacturing processes (especially thermal budget) to fabricate this vTFET in a CMOS compatible process.One important discovery of the research in this area is “ultradoping” which makes the abrupt doping profiles needed for efficientvTFETs far more manufacturable. This talk also will present how these Sandia results integrate with version 1.0b of the EES2 roadmap that will be issued in Summer 2025. Version 1.0a of the Roadmap is available at EES2 Roadmap Version 1.0 [https://eere-exchange.energy.gov/FileContent.aspx?FileID=f4234e29-cc0c-4a56-a510-86b616ab5535] .

Multilayer optical coatings play a vital role in the propagation of light in photonic devices through selective reflection, transmission, and absorption of specific wavelengths. Among transition metal oxides, Vanadium Oxide (VO2) shows significant promise due to its high corrosion resistance at low temperatures, high tensile strength, and high electrical conductivity. This work aims to enhance the performance and durability of optical coatings by depositing VO₂ thin films using Atomic Layer Deposition (ALD), a technique offering precise and conformal deposition of ultra-thin films with Angstrom-level thickness control at low temperatures, making it a preferred method of growing thin films on planar and nanostructured surfaces. Vanadium Oxide (VO2) films were synthesized on Silicon and Kapton Substrates by the ALD method using the precursor Tetrakis (ethylamino) vanadium (TEMAV). Results from XPS confirmed successful deposition, as the binding energies for vanadium (V2p ½ and V2p 3/2 orbitals were both present) and oxygen were both present, as well as residual traces of Carbon and Nitrogen. XRD measurements for the 7 nm sample and the 21 nm sample revealed that the films were amorphous, deposited at 150 °C. AFM results indicated mostly smooth surfaces with an RMS roughness value of between 0.2 and 0.3 nm. However, on a larger scale, that RMS roughness value increased to around 17 nm, indicating that there were signs of agglomeration in the deposition. Raman spectroscopy of the 21 nm sample exhibited spectral features corresponding to mixed oxidation states of vanadium, suggesting partial crystallinity post-annealing. Characterization of the 42 nm samples are still in progress. Post-deposition annealing at ~500 °C in ultra-high vacuum will be utilized to generate crystallization, then samples will undergo comprehensive determination of the structural and surface chemistry.

To achieve higher performance and lower power consumption in ULSI devices, transistors have been continuously miniaturized and integrated at higher densities, resulting in the reduction of Cu interconnect linewidths. However, as the linewidth approaches the mean free path of electrons in Cu (~40 nm), the effects of inelastic electron scattering at grain boundaries and sidewall interfaces become non-negligible, leading to increased resistivity. Furthermore, the conventional diffusion barrier TaN, used to prevent Cu penetration into interlayer dielectrics, has a much higher resistivity than Cu (Cu:1.68 µΩ·cm, TaN:135 µΩ·cm), and its thickness reduction is limited due to the need to maintain barrier integrity. As a result, the proportion of Cu in the interconnect cross-section decreases with scaling, causing a sharp increase in line resistance. Additionally, increased resistance at the via bottom due to the barrier layer also becomes problematic.

In this study, we focused on ZrN as a novel diffusion barrier material. ZrN possesses the lowest resistivity (13.6 µΩ·cm) among transition metal nitrides [1] and maintains its barrier properties even after annealing at 500 °C [2]. To deposit ZrN films, we employed thermal atomic layer deposition (ALD), which is suitable for conformal coating in narrow damascene trenches. Zr[N(CH₃)₂]₄ was used as the precursor, NH₃ as the reactant gas, and N₂ as the carrier/purge gas.

Figure 1 shows the thickness and resistivity of ZrN films deposited at 250 °C as a function of ALD cycles. Film thickness increased linearly with the number of cycles, indicating excellent controllability, although the resulting resistivity was not yet ideal. Figure 2 presents the growth per cycle (GPC) at various deposition temperatures, revealing a stable ALD window between 150 and 250 °C. Figure 3(a) shows the dependence of film density and resistivity on NH₃ supply time for ZrN deposited at 200 °C, which lies within the ALD window. Increasing the NH₃ supply time led to higher film density and lower resistivity. Since no significant change in film composition was observed by XPS (Fig. 3(b)), the densification is attributed to improved surface reactions during the NH₃ pulse. When a film deposited with a 5 sec NH₃ supply was etched using an Ar ion gun in the XPS chamber and its resistance measured, removal of the surface oxide layer significantly reduced the resistance (Fig. 4). Suppressing surface oxidation at elevated temperatures after deposition is expected to further reduce the resistivity.

[1] C. C. Wang et al., Journal of Materials Science, 30, 1627–1641 (1995).

[2] M.B. Takeyama et al., Japanese Journal of Applied Physics, 61 SJ0802 (2022).

Atomic Layer Etching (ALE) is a critical technology enabling atomic-scale precision in advanced semiconductor device fabrication. Although obtaining detailed etching characteristics from various fluorine-based gases is crucial for optimizing etch per cycle (EPC) and selectivity, experimental data on gases other than commonly used hydrogen fluoride (HF) or C-F combined gases remain limited. This study investigates silicon dioxide (SiO₂) ALE processes utilizing sulfur hexafluoride (SF₆) and nitrogen trifluoride (NF₃) gases, employing a combined thermal and remote plasma-assisted approach at a process temperature of 300°C. The selection of SF₆ and NF₃ gases was guided by their distinct environmental impacts, radical generation efficiencies, and their potential effects on etching characteristics.

In this study, a surface modification approach using trimethylaluminum (TMA), followed by selective removal with remotely generated fluorine radicals, was systematically evaluated. By combining thermal isotropic surface modification with highly reactive fluorine radicals generated via remote plasma, this method effectively leverages the advantages of both isotropic thermal etching and plasma-enhanced high EPC. Experimental results indicated that NF₃ gas generated significantly higher fluorine radical densities than SF₆ under identical thermal and remote plasma conditions, resulting in enhanced EPC. However, in the case of NF₃ gas flow rates above 20 sccm, the significantly higher density of fluorine radicals generated expanded beyond the ALE regime into conventional plasma etching territory, limiting uniform atomic-level control. In contrast, fluorine radicals generated by SF₆ remained within optimal quantities for true ALE conditions, even at a relatively high flow rate of 100 sccm. Additionally, the remote plasma-assisted method effectively minimized ion-induced surface damage, thus promoting superior etching quality.

Our findings highlight that selecting the appropriate gas (NF₃ or SF₆) based on specific process requirements is critical, as each gas offers distinct advantages. Future research will explore mixed-gas processes combining SF₆ and NF₃ to synergistically enhance their respective benefits and further optimize ALE performance.

AcknowledgmentsThis work was supported by the Electronics and Telecommunications Research Institute(ETRI) grant funded by the Korean government [25ZH1240]

Magnesium oxide (MgO) is a key material in electronic and optoelectronic devices due to its wide bandgap, optical transparency, and thermal stability. However, the performance of MgO-based multilayer systems is often limited by interfacial inconsistencies, especially when deposited via such techniques as sputtering, which introduce surface defects. Surface modification strategies have emerged to address these issues, particularly in enhancing compatibility with atomic layer deposition (ALD) processes.

This work explores the surface modification of sputter-deposited amorphous MgO films using 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hfacH), a fluorinated β-diketone. Although this compound has been reported as a small-molecule inhibitor in selected ALD processes, this study demonstrates that hfacH acts as a growth promoter for TiO₂ deposition on MgO with thermal ALD that utilizes TDMAT and H₂O as co-reactants. Water contact angle (WCA) measurements confirm that hfacH alters the MgO surface from hydrophilic to hydrophobic, yet TiO₂ nucleation is enhanced on the modified surface, challenging conventional interpretations of surface energy and precursor accessibility.

This study uses a suite of primary surface characterization tools, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to confirm successful TiO₂ deposition.

These findings challenge the prevailing notion of hfacH as a growth inhibitor and highlight its context-dependent behavior. The modified surface facilitates nucleation, likely due to altered surface energy and local chemical environment, suggesting a potential role for hfacH as a growth promoter. This study contributes to the understanding of molecular surface chemistry and offers new insight into improving interface quality in multilayer oxide systems.

By redefining the function of fluorinated ligands in ALD chemistry, this work opens opportunities for more controlled and efficient deposition strategies in advanced electronic device fabrication.

Fe₃GaTe₂ is a 2D van der Waals material that displays intrinsic ferromagnetism above room temperature along with strong perpendicular anisotropy, making it a possible candidate for spintronics and magnonics applications. Recent computational studies have shown that the Fe₃GaTe₂ Curie temperature becomes elevated and its magnetic properties are tunable at the monolayer, demonstrating the importance of obtaining ordered and defect-free thin film and monolayer structures of this material by using atomically-precise treatments.

To determine whether Fe₃GaTe₂ can be etched controllably in nearly atomic layer etching regime, we performed a chlorine gas dose followed by an acetylacetone dose on Fe₃GaTe₂ flakes exfoliated onto a silicon substrate. AFM and XPS after the chlorine dose at elevated temperature show a partially etched but rougher surface. The consequent acetylacetone dose at the same temperature shows further etching. We aim at exploring atomic layer etching of Fe₃GaTe₂ under further optimized and controlled conditions. We are also exploring the etching mechanism to determine why the chlorine dose causes the initial change.

High-k oxides such as HfO₂ and ZrO₂ are critical for advanced transistor integration, but their atomic-level patterning remains a bottleneck. We examined thermal atomic layer etching (ALE) of these oxides using NbF₅ and TiCl₄. Distinct material-dependent behaviors were observed: HfO₂ exhibited smooth, self-limiting etch per cycle, whereas ZrO₂ showed significant surface roughening and high Cl incorporation. Surface chemical analysis confirmed that the degradation in ZrO₂ arises from unstable chlorinated intermediates. Density functional theory (DFT) calculations support this interpretation, indicating stronger and more disruptive TiCl₄ interactions with ZrO₂ compared to HfO₂. ALE was further applied to nanohole structures with 150 nm diameter and 2000 nm depth. In this geometry, HfO₂ displayed topographically selective removal, attributed to crystallinity-related microstructural variations across the etched regions. These results demonstrate that both reactant chemistry and oxide microstructure govern ALE performance. The combined experimental and computational insights provide a framework for selective patterning of high-k dielectrics and offer process guidelines for enabling integration of HfO₂ and ZrO₂ in future nanoscale transistor and interconnect architectures.

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

At low Ar energy (30 eV), the surface layer evolves over cycles toward a pseudo-steady state: the modified layer thickens in early cycles and then stabilizes at sub-nanometer thickness as F incorporation balances volatilization. Carbon retention remains markedly low because carbon is preferentially eliminated as volatile C–N–containing species (e.g., CN, NCF). Si desorbs mainly as SiFₓ (SiF₄ dominant with substantial SiF₂), and N is removed as primarily as N₂. The per-cycle N/Si removal ratio converges to ~1.3, indicating that stoichiometric SiNₓ etching is obtained.

At higher Ar energies (50–70 eV), the etch response changes significantly: deeper F penetration together with preferential N sputtering reduces near-surface F passivation, creating an under-coordinated, Si-rich surface that readily binds CF₄-derived carbon. The resulting SiC network within the modified layer suppresses SiF_x formation, slows Si removal, and roughens the surface—ultimately leading to etch stop.

These results reveal an Ar–energy–dependent transition in carbon fate that determines whether SiN_x remains in a steady-etch regime or evolves toward SiC-mediated etch-stop. Building on this mechanistic picture, we will also present practical strategies to suppress SiC build-up and sustain steady etching. Our MLFF-based framework can be extended to other substrates and plasma etch chemistries beyond SiN_x.

Area-selective deposition (ASD) is an important strategy in improving the fidelity of and/or reducing the complexity of current semiconductor patterning processes. Dielectric on dielectric (DoD) deposition is of interest for fully self-aligned via flow; however known DoD processes are limited in terms of materials, selectivities, and processing ranges. A common strategy for achieving ASD is to use a blocking layer on the non-growth surface (e.g., a metal) to be able to deposit a target material selectively on the desired surface (e.g., a dielectric). The most well-established blocking layers are self-assembled monolayers (SAMs) based on long alkyl chains, such as dodecanethiol. While these have demonstrated extremely promising results, they present limitations such as restricted processing windows (e.g., temperature), a typical requirement for solution-phase processing, long exposure times, limited stability (temperature, time, chemical), and presence of defects (e.g., pinholes) resulting from disorder or alkyl chain distortions. We have investigated an alternative class of blocking layers based on aromatic thiol SAMs, sometimes referred to as small molecule inhibitors. We demonstrate how the atomic structure of the aromatic derivative influences SAM formation and blocking ability, and demonstrate improved blocking of typical atomic layer deposition oxide films over state-of-the-art dodecanethiol SAMs.

Carboranes, with their unique icosahedral boron-based cage structures, are promising molecular precursors for advanced thin film deposition techniques such as plasma-enhanced chemical vapor deposition (PECVD) and molecular layer deposition (MLD). Their high vapor pressures and chemical stability make them ideal for vapor-phase delivery in high- and ultra-high vacuum systems, enabling the fabrication of ultra-pure films. We report the following thermal properties of a series of carborane compounds: vapor pressure, enthalpy of sublimation, melting points, and specific heat capacities. Using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), optimized isothermal and dynamic measurement protocols were developed with benzoic acid as a calibration standard to ensure precision. Vapor pressure curves were constructed, and Antoine coefficients were derived to characterize the compounds over a temperature range suitable for deposition (0.1–10 Torr at <200 °C). The results provide critical insights into the vapor pressures, thermal stability and phase behavior of carboranes, advancing their application in next-generation electronic devices and novel material architectures.

Authors: MS Michael Stoll, PhD Gyanendra Bhattarai, PhD Rupak Thapa, PhD Mark Lee, PhD Nathan Oyler, PhD Michelle Paquette

Iridium metal films were deposited by atomic layer deposition (ALD) reaction between 1-ethylcyclopentadienyl-1,3-cyclohexadieneiridium(I) [(EtCp)Ir(CHD)] and molecular oxygen at 250 °C in the Anric AT650P ALD tool. The Ir growth rate was 0.5 Å/cycle. Ir films were growth on thermally oxidized SiO₂ with and without a 10 nm Ta₂O₅ interlayer. Ir films grown on Ta₂O₅ appear smoother and more continuous under SEM compared to films on SiO₂, consistent with a previous report by Schmitt et al. for Ir grown from ALD with Ir(acac)₃ and oxygen, who report that Ta₂O₅ delays Ir nucleation and yields higher substrate coverage compared to SiO₂ [1]. The resistivity of a 42 nm Ir film on Ta₂O₅ was ca. 12 μΩ-cm. Optical properties of iridium films were modeled using variable angle spectroscopic ellipsometry with interference enhancement [2]. An extinction coefficient >5 at 633 nm was obtained for Ir films thicker than 20 nm on Ta₂O₅.

[1] P. Schmitt et al. Coatings 2021, 11, 173. DOI: 10.3390/coatings11020173

[2] J. N. Hilfiker et al. Thin Solid Films 2008, 516, 7979-7989. DOI: 10.1016/j.tsf.2008.04.060