Atomic Layer Deposition Of Metal Oxides Research Articles

Parylene-AlOx Stacks for Improved 3D Encapsulation Solutions

The demand for ultra-tight encapsulation solutions with excellent barrier and high conformality properties has increased in recent years. To meet these challenges, thin-film barrier coatings have emerged as a promising solution. In this study, we investigate well-established silicon-based plasma-enhanced chemical vapor deposition (PECVD) and metal oxide atomic layer deposition (ALD) barrier coatings deposited at low temperatures (≤100 °C) regarding their abilities to address high-level 3D encapsulation applications. Various combinations of such layers are evaluated by measuring the water vapor transmission rate (WVTR) and considering the conformality properties. The impact and the benefits of the organic film integration, namely parylene VT4 grade, on the barrier performances is assessed. Among these combinations, parylene-AlOx stack emerges as one of the most effective solutions, obtaining a WVTR of 3.1 × 10−4 g m−2 day−1 at 38 °C and 90% relative humidity conditions.

Atomic layer deposition modified PIM-1 membranes for improved CO2 separation: A comparative study on the microstructure-performance relationships

The use of atomic layer deposition (ALD) technology has recently been extended to membrane-based gas separation, whereas how the ALD of metal oxides can tailor the membrane microporosity, and the contribution of the interactions between ALD-grown metal oxide clusters and gas molecules to the resultant gas separation performance, remain largely unclear. Here, a comparative study was performed by individually confining three different metal oxides, including Al2O3, ZnO, and TiO2, to the near-surface region of polymers of intrinsic microporosity (PIM-1) membranes in an attempt to clarify the relationships between microstructural properties and CO2 separation performance. Based on experimental characterizations and density functional theory (DFT) calculations, it is suggested that ALD of metal oxides on PIM-1 membranes can not only effectively narrow the micropore sizes, but also enable enhancement of the solubility of CO2 molecules. Meanwhile, gas permeation results revealed that the CO2 permeation behavior through either Al2O3- or ZnO-modified membranes is diffusion-dominated, while that of the TiO2-modified membranes is sorption-dominated, primarily due to the distinct differences in the loading amount of metal oxides and their interaction strength with the CO2 molecules. As a result, the TiO2-modified membranes achieved a significant increase in the CO2 permeability as the ALD cycle numbers increased together with no sacrifice of CO2/N2 and CO2/CH4 selectivities. This study is expected to lend important insight to the development of ALD-modified membranes for efficient CO2 separation.

Tailoring the Interfacial Interactions of Porous Polymer Membranes to Accelerate Atomic Layer Deposition: The Latent Path to Antifouling Membranes

Atomic layer deposition (ALD) is a powerful strategy to engineer hybrid organic-inorganic membranes with emergent functionalities. The combination of atomic-level thickness control, wide materials palette, and unprecedented conformality allow the physiochemical properties (e.g., hydrophilicity) of mesoporous polymer membranes to be precisely tuned. The nucleation of ALD materials growth on polymer surfaces relies on chemical interactions between the ALD metalorganic precursor and functional groups in the polymer structure and these interactions dictate the number of ALD cycles required to achieve a continuous coating. Strategies to enhance these interactions could enable desirable properties such as anti-fouling behavior to be imparted on inert polymer surfaces that lack the necessary functional groups for ALD nucleation. In this study, we demonstrate that the reactivity of polyacrylonitrile (PAN) membranes towards ALD metal oxide (MO) precursors with Lewis acid characteristics is enhanced by introducing Lewis base functional groups (amidoxime: Am) on the PAN backbone. The resulting Lewis acid-base interactions accelerates the MO nucleation in Am-PAN and reduce the number of deposition cycles required to achieve hydrophilicity compared to the untreated PAN membrane. Unveiling the reaction mechanism, the in-situ FTIR intensity changes established enhanced interaction dynamics between the ALD MO precursors and the Am-PAN membrane, unlike the PAN membrane. For similar MO cycles, through both spectroscopic and thermogravimetric analysis, we observe enhanced MO loading in the Am-PAN membrane compared to the PAN membrane. Here we have verified that strong Lewis acid-base interactions led to enhanced loading for a range of ALD MO materials including Al2O3, TiO2, SnO2, and ZnO. Most importantly, the Al2O3-Am-PAN hybrid membrane showed 23.3% higher antifouling capability compared to the pristine PAN membrane. Our approach expands the scope of design options for fouling-resistant porous hybrid inorganic-organic membranes and may reduce manufacturing costs of water treatment membranes.

In-Situ Resistance Optimization for ALD Nanocomposite Resistive Materials

Atomic layer deposition (ALD) made resistive nanocomposite materials can be used in many emerging applications such as charge draining layers for electron optic and MEMS devices, functional resistive layers in electron/ion amplifying microchannel plates (MCPs) which are used in photodetectors and mass spectrometry, HF resistance with tunable electrical conductivity coatings for stabilizing Li-ion battery cathode materials, selective solar absorbing coatings for concentrated solar power (CSP), and barrier and encapsulation coatings. Over the last decade, we have worked broadly on the research and development of resistive nanocomposite coatings. In this context, we have developed many nanocomposite materials with tunable resistance based on mixing conducting ALD metal and insulating ALD metal oxide or metal fluoride processes. These ALD nanocomposite materials show robust material performance in many of the applications listed above. Here we are particularly targeting a functional resistive coating that now commercially used for manufacturing large area ALD-MCPs. For this, there are two key topics that we are addressing a) Minimum thickness requirement for resistive coating which provides a stable targeted resistance behavior for MCPs, and b) Can we tune the temperature coefficient of resistance (TCR) of the ALD nanocomposite materials so that the ALD MCPs can operate over a wide temperature range without affecting their performance. To address these two topics, we have utilized in-situ resistance monitoring during the ALD nanocomposite process. With this method we have optimized resistive nanocomposite material ALD processes e.g. (M:Al2O3 where M= W and Mo) for MCPs and also targeted the desired TCR of the materials and achieved ultra-low TCR values. We believe these optimized coatings will reduce the ALD-MCP processing time substantially and enable ALD-MCPs and photodetectors to operate over a broader temperature range compared to existing ALD-MCPs.

Atomic Layer Deposition of Metal Oxides and Chalcogenides for High Performance Transistors.

Atomic layer deposition (ALD) is a deposition technique well‐suited to produce high‐quality thin film materials at the nanoscale for applications in transistors. This review comprehensively describes the latest developments in ALD of metal oxides (MOs) and chalcogenides with tunable bandgaps, compositions, and nanostructures for the fabrication of high‐performance field‐effect transistors. By ALD various n‐type and p‐type MOs, including binary and multinary semiconductors, can be deposited and applied as channel materials, transparent electrodes, or electrode interlayers for improving charge‐transport and switching properties of transistors. On the other hand, MO insulators by ALD are applied as dielectrics or protecting/encapsulating layers for enhancing device performance and stability. Metal chalcogenide semiconductors and their heterostructures made by ALD have shown great promise as novel building blocks to fabricate single channel or heterojunction materials in transistors. By correlating the device performance to the structural and chemical properties of the ALD materials, clear structure–property relations can be proposed, which can help to design better‐performing transistors. Finally, a brief concluding remark on these ALD materials and devices is presented, with insights into upcoming opportunities and challenges for future electronics and integrated applications.

Surface Engineering Suppresses the Failure of Biphasic Sodium Layered Cathode for High Performance Sodium‐Ion Batteries

AbstractIn the process of upgrading energy storage structures, sodium‐ion batteries (SIBs) are regarded as the most promising candidates for large‐scale grid storage systems. However, the difficulty in further improving their specific capacity and lifespan has become a major obstacle to promoting extensive application. Herein, by optimizing synthesis conditions, a biphasic‐Na2/3Ni1/3Mn2/3O2 cathode that exhibits an ultrahigh capacity of ≈200 mAh g‐1 without the involvement of anion redox reactions is successfully synthesized. Nevertheless, there is significant electrochemical performance degradation because of failure at the cathode‐electrolyte interface as revealed by comprehensive analyses. Further in‐depth research proves that the surface side reactions that occur at high operating voltages and the transition metal dissolution that occurs in low voltage are the root causes of electrode surface failure. Therefore, the metal oxide atomic layer deposition (ALD) protective layer is deliberately chosen to suppress such failures. The coating effectively blocks corrosion of the cathode material by the electrolyte and successfully anchors the transition metal ions on the particle surface. As a result, the cycle stability and rate performance of the electrode are improved considerably. This surface engineering strategy could provide concepts with broad applicability for suppressing the failure of sodium layered cathodes.

Re-examination of the Aqueous Stability of Atomic Layer Deposited (ALD) Amorphous Alumina (Al2O3) Thin Films and the Use of a Postdeposition Air Plasma Anneal to Enhance Stability.

Amorphous aluminum oxide (alumina) thin films are of interest as inert chemical barriers for various applications. However, the existing literature on the aqueous stability of atomic layer deposited (ALD) amorphous alumina thin films remains incomplete and, in some cases, inconsistent. Because these films have a metastable amorphous structure─which is likely partially hydrated in the as-deposited state─hydration and degradation behavior likely deviate from what is expected for the equilibrium, crystalline Al2O3 phase. Deposition conditions and the aqueous solution composition (ion content) appear to influence the reactivity and stability of amorphous ALD alumina films, but a full understanding of why these alumina films hydrate, solvate, and/or dissolve in near-neutral pH = 7 conditions, for which crystalline Al2O3 is expected to be stable, remains unsolved. In this work, we conduct an extensive X-ray photoelectron spectroscopy investigation of the surface chemistry as a function of water immersion time to reveal the formation of oxyhydroxide (AlOOH), hydroxide (Al(OH)3), and possible carbonate species. We further show that brief postdeposition exposures of these ALD alumina films to an air plasma anneal can significantly enhance the film's stability in near-neutral pH aqueous conditions. The simplicity and effectiveness of this plasma treatment may provide a new alternative to thermal annealing and capping treatments typically used to promote aqueous stability of low-temperature ALD metal oxide barrier layers.

Molecular Layer Deposition for Stable Lithium Metal Anode: From Composition Design to Mechanism Study

Energy storage systems with higher gravimetric energy density is highly desirable to fulfil the increasing requirement for various applications. One of the most promising direction is the development of next-generation batteries. Lithium (Li) metal is considered as the ‘Holy Grail’ for the anode materials owing to its ultrahigh capacity and low electrochemical potential [1]. Nevertheless, the application of Li metal as anode material has long been hindered due to some great challenges, such as poor cycling stability and short life span. The serious side reaction between metallic Li and organic electrolyte results in the formation of solid electrolyte interphase (SEI). On one hand, the SEI serves as a passivation layer on the surface which partially prevents further side reaction. On the other hands, the undesired dendrite growth cannot be prevented by SEI due to its heterogeneous and unstable properties [2]. Modification on the interphase to tune and stabilize its property has been considered as an approach to improve the cycling performance of Li metal anode. Recent studies on stabilizing SEI for Li metal anode can be divided into two categories, in-situ and ex-situ approach. In-situ approach refers to the design of electrolyte using electrolyte additives or adjusting the concentration of Li salt. Ex-situ approach refers to the construction of surface coating, including metal oxides, phosphates, and polymers on Li prior to electrochemical cycling [3]. An ideal SEI requires high ionic conductivity, good mechanical property, thin thickness, and high chemical stability. Nevertheless, it is still challenging to fulfil all the requirements of an ideal SEI for long-life and stable Li metal anode.Recent studies have demonstrated the application of Atomic layer deposition (ALD) and Molecular layer deposition (MLD) on the interphase modification of batteries. As a coating technique, ALD is can be applied to deposit inorganic materials such as various metal oxides. It shows unique advantages such as low depositing temperature, uniform film morphology and precise control over thickness in nanoscale [4-6]. As a derivative technique to ALD, MLD is widely used to fabricated inorganic-organic hybrid or pure polymer coatings. MLD still possesses the advantages of ALD and provides additional advantages such as tuneable mechanical property and adjustable coating composition. Our group previously reported ALD metal oxide and MLD metalcone as artificial SEI for metallic Li anodes [7-8].In this report, we demonstrate a type of MLD Polyurea (PU) coatings for metallic Li to modify the SEI layer [9-10]. The cycling life and stability has been improved upon the application of MLD PU. The surface chemistry of PU on Li is characterized by TOF-SIMS and XPS to investigate the evolution of SEI and the role of polymer coating. The results showed that MLD coating can effectively suppress the dendrite growth and remain stable under repeated Li plating/stripping. Furthermore, the nitrogen-containing polar groups in PU can effectively regulate the Li-ion flux and lead to a uniform Li deposition. In our following study, aluminium crosslinkers have been introduced into PU to achieve a hybrid nanoscale polymeric protective film with tuneable composition and improved stiffness. Owing to the improvement in the mechanical property, the protected Li can deliver much stable performance for more than 350 h with a higher cycling capacity of 2 mAh cm−2 without a notable increase in overpotential. Moreover, a stable charge/discharge cycling in Li–O2 batteries with the protected Li can be maintained for more than 600 h. Our study on MLD protected Li provides guidance on the rational design of electrode interfaces and opens new opportunities for the fabrication of next‐generation energy storage systems.[1] D. Lin, Y. Cui, et al, Nature Nanotechnology, 2017, 12, 194-206[2] X. Cheng, Q. Zhang, et al, Chemical Reviews, 2017, 117, 10403-10473[3] X. Cheng, Q. Zhang, et al, Adv. Sci., 2016, 3, 1500213[4] Y. Zhao, X. Sun, ACS Energy Lett. 2018, 3, 4, 899–914[5] Y. Zhao, X. Sun, et al, Joule, 2018, 2, 12, 2358-2604[6] Y. Zhao, X. Sun, et al, Chem. Soc. Rev., 2021, 50, 3889-3956[7] Y. Zhao, X. Sun, et al, Small Methods, 2018, 2, 1700417[8] K. R. Adair, X. Sun et al, Angew. Chem. Int. Ed., 2019, 58, 15797.[9] Y. Sun, X. Sun, et al, Adv. Mater., 2019, 31, 1806541[10] Y. Sun, X. Sun, et al, Adv. Energy Mater., 2020, 10, 2001139.

Vapor-Phase Grafting of Functional Silanes to Al2O3, ZnO, and TiO2 Surfaces Prepared By Atomic Layer Deposition

Atomic layer deposition (ALD) is a highly versatile surface functionalization technique that can conformally coat both planar and porous substrates. Surface modification of ALD films with organic species is an ideal way to tailor the surface properties for applications such as water treatment, sensors, gas capture and drug delivery. Here we demonstrate ALD metal oxide layers to establish a well-defined starting surface for vapor-phase surface organic functionalization. We investigated the surface chemistry of ALD Al2O3, TiO2, and ZnO and the grafting of six different bifunctional silanes at 100 ℃, 150 ℃ and 200 ℃. In situ quartz crystal microbalance (QCM) and Fourier-transform infrared (FTIR) spectroscopy measurements, and ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements show the uniform monolayer silane formation through self-limiting reaction. We observed a higher surface reactivity of grafted silane species at 100 ℃ compared to 200 ℃, which is attributed to temperature-dependent reactivity of the surface hydroxyls and the type of silane binding. The FTIR and XPS measurements also reveal that silanes react differently depending on the metal oxide surfaces.

Vapor-phase grafting of a model aminosilane compound to Al2O3, ZnO, and TiO2 surfaces prepared by atomic layer deposition

Atomic layer deposition (ALD) is a highly versatile surface functionalization technique that can conformally coat both planar and porous substrates. Here we use ALD metal oxide layers to establish a well-defined starting surface for vapor-phase surface organic modification. Vapor-phase (3-aminopropyl)triethoxysilane (APTES) surface silanization of ALD Al2O3, ZnO and TiO2 surfaces were studied at 100 °C, 150 °C and 200 °C. In situ quartz crystal microbalance (QCM) and Fourier-transform infrared (FTIR) spectroscopy measurements, and ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements showed uniform monolayer silane formation through self-limiting APTES reaction. We observed a higher surface density of grafted APTES species following silanization at 100 °C compared to 200 °C, and we attribute this to the temperature-dependent reactivity of the surface hydroxyls and changes in the mode of APTES reaction. The FTIR and XPS measurements revealed that APTES reacts with Al2O3 and ZnO exclusively through metal siloxy bond formation. However, APTES reacts with TiO2 through both siloxy bond formation and ammonium salt formation via the amine group.

Optimal design of novel precursor materials for the atomic layer deposition using computer-aided molecular design

Nanomaterials and nanostructures with multi-functional properties found widespread applications such as electronics, optics, and coatings that can be fabricated using Atomic Layer Deposition (ALD). ALD is a vapor phase deposition technique to generate thin films of metals and metal oxides on a substrate. In this process, a precursor, which often comprises of organic functional groups that surround the depositing metal, chemisorbs on the substrate or reacts with the surface sites and with each other. Precursor chemisorption on the substrate leads ALD to be a self-limiting process. Thus, the precursor(s) should be chosen in a way to enhance deposition based on the ALD conditions. For a given application, it is practically impossible to carry out a large number of experiments using numerous precursors with varied deposition conditions to find the one that maximizes the growth rate in ALD. In addition, only existing precursors can be tested experimentally. The overall objective of this work is to develop a computational tool for the in-silico design of precursor materials using adsorbate solid solution theory (ASST). In the first part of this paper, we apply the ASST to derive properties of the functional groups present in the precursor using a new Group Contribution Method (GCM). The GCM is successfully derived and compared with the experimental data from ALD. The method shows good agreement and is useful for the design of novel materials. In the second part of this paper, using the thermodynamic properties as obtained from GCM, we develop a computer-aided molecular design (CAMD) framework for the optimal design of novel precursor materials with enhanced deposition properties for the ALD of metal oxides and metals. The metaheuristic efficient ant colony optimization algorithm (EACO) developed in-house is used for both parts. CAMD is a combinatorial optimization methodology, where molecules with optimal desired properties are generated from functional groups. The precursor selection optimization problem is posed as a mixed integer nonlinear programming problem, which is solved using EACO. The ALD growth kinetics is used as an objective of optimization and a solution is validated with thermodynamic constraints. Along with the number of groups, the temperature is also included in the optimization framework as decision variables. Forty-one novel titanium precursor molecular structures for ALD are generated with growth rates ranging from 1.23 Å/cycle to as high as 1.65 Å/cycle. Thus, these precursors have shown growth rates higher than the known titanium precursors. ALD growth rate is found to be a function of the combination of the precursor functional groups as well as temperature with a complex correlation among them.

Ιn situ N2-NH3 plasma pre-treatment of silicon substrate enhances the initial growth and restricts the substrate oxidation during alumina ALD

The initial substrate inhibiting island growth and the formation of an interfacial layer with uncontrollable characteristics are the two main drawbacks of the Atomic Layer Deposition (ALD) of high-k metal-oxide gate dielectrics on silicon (Si). In this paper, we investigate the ALD of Al2O3 films from trimethyl aluminum and H2O, on fluorhydric acid (HF) cleaned, as well as on HF-cleaned and in situ N2-NH3 plasma pretreated Si between 0 and 75 cycles. The films and their interface were characterized via Scanning Transmission Electron Microscopy coupled to Energy-Dispersive X-ray spectroscopy. The initial deposition is clearly increased on the pretreated surfaces, obtaining a linear ALD regime even after 5 ALD cycles, compared to several tens of cycles needed on HF-cleaned Si. Furthermore, a SixNy layer is formed by the N2-NH3 plasma pretreatment, which acts as a barrier layer, reducing the oxidation of the Si substrate beneath it. This analysis provides a general framework for the understanding and determination of adequate surface pretreatments, able to combat the substrate inhibited initial growth and the Si oxidation during metal-oxide ALD on Si.

Memristive Devices Formed By ALD Metal Oxide Growth on a Hafnium Layer – Study of the Interfacial HfO2 Formation

Redox-based memristive devices (ReRAM) are considered for future energy-efficient non-volatile memory. Memristive cells enable high speed data access, low power consumption and high scalability, which is a key requirement for their use as artificial synapses in neuromorphic computing. Applications in networks of up to 106 devices require a scaling of the cell volume to about 103 nm3. For a typical valence-change-mechanism (VCM)-type switching cell with a metal oxide, like HfO2, sandwiched between two metal electrodes of different oxidation enthalpy, this means that the individual layer thickness is in the range of a few nanometers. Therefore, a control of the device properties demands for a control of interfacial reactions during device fabrication. Here, we are going to present a careful analysis of the oxidation of an Hf electrode layer during atomic layer deposition (ALD) of different metal oxides, these are Al2O3, TiO2, and HfO2. The in-situ encapsulation of a reactive metal electrode by means of an ALD metal oxide layer enables a better process control compared to stack structures formed by breaking the vacuum conditions.In this study, plasma and thermal ALD processes were used to grow ultrathin layers of Al2O3, TiO2, and HfO2 on top of Hf metal films, after transfer under UHV conditions. The growth of homogeneous metal oxide films of a few nanometer in thickness enabled a systematic analysis of the HfO2 formation at the Hf/metal oxide interface by means of angle dependent X-ray photoelectron spectroscopy (XPS). The results show that the properties of the interfacial HfO2 layer can be controlled by the metal oxide deposited on top as well as by the process chemistry and deposition temperature. In contrast to the TiO2 film, the Al2O3 layer clearly serves as oxidation barrier. In addition, the chemistry and structure of the HfO2 interfacial layer were analyzed from XPS and transmission electron microscopy analysis.Memristive device were fabricated from the Hf/HfO2/MOx stacks by depositing Pt top electrodes. The initial current of the devices scales with the pad size indicating the high quality of the insulating oxide layers. Consistent with VCM-type memristive devices the cells required an electroforming voltage to enable bipolar-type resistive switching. In this presentation, the trade-off between the two functionalities of oxidation protection versus enabling of a low electroforming voltage will be discussed.

Tuning ignition and energy release properties of Zirconium powder by atomic layer deposited metal oxide coatings.

Ultra-fine powders of reactive metals are promising fuels/additives for propellants. However, the metal surfaces make these materials very unstable in ambient atmosphere. This study explored the method of applying thin films of inorganic materials onto the surface of Zr powder and investigated the effects of different surface coatings on the energy release and ignition process of Zr. Thin films of Al2O3 and ZnO were deposited on a commercial micron-scale Zr powder by atomic layer deposition (ALD). Growth kinetics of ALD films on the Zr particles were studied using various tools. Chemical and structural characterizations revealed that the Zr particles were completely encapsulated by uniform Al2O3 or ZnO films. The thicknesses of the encapsulation layers could be precisely controlled. ALD Al2O3 coatings exhibited a unique surface-sealing effect, which inhibited the low temperature oxidation of Zr in ambient air. Laser and electrostatic discharge (ESD) ignition tests revealed that ALD Al2O3 coatings extended the ignition delay and reduced the ESD sensitivity of the Zr powder. In comparison, ALD ZnO coatings could not form effective gas diffusion barriers, therefore they could not change the oxidation process of Zr and only showed modest effects on ignition and ESD sensitivity of the Zr powder.

Tough and Immunosuppressive Titanium-Infiltrated Exoskeleton Matrices for Long-Term Endoskeleton Repair.

Although biodegradable membranes are essential for effective bone repair, severe loss of mechanical stability because of rapid biodegradation, soft tissue invasion, and excessive immune response remain intrinsically problematic. Inspired by the exoskeleton-reinforcing strategy found in nature, we have produced a Ti-infiltrated chitin nanofibrous membrane. The membrane employs vapor-phase infiltration of metals, which often occurs during metal oxide atomic layer deposition (ALD) on organic substrates. This metal infiltration manifests anomalous mechanical improvement and stable integration with chitin without cytotoxicity and immunogenicity. The membrane exhibits both impressive toughness (∼13.3 MJ·m-3) and high tensile strength (∼55.6 MPa), properties that are often mutually exclusive. More importantly, the membrane demonstrates notably enhanced resistance to biodegradation, remaining intact over the course of 12 weeks. It exhibits excellent osteointegrative performance and suppresses the immune response to pathogen-associated molecular pattern molecules indicated by IL-1β, IL-6, and granulocyte-macrophage colony-stimulating factor expression. We believe the excellent chemico-biological properties achieved with ALD treatment can provide insight for synergistic utilization of the polymers and ALD in medical applications.

Area-Selective Atomic Layer Deposition Using Si Precursors as Inhibitors

Short-chain aminosilanes, namely, bis(N,N-dimethylamino)dimethylsilane (DMADMS) and (N,N-dimethylamino)trimethylsilane (DMATMS), have been used as Si precursors for atomic layer deposition (ALD) of SiO2. In this work, the DMADMS and DMATMS Si precursors are utilized as inhibitors for area-selective ALD (AS-ALD). The inhibitors selectively adsorb on a SiO2 surface but not on H–Si, so that SiO2 becomes selectively deactivated toward subsequent ALD. The deactivation of the SiO2 surface by the inhibitors was investigated using various experimental and theoretical methods, including surface potential measurements, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy. Better inhibition was observed for ALD of Ru and Pt than for ALD of Al2O3 and HfO2. Through quantum mechanical and stochastic simulations, the difference in the blocking ability for noble metal and metal oxide ALD by the aminosilane inhibitors could be attributed to the inherently partial surface coverage by the inhibitors at their saturation and the reactivity of the subsequent ALD precursors. As silane inhibitors can be easily integrated with vacuum-based processes to facilitate high volume manufacturing of upcoming electronic devices, the current study provides a potential approach for the utilization of AS-ALD in pattern fabrication inside 3D nanostructures.

Solvothermal Synthesis of MIL-96 and UiO-66-NH2 on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats.

Metal-organic frameworks (MOFs), which contain reactive metal clusters and organic ligands allowing for large porosities and surface areas, have proven effective in gas adsorption, separations, and catalysis. MOFs are most commonly synthesized as bulk powder, requiring additional processes to adhere them to functional devices and fabrics that risk decreasing the powder porosity and adsorption capacity. Here, we demonstrate a method of first coating fabrics with metal oxide films using atomic layer deposition (ALD). This process creates conformal films of controllable thickness on each fiber, while providing a more reactive surface for MOF nucleation. By submerging the ALD coated fabric in solution during solvothermal MOF synthesis, the MOFs create a conformal, well-adhered coating on the fibers, resulting in a MOF-functionalized fabric, without additional adhesion materials that may block MOF pores and functional sites. Here we demonstrate two solvothermal synthesis methods. First, we form a MIL-96(Al) layer on polypropylene fibers using synthetic conditions that convert the metal oxide to MOF. Using initial inorganic films of varying thicknesses, diffusion of the organic linker into the inorganic allows us to control the extent of MOF loading on the fabric. Second, we perform a solvothermal synthesis of UiO-66-NH2 in which the MOF nucleates on the conformal metal oxide coating on polyamide-6 (PA-6) fibers, thereby producing a uniform and conformal thin film of MOF on the fabric. The resulting materials can be directly incorporated into filter devices or protective clothing and eliminate the maladroit qualities of loose powder.

Interfaces Formed by ALD Metal Oxide Growth on Metal Layers

Redox-based resistive switching random access memory (ReRAM) has the potential to fulfill the demands of future Green IT solutions for non-volatile memory circuits with high speed data access, high reliability, and low power consumption. A single valence-change-mechanism (VCM)-type switching cell is typically built from a transition metal oxide layer sandwiched between metal electrodes of different oxidation enthalpy. It is accepted in the scientific community that oxygen exchange and drift/diffusion processes play a major role in the resistive switching process. Thus, understanding oxidation/reduction reactions taking place already during the growth process is of utmost importance. For future three-dimensional (3D) integrated arrays of ReRAM cells atomic layer deposition (ALD) will become the method of choice to grow ultra-thin, conformal, homogeneous and dense oxide films at a thermal budget below 450 °C. For todays’ two dimensional stacked crossbar arrays, the metal oxide films can be grown on nearly inert Pt electrodes, while the oxygen exchange layer built from a reactive metal is deposited on top of the metal oxide. However, in the 3D ReRAM arrays the metal oxide might need to be grown on the reactive metal as well. In this study ultrathin ALD metal oxide layers are deposited on various metal electrodes by means of in-situ transfer from the sputtered metal into the ALD chamber. For the metal layers we utilized Pt, Hf, and Ta while for the metal oxides Al2O3, TiO2, and HfO2 were chosen. ALD metal oxide processes were performed by plasma assisted ALD in an Oxford FlexAlâ system. Due to the passivation property of the dense ALD metal oxide layer the metal oxide / metal interfaces could be analyzed ex-situ if the vacuum break was kept short. Angle resolved X-ray photoemission spectroscopy studies combined with X-ray reflectivity analysis reveal a clear picture of the interface formation and the oxygen gradient formed at the interface already during growth. The influence of process parameters and stack sequences on this ‘built-in’ oxygen gradient at metal / ALD metal oxide interfaces will be covered in this presentation. Among others, the possibility for PtOx formation during remote oxygen plasma ALD processes will be discussed.

Dielectric-MoS2 Interfaces Grown By Atomic Layer Deposition

Molybdenum disulfide (MoS2) has become a prototypical transition metal dichalcogenide (TMDC) atomic-layered material because of its unique materials properties. For example, bulk MoS2 exhibits an indirect band-gap of 1.3 eV, while a single monolayer has a direct band-gap of 1.8 eV. Recently, nanometer scale transistor devices have been made with MoS2 and graphene, yet much of this work relies on layered materials prepared using chemical vapor deposition (CVD) and mechanically exfoliation. Multiple reports have demonstrated the growth of MoS2 via CVD, but only a few studies have reported MoS2 growth using atomic layer deposition (ALD), which offers potential advantages for high volume semiconductor manufacturing. Here, we report the ALD of few-layer MoS2 films using MoF6 and various sulfur precursors on various ALD-grown dielectric surfaces and the ALD of metal oxides on the ALD MoS2 films at temperatures between 100-300 °C. In particular, we used in-situ quartz crystal microbalance measurements to investigate the nucleation of MoS2 ALD on oxide surfaces and vice versa. In general, self-limiting growth of MoS2 was observed on a wide range of dielectric surfaces including alumina, magnesia, and hafnia. The ALD MoS2 films were amorphous as-deposited, but crystallized into a layered atomic structure upon annealing at 800 °C. Using in-situ spectroscopy and electrical measurements, growth inhibition of MoS2 was seen depending on the growth substrate. These studies provide insight into the low-temperature ALD of MoS2 and provide guidance for the ALD of additional TMDC films.

Interfaces Formed by ALD Metal Oxide Growth on Metal Layers

Oxidation of metal layer surfaces due to atomic layer deposition (ALD) of thin metal oxide films is studied. For this, dense and smooth metal films of Pt, Ta and Hf were grown by off-axis sputtering and were in-situ transferred into the ALD system. Oxide films like HfO2, TiO2 and Al2O3 with a thickness of about 2 to 3 nm were deposited between 250 and 300°C by means of plasma assisted ALD utilizing a remote oxygen plasma as oxygen source. The ‘built-in’ oxygen gradients formed at different metal/ALD metal oxide interfaces were analysed by angle resolved X-ray photoelectron spectroscopy. The Pt metal films show no evidence of oxidation due to the PE-ALD process, independent of the grown metal oxide. In contrast, the thickness of the metal oxide layers formed at the surfaces of the Hf and Ta metal films depends on the ALD surface reactions as well as on the differences in oxidation enthalpy of the substrate metal and the metal oxide. The results are important for the design of high-density memristor arrays based on OxRAM devices.