ALD/ALE 2026 Session AF1-WeM: Plasma Enhanced and Low Temperature ALD

Atomic layer deposition (ALD) is renowned for its self-limiting characteristics, enabling precise atomic-level control and high conformality. During an ALD half-cycle, precursors may adsorb onto the substrate as physisorbed species before reacting with surface ligands to chemisorb. However, even under nominally saturated conditions, full surface coverage may not be achieved due to phenomena such as steric hindrance or retention of excess physisorbed precursors. To address this limitation, multiple precursor injections within a single half-cycle are employed to increase the amount of chemisorbed precursor. The observation that ALD growth differs when performed with discrete injections compared to a single injection suggests that adsorbed precursors undergo dynamic behavior on the timescale of a pulse. The temporal effects of precursor adsorption, rearrangement and desorption have received less attention in ALD, for which a single precursor pulse is typically assumed to be fully saturating.

In this work, ALD of HfO₂ with multiple-pulse sequencing was used to elucidate fundamental ALD mechanisms. A single long pulse of Hf precursor (tetrakis(dimethylamido)hafnium, TDMAH) was divided into several shorter sub-pulses to enable a direct comparison between single and multiple pulses during ALD with water as the counteractant at 200°C. The HfO₂ films grown by multiple pulse injection show a 26 % higher growth per cycle, as well as increased grain size, film density, and crystallinity. The improvement is attributed to a denser surface arrangement enabled by staggered precursor exposure. In situ infrared spectroscopy reveals differences in the types of hydroxyl groups consumed by precursors between single and multiple pulse modes. Hydroxyl species that were difficult to react in a single pulse (e. g. hydrogen-bonded OH) became more accessible through the multiple feeding of precursors.

To isolate the contribution from each pulse, the Zr precursor (tetrakis(dimethylamido)zirconium, TDMAZ) was introduced into the TDMAH pulse sequence as a “chemical tag” in HfO₂ ALD for defining the specific pulse. This approach enables direct observation of the fractional growth contribution from individual pulses under different pulse-time and pulse-sequence combinations. The results of X-ray photoelectron spectroscopy and in situ ellipsometry measurements between 150°C to 220°C indicate that during short pulses, only ~50% of the total growth occurs during the first pulse, resulting in a more sparsely populated surface that allows additional precursors to react in later pulses. Our results reveal that precursor residence time scales with precursor population, implying a coverage-dependent desorption mechanism. Low precursor loading, especially in the unsaturated regime, facilitates desorption and surface rearrangement, thereby enhancing the benefits from multiple-pulse strategy. Further mechanistic analysis of the single versus multiple pulse ALD schemes will be discussed.

2D transition metal dichalcogenides (TMDs) have shown to be promising candidates as alternative channel materials within scaled transistors in next generation electronics. As transistor dimensions continue to shrink and advanced device architectures evolve, there exists a need for the deposition of high quality, ultra-thin dielectrics onto semiconducting channels, typically via atomic layer deposition (ALD). However, deposition on pristine TMD basal planes is challenging due to the lack of dangling bonds and defect sites typically acting as ALD nucleation sites, thus resulting in nonuniform dielectric coverage.

In this work, we explore the effect of mild (low power, high pressure) H₂S plasma pretreatments to improve HfO₂ ALD on mono and few layer TMDs. First, we evaluate the effect of various plasma exposure duration times on monolayer WSe₂ at 250 °C. Minimal impact to WSe₂ following plasma exposure was confirmed via Raman, X-ray photoelectron, and photoluminescence spectroscopy. These plasma exposures were then applied to WSe₂ prior to ~ 3nm HfO₂ atomic layer deposition resulting in a significant improvement in dielectric nucleation and coverage on WSe₂ basal planes as compared to untreated control samples. To better understand the interplay between the plasma-pretreatment of the WSe₂ surface and the HfO₂ ALD process, we investigated the influence of the precursor (both Hf and O) dose times, number of precursor pulses, purge time, and stop flow pumping schemes on the microstructure of the HfO₂ deposited on pre-treated WSe₂. AFM, backscattered electron imaging, and Auger spectroscopy were utilized to determine an optimal combination of plasma exposure and HfO₂ ALD process parameters to yield uniform dielectrics across TMD basal planes.

After process optimization, we evaluated the minimum possible HfO₂ thickness allowed via our plasma pretreatment strategy while still maintaining fully coalesced films using AFM and cross-section TEM. Electrical measurements will demonstrate the capabilities of our technique in depositing high quality, ultra-thin dielectrics onto TMDs with minimal impact on TMD structural properties. This work will further enable the advancement of TMD based top gate and gate all around transistors with ultrathin dielectrics and addresses the concern of maintaining the integrity of underlying TMD channels during pre-treatments prior to ALD.

The need for integration of materials in the fabrication of devices sensible to thermal budgets, such as in flexible, organic and bio-inspired electronics, requires the development and study of low thermal ALD processes. On this scope several attempts have been performed to tackle this issue from different sides, involving the development of innovative precursors and engineering of growth processes, including the extension of ALD regime down to 80 °C in thermal ALD or employing plasma ALD to deliver the energy required for the chemical reactivity.

In this context, HfO₂ has been, and still is, the subject of many efforts, driven by the need of both innovative scientific challenges and functionalities and the existing markets. Recently, its integration in neuromorphic and magnetoionic devices, shifted the attention from qualifying the material properties in terms of compact, dense, defect-free, crystalline films, to consider the type and degree of defects, the selective incorporation of additional elements, typically C, N and OH, in amorphous films. Thus, the need to redefine low temperature ALD processes targeting HfO₂ with such specifications.

Within this framework, here, we compare three processes where thermal or plasma ALD is considered, employing two different equipment. In details, we consider TEMAHf and TDMAH as Hf precursors, H₂O as oxidant in the thermal ALD processes or Oxygen from O₂ in 300 W plasma ALD process, with temperature in the 80 – 100 °C range. We target four HfO₂ thickness, 3, 5, 10 and 20 nm, grown on SiO₂/Si substrate, to have the minimal set for accurately verify the ALD window regime and compare any change in the growth per cycle.

Grown HfO₂ layers are characterized from XRR, XPS and ToF-SIMS to evaluate any change in the electron density, elemental chemistry and stoichiometry. We observe, depending on the growth process, a stoichiometry below the nominal O:Hf=2:1 ratio in all cases, the lowest value in HfO₂ films grown using plasma ALD, with a slight evolution of the stoichiometry with the film thickness. Further, we observe from XPS a significant amount of OH groups depending on the ALD process, in-line with the electron density value from XRR. We explain such changes in terms of purging time and engineering of the oxidant sub-cycle within each ALD cycle, and considering the interplay between oxidation and hydroxilation phenomena.

Further, we integrate HfO₂ films in simple MIM capacitors to extract their dielectric constant (k) and voltage breakdown (V_BD). k value results around 17 in HfO₂ films from thermal ALD lowering to k = 12 in HfO₂ from plasma ALD, while V_BD varies in the range 3.0-5.5 MV/cm. We tentatively explain such changes in terms of defectivity and ionic species inside the grown HfO₂ films.

Finally, we briefly report on how the so-grown HfO₂ films enable, or not, magnetoionic functionality, depending on the ALD process employed.

Financial support from EIC METASPIN project, grant n. 101098651.

Germanium dioxide (GeO₂) is of particular interest in advanced microelectronics and is regarded as an ultrawide bandgap (UWBG) material for next-generation CMOS applications. With exceptional carrier mobility, it shows promise for high-frequency and high-power electronics,^[1,2] while its optical properties make it appealing for, e.g., anti-reflection coatings.^[3] Furthermore, two-dimensional (2D) GeO₂ has an inherently porous structure with molecular-sized holes and is thus of interest as a material for selective gas-separation membranes.^[4,5]

To transition these applications from research to industrial applications, it is necessary to develop scalable processes that enable the deposition of high-quality, defect-free GeO₂ films. Owing to its self-limiting nature, ALD emerges as the most promising solution, offering precise control over thickness and morphology. Yet, ALD strongly relies on the underlying chemistry, and especially the development of low-temperature processes is currently restricted by the lack of suitable Ge precursors.

To address this challenge, we introduce tetrakis-(3-dimethylamino)propyl germanium(IV) [Ge(DMP)₄], designed to be highly thermally stable, volatile (see TGA in Figure 1c), and reactive, to facilitate low-temperature ALD processing of GeO₂. Using this new liquid, non-pyrophoric, and monomeric precursor (as confirmed by single-crystal XRD and DFT structure analysis in Figure 1a,b), we developed a PEALD process for GeO₂ using O₂ plasma as the co-reactant. The process operates across a broad temperature range from 40 °C to 240 °C (Figure 2a) with a linear GPC of 0.24 Å at 150 °C. By utilizing different plasma pulse durations (t_plasma), the composition of the films can be adjusted: with t_plasma = 500 ms, the thin films consist of GeO₂, GeO, and Ge, with a higher degree of sub-stoichiometric GeO_x species at higher deposition temperatures, whereas stochiometric GeO₂ is obtained with t_plasma = 50 ms (Figure 2b, c). Initial characterization revealed that the composition directly influences the optical behavior of GeO_x thin films, as seen in the UV/Vis spectra of GeO_x thin films deposited using different plasma durations and deposition temperatures (Figure 3a). Further, transmission electron spectroscopy revealed closed and uniform thin films with an amorphous structure (Figure 3b). Thereby, the rapid nucleation behavior of the process allowed the deposition of a thin film using only eight PEALD cycles. This enabled down-scaling to a film thickness of ≤1 nm (Figure 3c), approaching the theoretical thickness required for the bilayer structure and providing a strong foundation to explore the scalable growth and utilization of bilayer GeO₂.

By identifying [Ge(DMP)₄] as a new and promising Ge precursor, PEALD processing of GeO₂ and GeO_x becomes feasible at near-room-temperature conditions, advancing the use of GeO₂ and GeO_x thin films for new applications based on their unique properties.

Binary, ternary, and multinary bismuth-containing oxide thin films have been synthesized by chemical vapor deposition and atomic layer deposition (ALD), and utilized in optical, ferroelectric, sensing, and catalytic applications. Binary bismuth oxide (Bi₂O₃) exists in several crystalline phases; however, deposition of a targeted single phase, as well as stoichiometric Bi₂O₃, by ALD remains challenging.

A recent study deposited Bi-rich α-Bi₂O₃ on Si(100) substrates by ALD using triphenylbismuth (BiPh₃) and ozone (O₃) as precursor and co-reactant, respectively, within an ALD window of 250-300 °C. Alternative substrates or annealing at temperatures between 700 - 800 °C was required to access additional phases (i.e., β and γ) and stoichiometric composition.

Herein, we report a thermal ALD process for Bi₂O₃ using methyldiphenylbismuth (MePh₂Bi) and O₃ as novel precursor and co-reactant, respectively. Saturation was achieved with a MePh₂Bi precursor delivery temperature as low as 90 °C, and a growth per cycle of ~ 0.6 Å/cycle within the range of 100 - 170 °C. MePh₂Bi, a liquid, methyl-substituted, and thermally stable precursor exhibits higher volatility than its solid BiPh₃ analogue, enabling deposition on temperature-sensitive substrates. Characterization by x-ray photoelectron spectroscopy, atomic force microscopy, and grazing incidence x-ray diffraction of the as-deposited films is presented.

SiO₂ is a commonly used dielectric material in semiconductor manufacturing, and aminosilanes are typically used as the Si precursor during radical-assisted atomic layer deposition (ALD) of SiO₂. This work explores the role of the initial surface Si−OH density, the substrate temperature, and the structure of the aminosilane precursor on the growth per cycle (GPC) for ALD on plasma-deposited SiO₂ substrates. Specifically, we studied O₂-plasma-assisted ALD of SiO₂ using two aminosilanes, di-sec-butylaminosilane (DSBAS) and dimethylamino trimethylsilane (DMATMS). The surface reactions during ALD were monitored using in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and the GPC was monitored using in situ four-wavelength ellipsometry. On an SiO₂ surface with a high initial Si−OH group density, in situ ATR-FTIR spectroscopy shows that ~30% more Si−OH groups are consumed by DMATMS than DSBAS because DSBAS contains a bulkier amino leaving group than DMATMS. In situ ellipsometry shows that at an ALD temperature of 100 °C, the GPC using DSBAS and DMATMS are ~1.8 and ~1.2 Å, respectively. The higher GPC for DSBAS shows that the initial aminosilane coverage does not influence the GPC. We speculate that during the O₂ plasma step, O radicals can easily insert into Si−H bonds in adsorbed DSBAS. However, for adsorbed DMATMS, surface Si−(CH₃)₃ groups must be combusted, and then converted to Si−OH groups from species generated in the O₂ plasma.

Mo-based materials (MoC_x, MoN_x, MoC_xN_y) exhibit high thermal and chemical stability as well as extremely low electrical resistivity. [1] Owing to these superior properties, Mo-based thin films are considered as promising diffusion barrier materials for advanced Cu and Ru interconnects. [2] In MoC_xN_y, C and/or N occupy interstitial sites within the crystal structure, enabling tunable properties. To enable their integration into advanced semiconductor interconnects, MoC_xN_y thin films were deposited by PEALD using a N-free Mo precursor with an H₂+N₂ mixture plasma reactant. By adjusting the H₂:N₂ ratio, the composition of C and N in the MoC_xN_y thin films was systematically controlled. Notably, N incorporation played a decisive role in increasing the growth rate from an extremely low value of 0.3 Å/cycle under pure H₂ plasma to ~1.5 Å/cycle with the introduction of a small amount of N₂, indicating that N₂ promotes surface reactions during MoC_xN_y growth. Under an H₂-rich plasma condition (H₂:N₂ = 8:1), typical ALD behavior was observed, including self-limiting growth with respect to both precursor and reactant pulse times. A stable ALD window was also identified at 225-250 ℃, within which a saturated growth rate of ~1.5 Å/cycle and linear thickness evolution with the number of cycles were achieved. Moreover, both the growth rate (1.5-1.9 Å/cycle) and electrical resistivity (590-1490 μΩ·cm) were effectively tuned by varying the H₂:N₂ ratio from 8:1 to 1:8. Structural and compositional analyses using XRD and XPS confirmed the formation of conductive MoC_xN_y thin films with controlled C and N incorporation. Electrical characterization also revealed a strong correlation between film composition and resistivity. Finally, the PEALD-MoC_xN_y thin films were applied as diffusion barrier for Cu and Ru interconnects as well as gate electrode and the results will be presented at the conference.

References

[1] Kang, W. et al. Journal of Vacuum Science & Technology a Vacuum Surfaces and Films (2023): 41 (6).

[2] Tripathi, C. C et al. Applied Surface Science (2008): 255 (6), 3518–3522.

Acknowledgements

This work was supported by the Technology Innovation Program (Public-private joint investment semiconductor R&D program (K-CHIPS) to foster high-quality human resources) (RS-2023-00236667, High-performance Ru-TiN interconnects via high-temperature atomic layer deposition (ALD) and development on new interconnect materials based on ALD) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (1415187401).This work was also supported by the Industrial Strategic Technology Development Program (RS-2024-00509266, Development of next-generation dielectric, electrode process equipment, and core materials for logic 1 nm or less and memory × nm level), funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). The precursor used in this study was provided by UP Chemical Co. Ltd., Korea.