ALD/ALE 2024 Session EM-TuP: Emerging Materials Poster Session

Brain-inspired parallel computing has been considered for the breakthrough of memory bottlenecks to meet the needs of simultaneously overflowing informative data. Numerous studies have been conducted on the development of reliable memristor arrays, energy-efficient computing, and various operational mechanisms for synaptic devices. However, for bioplausible neuromorphic computing, and temporal and spatial investigations as input signals toward leaky integrate-and-fire systems should be realized.

In this research, the importance of two-dimensioinal material based neuromorphic computing is addressed, and the basic components of the hardware neural network system are presented. Transition metal dichalcogenides (TMDCs) which is widely utilized for advanced van der Waals (vdW) based electronics is proper candidate for neuromorphic devices owing to atomically thin layered structure and superior electronic properties. Owing to the atomic structures, low-powered energy consumption can be realized.

Two-terminal devices of artificial and artificial synapses have been used as single components in neuromorphic computing, but they are associated with critical issues, such as current leakage and the absence of a third terminal as a fine synaptic weight modulator. To fully mimic biorealistic artificial neural networks, the integration of artificial neurons and synapses, hardware implementation of neurobiological functionality, and sensory neuromorphic computing are required. From single-component devices to architectures, the operational mechanism of each artificial component is unraveled, and the realization of bioplausible neuromorphic computing through integration is comprehensively discussed for future brain-inspired electronic systems.

Electron-enhanced atomic layer deposition (EE-ALD) was utilized for the growth of TiCN ternary nitride films with tunable composition at T< 140°C.TiCN EE-ALD was performed using sequential exposures of tetrakis(dimethylamido) titanium (TDMAT) and electrons together with the continuous presence of an ammonia (NH₃) reactive background gas (RBG) (Figure 1). The electrons can remove surface species via electron stimulated desorption (ESD).NH₃ can also be dissociated by the electron beam to produce H and NH_x radicals. The NH_x radicals may facilitate Ti nitridation and C removal from the film (Figure 2). The C content of the TiCN films can be tuned by varying the electron beam exposure time or the NH₃ RBG pressure.

Reducing the electron beam exposure reduces the production of active NH_x species and allows more carbon incorporation into the films during TiCN EE-ALD.Electron exposures were varied from 0.5 to 20 seconds. Electron exposures of 0.5 s resulted in a C:Ti ratio of 1:4, while electron exposures of 20 s resulted in a C:Ti ratio of 1:28, as measured by ex situ XPS (Figure 3).The resistivity of the films determined by 4-wavelength in situ ellipsometry is also affected by the carbon content.Films grown with a 0.5 s electron exposure had a resistivity of ~800 µΩ cm.Films deposited with a 20 s electron exposure time had a resistivity of ~200 µΩ cm.

TiCN films can also be grown by reducing the NH₃ partial pressure during TiCN EE-ALD.Lower NH₃ partial pressures lead to fewer active NH_x species present in the reactor and allow for the incorporation of more C into the films. NH₃ pressures were varied from 0 to 3.3 mTorr.Both the C:Ti ratio and the resistivity of the films varied with NH₃ pressure.The C:Ti ratio varied from 2:1 at no NH₃ to 1:10 at 3.3 mTorr NH₃, as measured by ex situ XPS.The resistivity varied from ~1000 µΩ cm for no NH₃ pressure to ~200 µΩ cm at an NH₃ pressure of 3.3 mTorr.

The use of a RBG during EE-ALD allows for compositional tuning of thin films.For example, TiN or TiO₂ films can be grown using TDMAT together with either NH₃ or O₂ RBGs.Varying the electron exposure or RBG pressure can also be utilized to control the film composition.Growth of binary or ternary ALD films is straightforward because EE-ALD with RBGs relies only on simple, volatile, gas phase precursors.

The atomic/molecular layer deposition (ALD/MLD) thin-film technique is strongly emerging as a unique designer’s tool for exciting new functional metal-organic materials. Current research demonstrates that this method is effective even at low deposition temperatures and capable of producing thin films that are highly stable and conformal. Stability and conformality are critical requirements of today’s 3D microelectronics and open vast potential for various industrial applications. In this context, we highlight the attractive characteristics of a new ALD/MLD process developed for Zn-organic thin films based on non-pyrophoric bis-3-(N,N-dimethylamino)propyl zinc [Zn(DMP)₂] and 1,4-benzene dithiol (BDT) precursors. This process yields air-stable Zn-BDT films with a notable growth rate of 4.5 Å/cycle at 60 °C. The reported process ability to produce highly conformal thin films was verified on lateral high-aspect-ratio (LHAR) test substrates.The Zn/S ratio (0.5) determined with RBS for a planar film was in alignment with the expected compositions. The depth-profile XPS analysis for a LHAR grown Zn-BDT film indicated a composition change along the cavity, which maybe arising from the different sticking coefficients of the precursors. The reported films were found to be electrically insulating and demonstrated remarkable stability under ambient conditions over extended storage periods, making them a promising candidate for barrier coating application in microelectronics. Also importantly, using state-of-the-art LHAR test structures we could demonstrate that by increasing the precursor pulse lengths it was possible to extend the essentially ideal film growth deep (PD=260 µm) into the high-aspect-ratio cavities (Figure 1).

Flexible thermoelectric (TE) heat-to-electricity conversion devices would be highly beneficial for wearable applications and also for any application in which the heat source is complex shape.[1] A TE device principle is simple, but it needs both n- and p-type semiconductor legs to function.

An efficient TE material should have simultaneously high electrical conductivity and low thermal conductivity, which is a challenge for conventional materials. The second challenge is that the currently employed inorganic TE materials are composed of heavy/rare/poisonous elements (Bi, Te, etc.), which are not environmentally sustainable. Simple metal oxides like ZnO are relevant material candidates, but their thermal conductivity is too high.

The combined atomic and molecular layer deposition (ALD/MLD) technique allows us to mix inorganic and organic precursor pulses in a sequential manner, thus enabling precise layer-engineered superlattice (SL) structures. In our previous works, we have deposited ZnO:organic SLs where monomolecular organic layers are embedded within nanoscale layers of the n-type ZnO semiconductor to block the phonon conduction at the resultant metal oxide/organic interfaces without affecting the electrical conductivity.[2] Organic layers also improve the flexibility of the film.

Here we present similar efforts for the yet-missing p-type semiconductor counterpart SnO. We use an amidinate-based tin(II) precursor which reacts well with water as the co-reactant under ALD conditions for the deposition of the SnO layers.[3] Thin films deposited at 220 °C results in phase pure SnO. Saturation for tin precursor happens at 12 s and for water at 2 s. The thickness of the films is measured by X-ray reflectivity (XRR) and the composition analyzed by Fourier-transform infrared spectroscopy (FTIR). For the organic component, we investigate various possibilities (hydroquinone, terephthalic acid, etc.); tentatively, terephthalic acid is found a promising organic component, as it has more oxidative character than e.g. hydroquinone.

References

[1] G. Marin, R. Funahashi & M. Karppinen, Textile-integrated ZnO-based thermoelectric device using atomic layer deposition, Advanced Engineering Materials22, 2000535 (2020).

[2] R. Ghiyasi, M. Milich, J. Tomko, P.E. Hopkins & M. Karppinen,Organic-component dependent thermal conductivity reduction in ALD/MLD grown ZnO:organic superlattice thin films, Applied Physics Letters118, 211903 (2021).

The information processing in actual human skin entails a system that dynamically integrates sensory compliance, threshold adjustment, and other complex nervous system functions, rather than being a mere collection of simple sensors. To attach AI diagnostic systems to the skin or, further, to mimic such systems for electronic skin, it is essential to produce neuromorphic hardware on flexible substrates. However, existing synthesis methods for MoS2, a material capable of superior and stable neuromorphic operation, involve high-temperature processes, posing significant limitations on direct growth onto flexible substrates.

In this study, MoS2 was directly synthesized onto flexible substrates via low-temperature plasma to fabricate a synaptic array and perform vector multiplication, which forms the basis of AI computations. The successful execution of vector multiplication was enabled by the rich grain boundaries of the synthesized MoS2, providing a stable resistive switching filament path, and demonstrated the maintenance of such hardware functionality even under bending.

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-2023-00235484, "Development of High Quality MX2 Materials and Processes through In-situ Defect Analysis") funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea)(1415187770)

Ruthenium (Ru) is highly regarded for its remarkable electrical conductivity, chemical stability, and catalytic activity, rendering it indispensable in electronic, energy, and catalysis applications. Nonetheless, the prohibitive cost associated with Ru precursor materials impedes its widespread utilization.

This study investigates the deposition of Ru films via Atomic Layer Deposition (ALD) to explore its potential for high-volume manufacturing. Employing batch processing of substrates enhances operational efficiency and cost-effectiveness. The Ru films demonstrate a growth per cycle (GPC) of 0.35 Å cy^−1 at 250 °C, with a density of 12.3 g/cm^3 and surface roughness below 0.7 nm. Electrical resistivity of all films falls below 15 μΩ cm, indicative of highly conductive films.

Grazing incidence X-ray diffraction (GI-XRD) analysis affirms the crystalline structure of the films as hexagonal P63/mmc metallic Ru, while scanning electron microscopy (SEM) underscores their excellent conformal coverage. Minimal impurity content, as evaluated through time-of-flight elastic recoil detection analysis (TOF-ERDA), is observed.

Scaling up the Ru deposition process yields a notable 14-fold enhancement in precursor efficiency, yielding films endowed with desirable physical properties. Thus, the exploration of Ru ALD not only tackles economic challenges but also paves the way for the integration of Ru-based materials into cutting-edge technologies, fostering innovation and progress across diverse domains.

References:

Aaltonen, T., Alén, P., Ritala, M. and Leskelä, M. (2003), Ruthenium Thin Films Grown by Atomic Layer Deposition. Chem. Vap. Deposition, 9: 45-49. https://doi.org/10.1002/cvde.200290007

Trivalent lanthanide ions (Ln³⁺) show attractive optical properties like luminescence emission from ultraviolet (UV) to visible and even near-infrared (NIR) range depending on the metal ion used. In addition, they can participate in a unique photon upconversion process that requires interaction between two or even more Ln³⁺ ions. Such Ln³⁺-doped materials could be implemented in various applications including solar cells, lasers, sensors, and bioimaging. The Ln³⁺ ions suffer, however, from weak light absorption and therefore inefficient direct excitation. Moreover, pure lanthanide oxides often display a concentration quenching, and in a consequence a lack of luminescence. These problems can be overcome by coupling species like organic ligands that can participate in energy transfer processes. In this sensitized luminescence, light is absorbed by the organic ligand, and energy is transferred to Ln³⁺ ions where the luminescence is generated. Furthermore, the presence of organic ligands can provide the separation between Ln³⁺ions to overcome the concentration quenching.

Molecular layer deposition (MLD) is a suitable tool to produce such luminescent organic-inorganic hybrid Ln³⁺-based materials with ultrahigh precision. This technique allows for accurate composition, thickness, and conformality control on a nanometer scale, which is specifically important for potential applications of luminescent materials as thin films and coatings. In this work, hybrid thin films were deposited using the β-diketonate Ln(thd)₃ precursors as a Ln³⁺ source (such as Nd³⁺, Eu³⁺, Tb³⁺, and Yb³⁺), and various aromatic dicarboxylic acids as organic linkers (including 1,4-benzenedicarboxylic acid and 2,6-naphthalenedicarboxylic acid). In addition, lanthanide fluorides (LnF₃) were incorporated into the films forming multilayer nanocomposite structures with more than one Ln³⁺ component showing strong luminescence emission. The final goal of this work is to create multilayer upconverting luminescent Ln³⁺-based materials.

Recently, there have been increasing efforts to develop thin film deposition of HEAs to improve the mechanical and thermal properties. One of well-established preparation technologies for HEA thin films is sputter deposition, which is known to be effective to accurately control the stoichiometry of HEA films by modulating various process parameters. To capture the effects of the sputtering parameters on the microstructure and mechanical properties of the film, the flow rate of Ar gas injected into the chamber (5, 7, and 8 sccm) was controlled. All films were identified as being of BCC phase with compositions of near equiatomic proportions, regardless of the gas flow rates. Nano-scale clusters were observed on the surfaces of all films, and nano-cracks were found in the film deposited at the Ar gas flow rate of 8 sccm, unlike the films deposited at the gas flow rates of 5 and 7 sccm. Detailed microstructural analysis of film deposition at an Ar gas flow rate of 8 sccm indicated that the void boundaries contribute to the formation of nano-cracks. The nano-indentation results indicated that the Ar gas flow rate 5 sccm specimen, with the smallest cluster size at the topmost surface, showed the highest hardness (12.21 ± 1.05 GPa) and Young’s modulus (188.1 ± 11 GPa) values.

The realm of two-dimensional (2D) dielectrics, instrumental in the advancement of electronics, offers unparalleled interface control, paving the way for the development of ultra-thin, high-performing electronic and optoelectronic devices. These materials, in synergy with 2D semiconductors, introduce devices boasting improved scalability, flexibility, and energy efficiency, pivotal for the miniaturization and performance enhancement of semiconductor technologies. Among these, Bi₂SeO₅ emerges as a frontrunner, characterized by its exceptional dielectric constant (κ ~ 16.5), remarkable thermal stability, and the absence of dangling bonds, which effectively reduces electron scattering at interfaces.

Previously, the fabrication of BiO_xSe_y has been confined to a select number of substrates, such as mica or STO, necessitating cumbersome transfer processes for practical device integration. Our research addresses this challenge by pioneering the growth of Bi₂SeO₅ on CMOS-compatible substrates including SiO₂ and TiN through the technique of Atomic Layer Deposition (ALD). This process facilitates direct chemical bonding between the precursor and the substrate, thereby inhibiting atomic migration. Utilizing precursors such as Bi(OR)₃, (RSi)₂Se, and O, we meticulously control injection and purge times to cultivate Bi₂O₃ and Bi₂Se₃ subcycles. These are combined into a super-cycle for the synthesis of Bi₂SeO₅, demonstrating an ALD specific self-limiting behavior.

In our endeavor, we have employed X-ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM) for comprehensive film characterization. The electrical properties were evaluated using Metal-Insulator-Metal (MIM) structured cells. Our findings not only confirm the direct growth feasibility of high-quality Bi₂SeO₅ films but also establish a foundation for their incorporation into sophisticated electronic devices, thereby narrowing the gap between material synthesis and practical application. This study underscores the potential of Bi₂SeO₅ in the evolution of electronic device fabrication, marking a significant milestone in the field of materials science.

TiO₂ has been intensively studied as a transparent conductive oxide and photoanode for photovoltaic and photocatalytic applications. The conductivity of TiO₂ can be improved by doping with group V elements such as Nb or Ta. However, effectively tailoring their properties by means of controlled doping amounts is challenging. Atomic layer deposition (ALD) offers an accurate tuning of the elemental composition by alternating binary ALD processes in a supercycle approach. This work aims at the supercycle deposition of Ta-doped TiO₂ by thermal ALD and the effect of doping and annealing conditions on the structural and electrical properties of Ta: TiO₂ thin films. Titanium (IV) isopropoxide (TTIP) and (tert.-Butylimido)-tris-(ethylmethylamido)-tantalum(V) (TBTEMTa) are used as titanium- and tantalum-precursors, respectively, and water as a co-reactant.For the characterization of the structural properties of the thin films, Raman spectroscopy and X-ray diffraction (XRD) are used. The elemental compositions of the films are characterized by X-ray photoelectron spectroscopy (XPS) and/or energy-dispersive X-ray spectroscopy (EDX). Finally, the electrical properties are investigated to study the effects of different doping concentrations. The results show that by using the supercycle atomic layer deposition we have successfully modified the electrical properties of TiO₂ by systematic doping with Ta, which makes it attractive for future applications.

Ferrocene is known for its stable and reversible redox properties which makes it a promising candidate for crafting precisely controlled redox-active layers via atomic layer deposition (ALD).^1,2 Current microelectronic manufacturing faces challenges with the inability to further scale to smaller features due to defects and edge placement errors with modern deposition techniques. This study explores the application of ferrocene-based precursors in selective vapor and solution deposition processes, aimed to develop redox-active organic layers to act as molecular binary switches for storing memory. Our approach uses ferrocene-appended molecules to achieve well-defined, high-quality layers on targeted metal substrates (Au or Cu) while avoiding non-growth regions. Small molecule synthesis, evaluation of precursor suitability, and characterization of films using Thermogravimetric Analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and Atomic Force Microscopy (AFM) techniques will be discussed.

(1) Ferrocene—Beauty and Function. Organometallics2013, 32 (20), 5623–5625.

(2) Parsons, G. N.; Clark, R. D. Area-Selective Deposition: Fundamentals, Applications, and Future Outlook. Chem. Mater.2020.

In an epsilon near zero (ENZ) material the real component of the permittivity vanishes at a specific frequency of light. Furthermore, when a light source, with a large enough intensity, near the ENZ frequency impinges on the material the refractive index is altered dramatically, the change is rapid and reversible - with the material being excited and returning to its original state on a timescale of a few picoseconds. This feature enables directional perfect absorption, large nonlinearity, near-zero refractive index, decoupling of electricity and magnetism, and infinite phase velocity. The driving component for the nonlinearities is believed to be due to the number of free electrons present in the material[1].

This study correlates the electrical (Hall, 4P/2P resistance), structural (EM, XRD, AFM), and compositional (XPS) characteristics of a series of AZO and GZO films to their ENZ outputs and investigates which growth parameters and corresponding properties demonstrate the greatest influence on the ENZ response. The optical data presented indicates an ability to tune the ENZ response towards the communication band. The development of efficient ENZ materials is a significant step towards producing efficient light-light interaction-based device technologies.

Metal-organic frameworks (MOFs) are a class of crystalline materials composed of metal ions/clusters as nodes bridged by organic moieties. These materials often exhibit remarkably high accessible specific surface areas, uniform and tunable pore volumes, and chemical modularity.MOFs have been synthesized mainly as powder or single crystals. However, to integrate these hybrid materials into microelectronic, thin films are needed. Different solution-based approaches, like layer-by-layer deposition,^[1] have been developed to grow MOF films but solvent contamination limits their applications like in electronics domains.^[2] Atomic/molecular layer deposition (ALD/MLD) approaches therefore circumvent this challenge as solvent-free MOF films with good quality, uniformity, and conformality are grown. In our group, we use ALD techniques to (a) directly deposit MOF thin films on solid substrates and (b) modify bulk MOFs by post-synthesis modification to adjust their properties, providing new functionalities while preserving the network structure.

(a) First direct growth of Copper-based MOFs by ALD/MLD is presented. Paddle-wheel frameworks, as copper terephthalate (Cu-TPA) and MOFs based on electrochemical active ligands as porphyrines, are successfully deposited using a home-made reactor. The obtained films are characterized using ellipsometry, UV-Visible, and IR spectroscopies, powder X-ray diffraction and scanning electron microscopy.

(b) To modify MOFs, in particular to incorporate metal cation in their reactive sites, vapor-phase infiltration (VPI) is also employed.^[5] Herein, post-synthesis modification of microporous porphyrin-based MOFs by VPI will be introduced Using pulses of a single precursor (diethylzinc or trimethylaluminium), isolated metallic entities are installed in the stable and microporous Zr(IV)-based porphyrinic MIL-173(Zr) bulk MOF. Indeed, two reactive sites are present: the porphyrin core (free base), and a phenolic group in the meta position of the linker. The structural characterizations and spectroscopic properties of the modified MOF are presented.

[1] Zhang & Chang, Processes, 8, 377 (2020).

[2] Lausund et al, Nat. Commun.7, 13578 (2016).

[3] Lee et al. Matter2, 404–415 (2020).

[4]Ahvenniemi & Karppinen, Chem. Commun.52, 1139–1142 (2016).

[5]. De et al.Inorg. Chem.59, 10129–10137 (2020).

The production of III/V semiconductors by Atomic Layer Deposition (ALD) faces significant obstacles that hinder their widespread application. The selection of suitable precursor materials proves challenging because they must react with III/V semiconductor surfaces in a self-limiting manner to meet the requirements of the ALD process [1]. Good results have been published using alkyl-silyl based precursors [2]. However, a crystalline structure has not been achieved, impacting the efficiency and reliability of the semiconductors. Crystalline semiconductors are important for electronic and optical applications.

In addition, the inadequate leak tightness of conventional ALD systems leads to high oxygen incorporation in the epitaxial layers, resulting in detrimental defects. Oxygen is a deep trap in the band gap of semiconductors that significantly reduce the lifetime of optical devices.

To overcome these obstacles, the Philipps-University Marburg and Dockweiler Chemicals collaborate jointly. The Philipps-University will use its extensive expertise in CVD processes to optimize processes and develop innovative techniques. For this purpose, a MOCVD system has been rebuilt by the Sempa Systems GmbH to enable processes in a leak-tight tool as required for deposition of high quality III/V semiconductors.

Dockweiler Chemicals will focus on developing tailored chemicals to enhance the properties of the deposited material, such as crystallinity. Through this collaboration between academic research and industrial innovation, we aim to jointly address the challenges of ALD-based III/V semiconductor manufacturing and open new perspectives for the semiconductor industry.

A promising material system to dive into ALD of III/V semiconductors is GaP, which is, for example, an ideal buffler layer for the deposition of compound semiconductors on silicon (Si) for various applications such as lasers on Si or tandem absorbers [3]. In particular, the nucleation of GaP on Si 300 mm (001) wafers requires a delicate process and is challenging to control using MOCVD. A self-limiting ALD process would inherently produce a uniform layer on arbitrarily large substrates. However, crystal quality of III/V semiconductors gown by ALD remain a challenge. First results using alkyl-silyl precursors will be compared with samples grown using standard MOCVD precursors in a close coupled shower head reactor.

1. Kääriäinen, T., Cameron, D., Kääriäinen, M.-L. & Sherman, A. , 2nd Edition. (Wiley, 2013).

2. Sarnet, T. et al.J Mater Chem C Mater4, 449–454 (2016) doi: 10.1039/C5TC03079J.

3. J Cryst Growth 315, 37–47 (2011) doi: 10.1016/j.jcrysgro.2010.10.036.

Besides extending EUV lithography (EUVL) technology by utilizing optic systems with numerical apertures exceeding 0.55 (higher-NA EUVL) or even surpassing 0.75 (hyper-NA EUVL), reducing lithography wavelength (λ) from 13.5 to 6.7 nm has been of interest to accommodate the extreme downscaling of device critical dimension [1]. Alongside advancements in light sources and optical systems, the adoption of novel resist platforms is an important key factor for the readiness of 6.7 nm-based technology due to a reduction in material sensitivity [2].

Herein, we investigate the potential of a Zn-based hybrid thin-film system that is synthesized via ALD analog as a part of the dry resist platform for beyond EUV (BEUV) lithography technology. The growth characteristics and material stability of Zn-based hybrid thin films are evaluated, with the highest growth rate observed at 75°C (Figure 1a). Upon being exposed to electrons with energy similar to those of photons at 6.7 nm wavelength (~180 eV), the Zn-based inorganic-organic hybrid thin films exhibited negative tone characteristics when developed using diluted KOH solution (Figure 1b). The low-energy electron beam lithography (EBL) will be employed to rapidly assess the material sensitivity, with height profiles obtained using an atomic force microscope (AFM). Furthermore, we extend our focus on understanding the chemical alternations of the Zn-based hybrid thin films when exposed to electrons with energy similar to or lower than those of BUEV photons. The experiment is conducted using our unique in-situ FTIR system equipped with an electron flood gun and a residual gas analyzer (RGA). The in-operando RGA spectrum (Figure 2) indicates H₂, H₂O, CO, and CO₂ as byproducts produced during the electron exposures. The crosslinked mechanisms of Zn-based inorganic organic hybrid thin films are elucidated through in-situ IR absorbance and in-operando RGA analysis. A comparison between Zn-based and Al-based hybrid thin film patterning characteristics will also be conducted. This study highlights the potential of novel metal-containing hybrid thin films as a part of the dry resist platform and offers valuable insight for the development of novel resist technology for beyond EUVL applications, thus contributing to the ongoing advancement of lithographic techniques.

This research is supported by SRC, the U.S. Department of Energy Office of Science Accelerate Initiative Award 2023-BNL-NC033-Fund, and NRF Korea.

[1] IRDS International Roadmap for Devices and System – 2023 Update Lithography & Patterning

[2] C. Anderson et al., Proc. SPIE 8322, 832212 (2017).

The logic of chemical synthesis (Nobel prize in Chemistry 1990) originally for organic molecules is nowadays observed in inorganic synthesis, especially for the latest electronic device, known as atomic layer deposition (ALD), selective-area deposition, and atomic layer etching. To extend the synthetic strategy toward synthesis of atomically precise dot supper-lattices which cannot be obtained by other methods like molecule beam epitaxy (Nobel prize in Physics 2000), we herein propose an oxide-based combination of ALD and atomically precise dot deposition [ref. 1-3], because we can expect the following four advantages: (1) the strong ionic covalent bonds of oxides keep high melting points even on nanoscale dots to maintain the atomically precise structure without protective ligands [ref. 2,4]; (2) ALD precursors easily chemi-adsorb on the bare surface of oxide dots; (3) the separated oxide dots due to the original organic spacers are conformally solvated by dielectric ALD oxides to show redox reactions as if in solution [ref. 5]; (4) the periods of phononic contrast between the oxide dots and amorphous ALD oxides matches the wavelength of THz phonon over room temperature, less than 1 nm, to confine heat conduction [ref. 6]. In this research, we first confirmed the linear thickness growth based on the number of super cycle, second demonstrated the redox response of oxide dot inside a flash memory structure [ref. 7], and finally evaluated the effect of ALD cycle on thermal conduction using the previous method [ref. 8]. These results suggest the multistep inorganic synthesis may be able to design a new type of electron-phonon interaction or coupling.

[ref. 1] Satoh et al. Nat. Nanotech. 2008, 3, 106–111.

[ref. 2] Satoh et al. Sci. Rep. 2013, 3, 1959.

[ref. 3] Satoh, Interdiscip. J. Chem., 2017, 1, 52-57.

[ref. 4] Satoh, Soft Matter Nanotechnology, Wiley-VCH, 2015, Chapter 8, 217–232.

[ref. 5] Satoh, Chem. Lett. 2014, 43, 629–630.

[ref. 6] Satoh, Solid State Phenomena, 2017, 257, 156-159.

[ref. 7] Farmer, Gordon. J. App. Phys. 2007, 101, 124503.

[ref. 8] Satoh et al. MRS Adv. 2020, 5, 481-487.

Purely inorganic, or organic thin films can be fabricated using either atomic or molecular layer deposition (ALD and MLD). By combining these two methods, one can fabricate hybrid thin films, which contain both organic and inorganic layers, and thus will drastically improve the applicability of the thin film technology. Even though this deposition method has been used extensively experimentally, the reaction paths occurring during the thin film deposition process are still widely unknown. Density functional theory (DFT) -based modeling answers this problem, as we can study the growth mechanisms of hybrid thin films on an atomic-level accuracy.

Hybrid thin films under investigation were studied using gas-phase and surface reaction models, of which examples are presented in Figure 1. The surface models consisted of two differently ethyl-saturated zinc oxide surfaces, which account for the imperfection of the thin film growth, and the adsorbing organic precursor 4-Aminophenol. The gas-phase models consisted of diethyl zinc and an organic molecule from a variety of aliphatic and aromatic compounds.

Modelling of the reaction pathways was conducted using the GPAW program. DFT was used with the PBE exchange and correlation functional. Van der Waals correction TS09 was used due to weak interactions caused by the ethyl ligands. All the reaction barriers were calculated using the nudged elastic band -method with a climbing image (CI-NEB).

4-Aminophenol reacts faster and more strongly with its hydroxyl than its amino group, which will dictate the alignment of 4-Aminophenol in the film structure. 4-Aminophenol adsorbed to the surface through physisorption, and after that, the hydrogen will migrate to ethyl ligand within a complicated branching reaction chain. This process, of which an example is illustrated in Figure 2, utilizes primarily a ligand exchange but also a dissociation reaction. The amount of ethyl ligands partaking in the growth reaction of the hybrid thin film was approximately 20 to 25 percent. Different macrostructures of these hybrid thin films were compared and the effect of oxygen on the growth reaction mechanisms was discovered.

Gas-phase models were utilized in the screening of over a dozen compounds to discover the relative reactivity of different functional groups present in aliphatic and aromatic organic precursors. Our results on both the reactivity and bond strength between different functional groups agree qualitatively with experimental results. Thus, this rapid screening process enables the feasibility prediction of the potential organic precursors, and it can be expanded to hundreds of precursors.

Vapour phase infiltration (VPI) has emerged as a promising technology for an enhanced range of options in the synthesis of metal-organic hybrid materials. In the field of polymeric gas separation membranes, a common problem is long-term stability, as filtered gases can lead to material deterioration or swelling [1]. In addition to increasing the membrane stability, VPI has also been shown to beneficially impact the separation performance, e.g., when infiltrating polymer of intrinsic microporosity 1 (PIM-1) with trimethylaluminium (TMA) [2].

In this study, VPI of poly(1-trimethylsilyl-1-propyne) (PTMSP) with TMA is investigated. PTMSP is chosen due to its exceptionally high free volume and its organic nature, featuring a carbon-carbon double bond as functional group. Saturation of the precursor inside the polymer is reached after already 60 s infiltration time inducing significant densification of the material as observed by transmission electron microscopy (TEM). Water contact angle measurements indicate a shift towards hydrophilic behaviour after infiltration. Depth profiling using time-of-flight secondary ion mass spectrometry (TOF-SIMS) shows accumulation of aluminium in the PTMSP polymer. However, significantly more aluminium accumulation is seen in the gradient layer between PTMSP polymer and SiO₂. The highest aluminium intensity is measured for the interfaces of PTMSP-gradient layer and gradient layer-SiO₂. A reaction pathway for the infiltration is proposed and supplemented by density functional theory (DFT) simulations. Infrared spectra obtained from experiments and simulation support the presented reaction pathway showing, among others, increases in CH₃ and OH intensities. In terms of membrane performance, a beneficial influence on selectivity is observed for infiltration times up to 1 s. Prolonged infiltration leads to greatly reduced permeance values, falling even below the detection limit of the measurement device. These results outline the versatility of the VPI technique, being applicable for both gas-barrier and membrane applications in the future.

[1]: N. A. Ahmad, C.P. Leo, A.L. Ahmad and M. Nur Izwanne, Separation and Purification Technology, 2019, 212, 941-951.

[2]: E. K. McGuinness, F. Zhang, Y. Ma, R. P. Lively and M. D. Losego, Chemistry of Materials, 2019, 31 (15), 5509-5518.

The kinetics of vapor phase infiltration (VPI) can be very complex due to various operational factors and intrinsic properties affecting precursor sorption and diffusion. These factors include the precursor’s vapor pressure, process temperature, precursor size, the density of reactive polymer groups, precursor-polymer reaction rates, and polymer free volume. Understanding the transport and reaction mechanisms is crucial for the successful optimization of VPI processes and their application to new precursor-polymer systems.

In this work, we investigate diethyl zinc (DEZ) infiltration into highly crosslinked polyacrylates, a class of materials that has not been previously studied for VPI. Our findings reveal that in such a densely crosslinked system, diffusion becomes the limiting factor. By copolymerizing trimethylolpropane triacrylate (TMPTA) with its ethoxylated counterpart (ETPTA), which has longer and more flexible side chains, we could significantly enhance DEZ diffusion, highlighting the role of polymer free volume in VPI kinetics. X-ray photoelrctron spectroscopy (XPS) depth profiles showed that while only partial Zn infiltration is achieved for a 200 nm homopolymer film of pTMPTA even after 50 h of DEZ exposure, adding 10 wt.% of ETPTA is enough to allow for full infiltration of the polymer layer. At 25 wt.% of ETPTA, the time required to achieve complete infiltration with DEZ is reduced to 15 h, and at 75 wt.% it is as short as 2 h. In a similar manner, reducing the UV polymerization time of the photopolymers and thus decreasing their crosslinking density also improves DEZ diffusion. The observed transport behavior is in good agreement with the theoretical reaction-diffusion model for VPI developed by Ren, McGuinness et al.[1].

The polyacrylate system studied in this work is of particular interest for VPI because its photopatternability opens new avenues for the creation of hybrid organic-inorganic nano- and microstructures by integrating VPI with existing polymer patterning techniques. As a proof of concept, we demonstrate the selective DEZ infiltration of a photolithographically patterned p(TMPTA-c-ETPTA) copolymer film on poly (methyl methacrylate) (pMMA). Energy-dispersive X-ray spectroscopy (EDX) elemental mapping and XPS depth profiles confirmed that Zn was present only in the patterned copolymer structures, but not in the poorly infiltrating pMMA.

Overall, our findings advance the understanding of VPI process kinetics, and lay an important foundation for applying VPI to highly crosslinked polyacrylates for top-down property modification of photopatternable polymer structures.

[1] Y. Ren, E.K. McGuinness, et al., Chem. of Mat. 33 (2021)

Membranes are used at large scale to purify surface or wastewater and are increasingly applied in nanofiltration (pore size range 1-10 nm) of organic solvents. However, mass organic solvent streams containing small molecules are still treated with traditional energy-intensive technologies like distillation. Here, polymeric membranes suffer from swelling, which can change pore sizes significantly due to solvent-membrane interaction. In contrast, ceramic membranes are hydrophilic and thus more robust yet challenging to reproduce commercially. In this study we have used Molecular Layer Deposition (MLD) to modify high-porosity Al₂O₃-based ceramic membranes with average pore size of 20 nm, by coating their internal surface with hybrid layers grown by MLD. Objectives were to 1) narrow the membranes’ pore size and pore size distribution by depositing a TiO₂ layer (acting as an intermediate) from TiCl₄ and EG (ethane-1,2-diol). 2) To investigate the hydrophilicity of hybrid layers grown by MLD from TiCl₄ and three different organic co-reactants, being heptanol, 3-aminopropyltriethoxysilane (APTES), and n-phenyltrimethoxy-silane (n-PTMS). Depositions were carried out at temperatures ranging from 100 to 200 °C, pulse times ranging from 0.1 to 0.4 sec for TiCl₄, 1 to 20 sec for the co-reactants, and 60 to 120 sec for N₂ purging. Typically, using MLD, the average pore size of the bare ceramic support (≈20 nm) could be reduced below 2 nm, as determined by permporometry, for 10 nm TiO₂ layer, see Fig. 1. The three hybrid layer types were also grown with identical process recipes on planar silicon substrates. This way, they could be tested on their surface hydrophilicity and chemical stability in different solvents (see Fig. 2),

• Surface hydrophilicity was measured by way of measuring the respective Water Contact Angle (WCA). These WCA values were 109° (heptanol), 106° (APTES), and 114° (n-PTMS).
• Chemical stability was measured by immersing the samples in solvents with decreasing polarity: water > acetone > ethyl acetate > hexane. The n-PTMS-based hybrid layer showed the most promising results:exhibiting excellent stability against hexane (non-polar), without layer degradation or change in its wetting performance (WCA constant at 114° after 24 hrs and seven days). While the heptanol- and APTES-based layers reacting with solvents lost hydrophobicity (WCA reducing to < 90°).

The results obtained so far provide new insights into the effect of using different organic co-reactants in MLD to optimize pore size and surface hydrophilicity to improve ceramic membranes’ performance in organic solvent nanofiltration.

The advancement in enhancing traditional organic or inorganic materials has reached a plateau, necessitating a significant step forward by concurrently exploiting the properties of both organic and inorganic materials. Over the last decade, vapor phase infiltration (VPI) has gained substantial attention as a versatile top-down approach for crafting hybrid materials. Various hybrid systems, incorporating a blend of metal oxide and polymer, have been developed. Of particular interest is In₂O₃, an essential component in materials employed for transparent electrodes such as indium tin oxide or indium zinc oxide, pivotal for the electronics industry.

Our research focuses on the creation of a hybrid material based on indium oxide and Parylene C and further polymers of technological relevance. The polymer in our approach serves as a dielectric and flexible polymeric matrix. To gain a deeper understanding of this hybrid material system, we examine the impact of the oxygen source on VPI of indium oxide into Parylene C using trimethylindium (TMIn) and three distinct oxygen-containing precursors, water (H₂O), hydrogen peroxide (H₂O₂) and ozone (O₃). Through the application of these processes to the polymer, we analyze the hybrid Parylene C/In_xO_y samples by transmission electron microscopy (TEM), depth-profiling with secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectrometry (XPS). The study contrasts the differences between the various precursor combinations.

Our findings reveal that employing hydrogen peroxide as the second precursor in a TMIn-based VPI process is advantageous compared to using water. The reactivity of H₂O₂ contributes to higher growth rates, deeper infiltration, and lower onset temperatures for crystallite formation of indium oxide in the TMIn/H₂O₂ process compared to TMIn/H₂O, while ozone degraded the polymer chains and a lowered its mechanical flexibility, limiting the applicability of TMIn/O₃ process. The resulting hybrids are conductive and exhibit sheet resistance values as low as 3 kΩ/□ without additional annealing. Furthermore, these hybrids display a surprising ability to heal ruptures and defects in the conductive layers on their surface. Given that Parylene C is a dielectric and indium oxide is a wide band gap semiconductor, the amalgamated flexible conductive hybrids hold great promise for applications in electronics, such as flexible electrodes in wearables, displays, solar cells, or medical sensors.

Metal oxide nanocoatings show strong application potential in many areas including anti-corrosion protection, surface engineering of polymers, medical materials, or electrode/electrolyte interfaces in all-solid-state batteries, to name just a few. Precise tuning of the functional properties of coatings, however, creates the need to fabricate materials that are more complex than binary ones. ALD is an excellent tool for fabrication such conformal multi-component thin-film coatings, enabling strict control of their thickness and composition or doping profiles. However, the ALD of such complex layers is usually far from satisfying the rule of mixtures and does not provide a linear dependence for growth-per-cycle and composition on the relevant parameters specific to the growth of binary materials. Therefore, surface engineering using multi-component ALD oxides requires in-depth experimental knowledge on layer growth using various substrates.

For our research we used ALD Beneq TFS-200 equipment. We experimentally tested thermal ALD of Al₂O₃, ZrO₂ and ZnO thin films, as well as their mixtures (three different ternaries) and nanolaminates. For deposition of complex films we used a supercycle approach. The processes temperature range was 100-300°C. The films were generally only several tens of nanometers thick, and most were less than 20 nm. Silicon and gallium arsenide were used as substrates. We used trimethylaluminium, tetrakis(ethylmethylamino)zirconium (IV) and diethylzinc as chemical precursors for Al, Zr and Zn, respectively. We alternatively used deionized water or ozone as oxygen precursors. We proved the possibility of controlling the thickness and composition, though they typically deviated from the properties expected by the rule of mixtures. Thickness and composition of the films were analysed by a combination of spectroscopic ellipsometry, transmission electron microscopy energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. For selected films we analysed also coating tightness, surface topography and electrical properties using atomic force microscopy (by tapping mode and conductive AFM).

Acknowledgements:

Research financed under the ARISER project (Large Area Magnetron Sputtered All-Solid-State Batteries with ALD Buffer Layers), contract number: M-ERA.NET3/2021/99/ARISER/2022, KLAWIATURY project, contract number: POIR.01.01.01-00-2438/20-00, and subsidy of Łukasiewicz Research Network.

Taivo Jõgiaas, Oliver Vanker, Toomas Daniel Viskus, Sergei Tolbin, Aivar Tarre, Kaupo Kukli

Institute of Physics, University of Tartu, W. Ostwald str. 1, EE50411 Tartu, ESTONIA

Silicon oxide can be deposited from various sources using atomic layer deposition, but a comprehensive comparison of related resultant thin film properties is hard to find. A silicon source exploited, providing effective growth, has been organometallic compound hexakis(ethylamino)disilane (AHEAD) [1-3]. Oxygen source has been water, ozone or oxygen plasma.

In this work we compare some physical properties of SiO₂, such as density, refractive index, resistive switching, hardness and Young’s modulus of ALD deposited thin films and a thermally grown one. Two different processes were used for ALD SiO₂ films. Thermal ALD process used AHEAD and ozone as precursors and O₂ plasma process used AHEAD and O₂ plasma. The ozone process was also characterized in more detail by using quartz crystal micro-balance.

As an example of the results, mechanical characterization using instrumented nano-indentation was performed. The thermally grown SiO₂ had about 25 % higher hardness and modulus compared to fused quartz glass (the calibration standard for the indentation device). The SiO₂ from AHEAD-ozone process on the other hand showed approximately 2-3 times lower hardness and 20-40 % lower modulus compared to fused quarts glass, somewhat depending on the ALD growth temperature. The difference these two SiO₂ films could indicate internal stresses in the films, which influence mechanical behaviour of materials.

From applications point of view, the internal stresses in layers used in a device or structure may not regarded as advantageous. For instance, if crystalline layered structures were sandwiched between such stressed layers, occasional relieve of the stress could affect the crystalline structure and cause device/structure malfunctions. Complementarily, other results like resistive switching properties will be reported.

The conventional preparation of thin film composite (TFC) reverse osmosis membranes involves interfacial polymerization (IP) atop a porous, polymeric support. This process can create selective polyamide films 100+ nm thick, with complex, yet relatively inhomogeneous morphologies. While commercially successful, IP is limited in its ability to control the selective layer composition, thickness and morphology which hinders transport properties and operational lifetime.

To overcome the limitations, we have developed TFC reverse osmosis membranes using all-organic molecular layer deposition (MLD) rather than IP. Key to fabrication was the ability to grow the MLD film across the pore openings of a porous substrate despite the conformal nature of MLD. To do so, we developed a sacrificial polyvinyl alcohol (PVA) pore filling process that enabled growth atop porous polyethersulfone supports while avoiding undesirable infiltration effects. Selective polyamide MLD films were made from m-phenylenediamine and trimesoyl chloride precursors at 3 Å/cy, 120°C. Vapor phase infiltration of the organic precursors into the PVA was investigated with in-situ ellipsometry. In crossflow desalination tests, MLD TFC membranes had improved water flux and NaCl rejection compared to commercial IP membranes. Improvements in performance were attributed to differences in the synthesized materials: compared to IP, MLD films were dense, highly crosslinked, and morphologically conformal, without nanoscale voids. Furthermore, a tradeoff between flux and salt rejection could be tuned by varying the thickness and composition of the MLD selective layer. This research has broad implications for this and other applications which use TFCs, such as gas separations and nanofiltration.

Even though semiconductor industry is doing a great job in tearing down scaling walls to increase the longevity of traditional Si-technology, new materials solutions are inevitably needed to harness the greatness promised by future ICT. This includes true edge computing for the ultra-connected smart society, as well as novel devices for neuromorphic computing. It is only natural that different technologies and device architectures are developed for different use - and with this more dedicated materials solutions for different applications.

These new materials will still need to adhere to many of the same processing constraints that we know from Si-ICT, such as low thermal budget, extreme conformality and thickness control - making ALD the most (if not the only) viable synthesis technique also as we look towards the next generations.

In this work we present how we are purposefully crafting an ALD catalogue of complex oxides with a wide range of functional properties. This includes materials that exhibit ferromagnetic, ferroelectric, metallic and semiconducting behavior, as well as combinations of these functionalities. We show how the quirks of ALD-processes and precursors can be exploited to enable low-temperature epitaxy of these materials. We point at cation substitution with high compositional control, strain engineering by careful choice of substrate and strategic process design to enable high quality crystal growth.

Materials such as RENiO₃ (RE = La - Lu), BaTiO₃, NiFe₂O₄ and BaSnO₃ are used as examples of materials with different functionalities, and where distinct ALD process design is needed to facilitate crystal growth. We introduce some ideas that aid ALD process design for complex oxides - letting us quickly tune in to optimal conditions.

We strongly advocate for the pivotal role of ALD in advancing ICT through the utilization of dedicated materials for emerging technology paradigms. However, this expansion imposes new challenges on ALD process development, necessitating innovative solutions. Through this work, we address these challenges and propose strategies to overcome them.

Molecular layer deposition (MLD) of siloxane-based polymers can be highly useful because these polymers (for example, polydimethylsiloxane) have vast applications in different fields. However, previous literature reported that MLD of siloxane-based polymer using homo-bifunctional siloxane-bridged silane molecules was ineffective due to the lack of film growth.¹ In this work, the growth of siloxane-based polymers in MLD using siloxane-bridged (Si-O-Si) silane precursor and water was investigated and compared to the growth of carbosilane-siloxane film using methylene-bridged (Si-CH₂-Si) silane precursor and water. Our results show that the film deposited with methylene-bridge precursor can grow beyond 1 nm whereas the film deposited with siloxane-bridged ultimately stops growing after the film thickness reaches only a few angstroms. This finding suggests that the rigidity of the bond angle between the functional groups may play a role in mitigating the occurrence of “double reaction” – that is, the functional groups on both ends of the precursor react with the surface of the substrate – which results in promoting film growth. The findings reported in this work also provide insights into the growth mechanism of ultra-thin organic thin films.

1: Pinna, N; Knez, M.; Atomic Layer Deposition of Nanostructured Materials; Wiley-VCH, 2011, p. 99-102

Today’s nanofabrication techniques require multistep and costly processes in order to fabricate complex, multi-material nanostructures. Performing atomic layer deposition (ALD) using two different organometallic precursors within polymeric templates can offer a new pathway for doped three-dimensional nanostructures fabrication.

In recent years, sequential infiltration synthesis (SIS), an ALD-derived process, has been shown to enable inorganic materials growth within polymers. Sequential polymer removal results in a polymer-templated inorganic nanostructure. While SIS shows great potential in fabricating a large variety of structures and materials, it is currently limited in its ability to fabricate composite materials with precise control over the ratios between the two materials.

In our research, we utilized SIS properties to achieve doped materials. We demonstrated a simultaneous growth of two different metal oxides using a co-injection SIS process within polymeric films. Tuning the partial pressure of each precursor through the pulse duration allowed us to control the ratio between the two materials, thus controlling the dopant percentage. We used epoxidized isoprene films as the polymeric template and DEZ (diethyl zinc), and TMA (trimethyl aluminum) as the organometallic precursors to fabricate Al-doped ZnO films. We performed structural characterization using scanning and transmission electron microscopy (SEM and TEM, respectively) to characterize the films and energy-dispersive X-ray spectroscopy (EDS) TEM to quantify each element and evaluate the doping percentage. To demonstrate the fabrication of 3D doped nanostructures we applied a co-injection SIS process on polystyrene-block-poly(epoxyisoprene) (PS-b-EPI) and achieved selective growth within the EPI domain. Following polymer removal a three-dimensional doped nanostructures was achieved. Co-injection SIS offers a simple solution for three-dimensional doped and composite nano-materials in a direct process, while still offering us precise control over the ratio between two materials.

In the last few decades, there has been a phenomenal rise and progress in the field of III–Nitride semiconductors. Now, the materials group III-N is changing the path of high-performance integrated circuits (IC) technology in high power and high frequency regimes and has received much attention due to its wide and direct bandgap, high electron mobility and high breakdown electrical field.

III-N semiconductors play a prominent role in the industry, primarily through chemical vapor deposition (CVD) techniques like Metalorganic Vapour-Phase Epitaxy (MOVPE). These conventional deposition methods typically involve the reaction of ammonia (NH3) with industrially relevant precursors such as trimethylaluminum (TMA), trimethylgallium (TMG) at elevated temperatures (750-900 °C) [1].
Plasma-assisted Atomic Layer Deposition is emerging as a reliable alternative to standard CVD techniques. It presents itself as a solution for achieving highly conformal coatings and low-temperature processing.

This work is based on the optimization and engineering of the plasma ALD of c-axis highly oriented aluminium nitride (AlN) films we proposed [2]. The pathway was further extended to facilitate low temperature (<450 °C) deposition of other III-N materials, such as GaN and InN semiconductors with wide and small bandgap respectively. In this work we utilized a gas mixture of H2/Ar/N2 as nitrogen precursor for nitride thin films initiated by a plasma source. The mix of Ar and H2 allows both to stabilize the plasma phase and to induce an optimized reducing of the ligands of the organometallic precursors reducing the carbon contaminants measured in the materials. We observed by XPS stoichiometric III-N films with no detectable carbon contaminants and also a low level (<5%) of oxygen contaminants in the nitride films. The XRD and TEM analysis confirm privileged hexagonal crystalline structure of the thin film below 100 nm thickness.
Insights on the ALD set-up and specific sequence of the deposition process will be presented for the growth of aluminium nitride (AlN), gallium nitride (GaN), indium nitride (InN), and ternary III-N films.

Those results have the potential to pave a way for both buffer-oriented seed films for post-growth, and also films with tailor made electronics properties for the next-generation of III-Nitride/CMOS components and functional coatings with piezoelectric and semiconducting properties for MEMS applications with transducing capabilities.