ALD/ALE 2022 Session AF3-MoA: Plasma Enhanced ALD

Metal nitrides films stand out as candidates for many strategic industrial applications as they exhibit superior functional properties such as mechanical, electrical and thermal properties. Complementary chemical vapor deposition techniques from High Temperature Chemical Vapor Deposition (HTCVD) to Thermal and Plasma Enhanced Atomic Layer Deposition (T-ALD and PEALD) have been investigated to fabricate metal nitrides thin films. Coupling or combining these techniques might open new opportunities. In each case, one of the major challenges is the synthesis of high quality, pure (with no oxygen contamination) material. Among ALD developments, efforts have been focused on the exploration of thermal stabilities of different precursor molecules, chemical reactions as well as growth processes sequences and conditions.

In this presentation, the examples of various metal nitride deposition process development with special focus on Aluminum nitride will be presented. AlN is a multifunctional material, which has been widely investigated for many potential applications in recent years, due to its high melting point, excellent thermal conductivity and good chemical stability and behavior towards oxidization and abrasion with respect to other nitrides. It is a semiconductor material with a wide bandgap, offering transparency even in the UV region. Moreover, its oxidation rate is low at temperatures below 1100°C. AlN films are attractive for applications in energy, aeronautics, electronic or optoelectronic devices. For instance, thin films are investigated in piezoelectric based applications, as passivating and protective coatings for metallic architectures, as AlN substrate in high power applications.

We report on the optimizing routes and strategies via coupling deposition processes to obtain the best film properties on various systems.

The versatile properties of molybdenum oxide strongly depend on the structural features and in particular on its crystallinity, composition and morphology. This makes it an interesting material class for a variety of applications, i.e., (opto)electronics, (photo)catalysis and gas sensors. Moreover, the performance of these applications is significantly enhanced by the implementation of the active material in thin film form. Therefore, the development of atomic layer deposition (ALD) processes for the fabrication of nanostructured molybdenum oxide thin films has grown steadily in the recent years. As a consequence, the demand for suitable molybdenum precursors with improved physico-chemical properties is rising as the library for appropriate molybdenum precursors is rather small.

In this study, molybdenum oxide thin films were deposited using a new plasma-enhanced ALD (PEALD) process employing the molybdenum precursor Mo[(N^tBu)₂(^tBu₂DAD)], recently developed in our group, and oxygen plasma. The process yielded a growth rate of 0.75 Å/cycle on Si(100), which is in the range of other PEALD processes reported for molybdenum oxide. Furthermore, the linear dependence of the thickness on the number of cycles was confirmed within a temperature window between 100°C and 240°C. X-ray diffraction (XRD) patterns show that on the lower end of the temperature window the films appear to be amorphous while crystallization starts at the higher end (240°C), yielding nanocrystalline β-MoO₃ thin films. Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS) analyses revealed the formation of pure films, while SEM analysis showed the smooth morphology of the films with crystallite formation at higher deposition temperatures. Furthermore, sensor substrates were coated with molybdenum oxide to investigate the resistive hydrogen sensitivity of the thin films with respect to their morphology. This study demonstrates that the recently developed molybdenum precursor is suitable for the utilization in ALD applications. The resistive response of the deposited MoO_x thin films towards hydrogen gas reveals the potential of this material for thin film hydrogen gas sensors. Moreover, the overall quality of the films makes it also promising for implementation in other applications such as catalysis and optoelectronics.

Among the layered metal dichalcogenide materials, semiconducting tin disulfide (SnS₂) is a potent candidate for photocatalysis, field-effect transistors, lithium-ion batteries, and gas sensing applications. Hence, there is a demand for a scalable technique to uniformly and conformally deposit SnS₂ thin films, preferably at low temperatures. Here, we present a plasma-enhanced atomic layer deposition (PE-ALD) technique to deposit crystalline SnS₂ using tetrakis(dimethylamino)tin (TDMASn) precursor and H₂S/Ar plasma at temperatures as low as 80°C.

TDMASn precursor was previously combined with H₂S to deposit tin sulfides via thermal ALD.¹ We employed H₂S plasma as the reactant inspired by previous reports demonstrating a significant effect of using plasma on the ALD growth characteristics and material properties.² The new PE-ALD process is self-limiting with a growth per cycle of 0.45-0.15 Å/cycle in a temperature range of 80 –180 °C. (Fig. 1) In contrast to the thermal ALD process which deposits amorphous SnS₂ thin films at 80°C and a mixture of SnS and SnS₂ phases at 180°C, crystalline SnS₂ thin films are deposited with the PE-ALD process in the temperature range of 80-180°C.(Fig. 2) Moreover, scanning electron microscopy analysis shows an evolution in thin-film morphology from grain-like structures with size in the range of 30-50 nm to out-of-plane oriented structures for SnS₂ deposited by the PE-ALD process at 80°C and 180°C, respectively. (Fig. 3) Optical transmission measurements detected an indirect bandgap in the range of 2.1-2.3eV in all the as-deposited SnS₂ thin films. (Fig. 4)

SnS₂ nanostructures with different morphologies have been previously investigated as anode materials in lithium-ion batteries to counter the problems of poor capacity retention associated with significant volume changes during cycling. Thus, we compared the electrochemical performance of SnS₂ thin films with three different morphologies as anode material in Lithium-ion batteries. The SnS₂ thin films with out-of-plane orientation structures exhibit better cycling stability with a capacity retention of 77% in contrast to the amorphous films which show 34% capacity retention after 100 cycles. (Fig. 5) We assume these out-of-plane orientation sites facilitate the diffusion of Li⁺ ions thus limiting the pulverization and retaining the capacity.

1. Ham, G.; Shin, S.; Park, J.; Choi, H.; Kim, J.; Lee, Y.-A.; Seo, H.; Jeon, H., ACS Applied Materials & Interfaces 2013,5 (18), 8889-8896.

1. Kuhs, J.; Dobbelaere, T.; Hens, Z.; Detavernier, C., Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2017,35 (1), 01B111.

Electrocatalysts often suffer from poor stability under operating conditions due to (electro)chemical susceptibilities and/or poor adhesion to the support structure. For the realization of highly active and stable catalytic layers, catalyst-support integration and interface engineering play important roles. Interface engineering is also decisive for the integration of electrocatalysts with semiconductor light absorbers for solar-to-chemical energy conversion. For designing surfaces and interface layers of energy conversion devices, plasma-enhanced atomic layer deposition (PE-ALD) has emerged as a powerful method. These processes are typically developed with the aim of ensuring saturated surface oxidation reactions. However, exploring less aggressive process parameters opens new opportunities to precisely tailor the functional properties of active catalysts.

Here, we elucidated non-saturated oxidation of cobaltocene precursor by varying the plasma exposure time and plasma power to precisely control structural, mechanical, and optical properties of biphasic CoO_x thin films, thereby tailoring their catalytic activities and chemical stabilities.[1] Short pulses and low plasma power facilitate the formation of porous, unstable Co(OH)₂ layers with high electrochemical activity, while long pulses and high power yield stable, inactive Co₃O₄ layers. The best combination of stability and activity is observed for intermediate plasma exposure times leading to the formation of biphasic films consisting of a Co(OH)₂ surface and Co₃O₄ interface layer. The underlying reason for the formation of a porous Co(OH)₂ surface layers is the incomplete decomposition of the precursor at either short pulse durations or low plasma power, which also leads the incorporation of carbon impurities. The corresponding change in the chemical composition is reflected in the respective growth chemistry, which is characterized by reduced precursor adsorption and changes in the growth per cycle. The gained mechanistic insights were applied in a two-step growth process to intentionally engineer bilayer films consisting of a stable Co₃O₄ interface layer with a catalytic Co(OH)₂ surface exhibiting improved electrochemical performance without sacrificing chemical stability. This work highlights that unsaturated oxidation allows access to different material phases with tailored properties for engineering active catalysts and their interfaces.

[1]. Kuhl, M. et al. Designing multifunctional CoO_x layers for efficient and stable electrochemical energy conversion, chemrxiv, DOI:10.26434/chemrxiv-2022-23ck4 (2022).

Fabrication of the high-quality insulating film with reduction of the thermal budget in a process is required for the metal-oxide-semiconductor (MOS) transistor fabrication with next-generation semiconductor material such as Ge and SiGe. Hafnium dioxide (HfO₂) is one of the promising candidate materials due to its high dielectric constant (high-k-value), thermal stability, and a high-quality interface between HfO₂ and SiO₂ for reduction of the gate leakage current with miniaturization of fabrication scale. To deposit a high-quality gate dielectric film on a high aspect ratio channel, the atomic layer deposition (ALD) method has been adopted with high coverage and thickness control. We have already successfully deposited high-quality SiO₂ films using defect-free neutral beam enhanced ALD (NBEALD) at low substrate temperature (30 ºC).In this study, we demonstrated a high-quality amorphous HfO₂/SiO₂/Si structure using low-temperature NBEALD.

The NBEALD was carried out in a large chamber for an 8-inch diameter. A precursor and carrier gases were Tetrakis(ethylmethylamino)hafnium (TEMAH) and Ar, respectively. The Si(100) substrate was cleaned with sulfuric acid hydrolysis (4 sulfuric acid:1 hydrogen peroxide) and 1% hydrofluoric acid, and then high-quality SiO₂ with a thickness of 1.6 nm could be initially formed after oxygen NB irradiation. After that, samples were grown under the following ALD growth conditions: gas supply (5 sec), purge (5 sec), oxygen NB irradiation (20 sec), oxygen gas purge (5 sec). Oxygen plasma for oxygen neutral beam was discharged at 1300 W, while the bias was not applied, and the stage temperature was at the room temperature, 30 ºC. Different thicknesses of HfO₂ films were deposited by using 50, 100, 150, and 200 cycles to evaluate the film characteristics. The surface roughness and crystalline state of deposited HfO₂ films were evaluated by atomic force microscopy (AFM) X-ray diffraction (XRD) measurements. The surface roughness increased from 0.3 to 1.1 nm with increasing growth cycle. XRD results were measured by the θ-2θ scan. Any typical diffraction peaks were not observed for all the samples. Therefore, we could successfully form high uniformity amorphous HfO₂/SiO₂ thin films even at low temperatures in an in-situ environment. It is suggested that this technique can contribute to the development of the MOS transistor fabrication process with a small heat budget in the future. Finally, we can discuss the electrical characteristics of nano-devices.