Metal Atomic Layer Deposition Research Articles

Chemical mechanism for nucleation enhancement in atomic layer deposition of Pt by surface functionalization

Atomic layer deposition (ALD) of metals on oxide substrates often yields islandlike growth, undesirable toward conductor applications. Thus, enhancement of initial nucleation of metal ALD is of great importance. One potential solution reported is to functionalize the substrate surface with small molecules, such as trimethylaluminum (TMA) and aluminum chloride (AlCl3) before metal ALD, so that nucleation delay is reduced, and the coverage and conformality are enhanced. In this study, density functional theory calculations are utilized to elucidate the chemical mechanism toward a change of the reactivity of the surface toward metal ALD upon functionalization by TMA or AlCl3. Surface adsorption of a typical Pt ALD precursor, trimethyl(methylcyclopentadienyl)platinum (MeCpPtMe3), on an OH-terminated SiO2 surface is compared to its adsorption on AlCH3-terminated and AlCl-terminated surfaces. Considering rate-limiting steps on each surface, greater rates for adsorption of Pt are found for the organometallic-functionalized surfaces than the bare SiO2 surface, confirming the chemical enhancement effect by functionalization.

Low Resistivity Metallic Films by Thermal Atomic Layer Deposition for High Aspect-Ratio Interconnects

Scaling interconnects to increase device density is a critical bottle neck for a range of applications from complementary metal oxide semiconductor (CMOS) to microelectromechanical (MEMS) switches and other devices. Currently, Cu is the interconnect metal of choice to fill vias but comes with significant challenges. To perform the fill, first a diffusion barrier is applied to ensure Cu leakage does not cause electrical breakdown between vias, then a metal seed layer is used to ensure smooth and dense Cu electroplating. Uniform seed resistivity, which can be correlated to thickness, is critical to produce low resistivity interconnects, free of voids. Unfortunately, state-of-the-art processes for high quality metallic films are limited to line-of-site techniques like chemical vapor deposition (CVD) or physical vapor deposition (PVD), limiting possible device pitch and architectures. When the aspect ratio increases above ~8:1 or there is a shadow, typically one of the layers is missing, resulting in insufficiently thin seed for successful Cu electroplating nucleation. This deficiency creates a void or produces a thickness gradient, resulting in pinch-off at the top. Ultimately, the device fails due to high line resistance or full dielectric breakdown. Fortunately, atomic layer deposition (ALD) can produce high density and low resistivity metal films for both Cu diffusion barrier and Cu electroplating seed applications. Here we report on a total solution to this problem using only thermal ALD. Adoption of ALD of metals has been slow in the market as traditional ALD reactors can be too slow and inefficient for production requirements or require plasma for sufficient film quality. Plasma enhanced ALD (PEALD) can produce quality films at a reasonable thermal budget, unfortunately every plasma process will have a limit to the aspect ratio that can be coated conformally and there is additional risk of surface plasma damage. Forge Nano has demonstrated single wafer thermal ALD technology to improve the speed and efficiency of ALD metal films, enabling a production worthy process. In this work a complete Cu barrier-seed stack solution has been demonstrated on Si vias ranging from 4:1 to 25:1 aspect ratio, showing successful Cu electroplating. A SiO2 ALD film, with a deposition rate of 10 nm/min, is first deposited to provide a dielectric barrier, then a thin TiN layer is applied as a Cu diffusion barrier, followed by a low resistivity Ru film for Cu adhesion. A novel TiN thermal ALD process at 300°C has been developed which decreases the as-deposited resistivity from ~1200 to <300 uΩ·cm at a deposition rate >1 nm/min. In addition, a high-quality Ru film has been developed with resistivity values <20 uΩ·cm and can be deposited on SiO2, HfO2, Pt, and TiN. An example of success as a total Cu barrier/seed stack for Cu electroplating in Si trenches is shown in Figure 1. When compared to PVD Ti/W barrier and Cu seed, the ALD stack produces dense, void free nucleation of Cu that remains well adhered to the via. However, the PVD stack has voids at the bottom of the trench from poor adhesion of electroplated Cu and narrowing at the top from a resistivity gradient within the trench, resulting in eventual pinch-off. From this comparison, it can be observed that the ALD Ru provides sufficient adhesion for Cu electroplating and that resistivity of the ALD Ru/TiN stack is sufficiently low and consistent for conformal and dense Cu electroplating. We expect this work to open up higher aspect ratio interconnects and possible new device architecture to the market. Figure 1

ALd of Metal Fluorides–Potential Applications and Current State

AbstractMetal fluoride thin films are important materials in a multitude of applications. Currently, they are mostly used in optics, but their potential in energy harvesting and storage is recognized as well. Atomic layer deposition (ALD) is an advanced thin film deposition method that has an ever‐increasing role in microelectronics. The assets of ALD are its capability to produce uniform, stoichiometric, and pure films with precise thickness control even on top of complicated structures, such as high aspect ratio trenches. These characteristics can be beneficial in applications of metal fluoride thin films but so far ALD of metal fluorides has remained much less studied and used than ALD of metal oxides, nitrides, sulfides, and pure metals. This review aims to motivate research on ALD of metal fluorides by surveying potential applications for ALD metal fluoride thin films and coatings. The basics of luminescent applications, antireflection coatings, and lithium‐ion batteries will be discussed. Next, the fundamentals of ALD will be presented followed by a comprehensive summary of the metal fluoride ALD processes published so far.

Modeling the Atomic Layer Deposition of Metal Thin Films: Studying the Surface Reaction Mechanism By Density Functional Theory Simulations

Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. The atomic scale understanding of ALD is vital and essential to design and optimize the deposition process, where density functional theory (DFT) calculations play an important role in providing detailed reaction mechanism, theoretical screening of suitable precursors and estimated growth-per-cycle (GPC).In this study, we show our recent works on DFT calculations of the growth mechanism study of Cobalt (Co) thin film by plasma-enhanced ALD1,2. We first addressed the surface reaction mechanism at the metal precursor pulse and plasma half-cycle on NHx-terminated Co surfaces, which corresponds to the steady growth for the PE-ALD. The adsorption and reactions of metal precursors (CoCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. The plausible reaction pathways include: precursor adsorption, hydrogen transfer, CpH formation and CpH desorption. The direct Cp dissociation mechanism is not considered on these NHx-terminated metal surfaces due to experimentally observed minimal C impurities at the deposited metal thin films, which indicates that most of the Cp ligand is removed completely. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method.The reactions at the initial stages on typical H:Si(100) surface are investigated to gain atomic insight on the effect of different substrates on the elimination of Cp ligands. Our work is important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.In addition, we present our recent works3, 4 on the thermal ALD of Co using reducing agent Zn(DMP)2 and thermal ALD of Fe4Zn9 thin film with diethyl zinc (DEZ). Our DFT results show that by selecting suitable reducing agent, we can control the deposited thin films to be Zn-free high quality Co thin film or desired intermetallic thin film with high Zn content.These DFT results are important to understand the reaction mechanism for ALD and provides new knowledge in depositing the thin films by adjusting the reducing agents.

Novel liquid cobalt precursor for metal atomic layer deposition

A novel liquid cobalt (Co) precursor, bis (diisopropylbutanamidinate)cobalt (DIPRoBA-Co), is proposed as the liquid Co precursor for metal atomic layer deposition (ALD). This precursor was synthesized and the Co ALD process using the obtained precursor was examined. This precursor is liquid and successfully achieves Co ALD with a liquid precursor instead of a solid one. Transmission electron microscopy observation shows that 5–6 nm thick Co metal film grows on a plasma silicon dioxide substrate step structure by this ALD using an ammonia, hydrogen and DIPRoBA-Co gas system with 80 cycles at 220 °C substrate temperature.

Combined effects of the deposition temperature and metal electrodes on ferroelectric properties of atomic-layer-deposited Hf0.5Zr0.5O2 films

The combined effects of the atomic-layer-deposition (ALD) temperature (220 °C–280 °C) and metal electrodes (TiN and Mo) on the ferroelectric properties of Hf0.5Zr0.5O2 films were studied. Regardless of the metal electrode, a tetragonal–orthorhombic–monoclinic phase evolution sequence was observed with increasing ALD temperature after post-metallization annealing. However, the phase transition temperature slightly changed depending on the metal electrode, which was predetermined based on the as-deposited states. Additionally, the out-of-plane orientation of the final orthorhombic grains was highly dependent on the crystallographic alignment of the metal electrode grains, resulting in different values of the maximum remanent polarization. In terms of long-term reliability, the fatigue characteristics were highly dependent on the electrode characteristics, and the wake-up characteristics were mainly affected by the initial phase distribution determined by both the ALD temperature and metal electrode.

Atomic layer deposition of CoF2, NiF2 and HoF3 thin films.

The present study describes atomic layer deposition (ALD) processes and characterization of CoF2, NiF2, and HoF3 thin films. For CoF2 deposition CoCl2(TMEDA) (TMEDA = N,N,N',N'-tetramethylethylenediamine) and NH4F were used as precursors. CoF2 deposition was studied at 180-275 °C, resulting in a growth per cycle (GPC) of 0.7 to 1.2 Å. All the films consist of tetragonal CoF2 according to XRD. The impurity contents were measured with ToF-ERDA and less than 1 at% of N and Cl were detected in the films, indicating effective reactions. In addition, the F/Co ratio is close to 2 as measured by the same method. The saturation of the GPC with respect to precursor pulses and purges was verified at 250 °C. The common feature of ALD metal fluoride films - remarkable roughness - is encountered also in this process. However, the films became smoother as the deposition temperature was increased. CoF2 deposition was also demonstrated on graphite substrates. NiF2 deposition was studied at 210-250 °C by using Ni(thd)2 and TaF5 or a new fluoride source NbF5 as the precursors. Tetragonal NiF2 was obtained, but the oxygen and hydrogen contents in the films were remarkable, up to ∼11 at%, as measured by ToF-ERDA. This was observed also when the films were in situ capped with YF3. NbF5 was shown to be a potential fluoride precursor by combining it with Ho(thd)3 to deposit HoF3 films. Orthorhombic HoF3 was obtained at deposition temperatures of 200-275 °C. The films deposited at 235-275 °C are pure, and the Nb contents in films deposited at 250 and 275 °C are only 0.21 and 0.15 at%. The main impurity in both films is oxygen, but the contents are only 1.5 and 1.6 at%. The saturation of the GPC with respect to precursor pulses was verified at 250 °C. The GPC is ∼1 Å.

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.

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.

Precise Pore Size Tailoring and Surface Chemistry Modification of Polycarbonate Membranes Using Atomic Layer Deposition

Physical and chemical surface modifications are commonly used in many applications such as energy generation and storage, microelectronics, water purification, gas separation, and medical/plant biology. In recent several years, sequential infiltration synthesis (SIS), a variant of atomic layer deposition (ALD), has emerged as a promising technology that uses self-limiting reactions to create hybrid organic/inorganic materials within the polymer-free volume. Herein, we employed ALD/SIS of Al2O3, SnO2, Ga2O3, TiO2, and In2O3 to precisely tune the pore size and surface characteristics of track-etched polycarbonate membranes. We used in situ Fourier transform infrared spectroscopy to elucidate the chemistry for Al2O3 SIS in polycarbonate and in situ quartz crystal microbalance measurements to probe the extent of Al2O3 infiltration during SIS in polycarbonate thin films. Next, we used scanning electron microscopy, thermogravimetric analysis, and water permeance measurements to probe the pore size reduction and conformality of SIS Al2O3 in the track-etched polycarbonate membranes. We then used water permeance measurements to compare the water flux for a series of membranes with similar pore size but different pore wall chemistry and compared these results with water contact angle measurements. Interestingly, the hydrophilic metal oxide coatings yielded higher water flux values compared to hydrophobic coatings. Finally, we examined ion-selective transport in the modified track-etched polycarbonate membranes and rationalized the results using zeta potential measurements of the ALD metal oxides. Keywords: atomic layer deposition; sequential infiltration synthesis; metal oxides; hydrophilicity/hydrophobicity; permeability; surface charge; ion conductivity/selectivity.

Removal of Surface Carbonate from Lithium-Ion Battery Cathode Materials via Vapor-Phase Fluorination

Ultrathin metal-fluoride barrier coatings on the surface of lithium-ion battery (LIB) cathodes can improve cycling stability and prevent corrosion by acidic byproducts in the electrolyte. Atomic layer deposition (ALD) is an effective method to deposit ultrathin metal fluoride coatings on LIB cathodes. Although numerous studies have demonstrated the benefit of ALD metal fluoride coatings to LIB performance, comparatively few works have examined the effect of individual ALD precursors on the cathode surface. This paper uses X-ray photoelectron spectroscopy (XPS) measurements to elucidate the surface chemical changes on LIB cathode material surfaces upon exposure to the ALD metal fluoride precursor, hydrogen fluorine pyridine (HFPy). We found a decrease in surface carbonate and an increase in surface fluoride after HFPy exposure suggesting the conversion of lithium carbonate (Li2CO3) to lithium fluoride (LiF). This conversion is desirable given that Li2CO3 degrades LIB performance, whereas LiF provides an excellent physio-chemical barrier against chemical attack during cycling. Scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, and XPS measurements following HFPy exposure to Li2CO3 powder revealed the formation of a conformal LiF shell around the Li2CO3 particles. Finally, we confirmed the complete conversion of ∼7 nm ALD Li2CO3 films on silicon from HFPy exposure using XPS and spectroscopic ellipsometry. The elimination of problematic Li2CO3 from LIB cathode surfaces and conversion into a protective LiF coating via a single precursor vapor treatment may provide a cost-effective method for enhancing LIB performance.

(Keynote) Status & Challenges in Today's Atomic Layer Processing Market

The global market for atomic-layer deposition (ALD) and chemical-vapor deposition (CVD) metal precursors for semiconductor manufacturing in 2020 was worth ~US$640M [1]. Driven by advanced logic, DRAM and 3D-NAND memory chip fabrication needs, demand for ALD and CVD metal precursors is forecast to see a 2020-2025 compound annual growth rate (CAGR) of ~10%. In the short-term forecast, sees the global semiconductor market as very healthy with 15-20% Year-over-Year (YoY) growth in 2021 revenues [2], and overall critical materials market growth >7% YoY. The greatest material demand increases are seen in ALD/CVD metal precursors, and in specialty wet cleans that often are integrated with metal deposition and etching. All ALD/CVD metal precursors are in healthy demand, since ALD is critical for fabs running 22-45nm nodes as well as for fabs at the leading edge at 16/14 nm and below. In particular, cobalt (Co) and hafnium (Hf) precursors saw 18-20% year-over-year (YoY) growth in 2020, and are forecasted to see strong demand through 2025. Zirconium (Zr) used for DRAM capacitors saw a lower growth but is a big segment on its ow. Ruthenium (Ru) metal is slowly replacing some of the Co and tungsten (W) interconnects on the most advanced logic chips, with anticipated precursor revenue growth to US$10M in 2025 for this highly strategic material. Metals to observe the next years include molybdenum (Mo), niobium (Nb) and lanthanum (La). The long-term revenue outlook for precursors continues to be good driven by: replacement of PVD processes to meet challenging specification for 3D and high aspect ratio device architecture, driven my lateral and stacked scalingReplacement of high themperature diffusion processes (oxidation and nitridation) with low thermal budget plasma ALD processes.Implementation of leading-edge logic nodes with a higher penetration of ALD processes in automotive and the EV transition of the global vehicle fleet.Growing need for ALD films in new semiconductor device technologies and new products: Spin-transfer torque MRAM (STT-MRAM), Resistive RAM (ReRAM), Ferroelectric FET (FeFET) and RAM (FRAM) and Cross-Point memoryImplementation of ALD in emerging new technologies: Photovoltaic, Display, MEMS, Power Electronics, LED/µLED, Optical, Lithium Battery (EVs, Mobile), Parts and powder coating, Medical / Healthcare. New precursors will be needed to meet improved performance for 3D devices and lower thermal budgets as well as reduced cost of ownership. New metals and new metal precursors, mainly for interconnects and contacts, are being continuously researched (e.g. W, Co, Ru, La, Mn).ALD materials and Original Equipment Manufacturer (OEM) tools are needed for multi-patterning lithography used with Deep-UV (DUV) Immersion and with Extreme-UV (EUV) in leading logic and memory fabs. Multi-patterning typically uses low-temperature Plasma-Enhanced ALD (PEALD), either in clusters of 8-16 single-wafer chambers, or in “Spatial” high-throughput tools. It is anticipated that fab investments in Asia will allow South Korean and Chinese OEMs to win near-term ALD orders, and they may soon compete with US, EU, and Japanese OEMs in the global tool market for standard processes that are commoditized. Recent actions taken by the new US administration has boosted logic fab investments in the US and in total USD 70 B leading edge fab investments are on the horizon the next three years, which will generate a need for both ALD equipment and a healthy precursor supply chain on all materials above going forward.

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.

Atomic Layer Deposition of GdF3 Thin Films

Metal fluorides, especially rare earth fluorides, have suffered from a lack of atomic layer deposition (ALD) processes. For long, only YF3 and LaF3 ALD processes were found in the literature for the rare earth fluorides1, 2. Recently, articles describing ALD of SmF3, EuF3 and TbF3 expanded the selection3, 4. ALD-made rare earth fluorides can be exploited in luminescent applications — both as dopants and host structures — but also in antireflection applications.Here we present an ALD process for GdF3. Having a refractive index of 1.59 and low absorption of light at wavelengths down to short UV, GdF3 films have found use in UV-antireflection coatings. In this study, GdF3 films were grown with Gd(thd)3 and HF as precursors. HF was produced in situ by heating solid NH4F. The process yields orthorhombic GdF3 at the studied deposition temperature range of 285–375 °C. The growth rate is low (~0.24 Å/cycle) and stays quite constant across the deposition temperature range, which is in a striking contrast to most of the ALD metal fluoride processes using TiF4 as the fluorine source1, 2. The saturation of the growth rate with respect to precursor doses and purges was verified at 300 °C. According to ToF-ERDA, the films are stoichiometric and contain low amounts of impurities. For example, the nitrogen content in the films is less than 0.04 at-%. The films also show high transmittance at the wavelength range of 190 –1100 nm as measured by UV-Vis spectrometry.1. T. Pilvi et al., Chem. Vapor Deposition. 15, 27 (2009)2. T. Pilvi et al., Chem. Vapor Deposition. 14, 85 (2008)3. PA. Hansen et al., Commun. Chem. 3, 162 (2020)4. E. Atosuo et al., J. Vac. Sci. Technol. A 39, 022404 (2021)

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.

A new route of synthesizing atomically thin 2D materials embedded in bulk oxides

Conventional mechanical or chemical exfoliation approach of 2D material synthesis is largely dependent on the inherent structure of the parent material, i.e., whether it is a layered structure or a 3D bulk structure with embedded 2D substructures. A recent experiment demonstrated that unprecedented atomically thin metal oxides without bulk layered structures can be synthesized by using liquid metals. Supported by an experimental realization of atomically thin W layers through the metal atomic layer deposition method, we propose a new type of transition metal (TM)-based 2D materials that can be stabilized at the oxide interfaces with oxide substrates and overlayers. Based on the ab initio density functional theory calculations, we show that most of the TM elements can form unprecedented atomically thin 2D materials by the surface oxygen passivation, which is available from the oxide substrate and the overlayer. The stabilized 2D TM layers show diverse electronic and magnetic properties. Our results suggest a novel way to extend 2D materials study and a possible application of those 2D TM layers embedded in oxides.

Cobalt Metal ALD: Understanding the Mechanism and Role of Zinc Alkyl Precursors as Reductants for Low-Resistivity Co Thin Films

In this work, we report a new and promising approach toward the atomic layer deposition (ALD) of metallic Co thin films. Utilizing the simple and known CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) precursor in combination with the intramolecularly stabilized Zn aminoalkyl compound Zn(DMP)2 (DMP = dimethylaminopropyl) as an auxiliary reducing agent, a thermal ALD process is developed that enables the deposition of Zn-free Co thin films. ALD studies demonstrate the saturation behavior of both precursors and linearity depending on the applied number of cycles as well as temperature dependency of film growth in a regime of 140–215 °C. While the process optimization is carried out on Si with native oxide, additional growth studies are conducted on Au and Pt substrates. This study is complemented by initial reactivity and suitability tests of several potential Zn alkyl-reducing agents. For the CoCl2(TMEDA)–Zn(DMP)2 combination, these findings allow us to propose a series of elemental reaction steps hypothetically leading to pure Co film formation in the ALD process whose feasibility is probed by a set of density functional theory (DFT) calculations. The DFT results show that for reactions of the precursors in the gas phase and on Co(111) substrate surfaces, a pathway involving C–C coupling and diamine formation through reductive elimination of an intermediate Co(II) alkyl species is preferred. Co thin films with an average thickness of 10–25 nm obtained from the process are subjected to thorough analysis comprising atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry/nuclear reaction analysis as well as depth profiling X-ray photoemission spectroscopy (XPS). From XPS analysis, it was found that graphitic and carbidic carbon coexist in the Co metal film bulk. Despite carbon concentrations of ∼20 at. % in the Co thin film bulk, resistivity measurements for ∼22 nm thick films grown on a defined SiO2 insulator layer yield highly promising values in a range of 15–20 μΩ cm without any postgrowth treatment.

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.

Modelling the Atomic Layer Deposition of Cobalt and Ruthenium on NHx-Terminated Metal Surfaces

Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit very thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. Cobalt (Co) and Ruthenium (Ru) are used as a seed layers for metallization of interconnects. They are also potential materials for the electrode in dynamic random-access memory (DRAM) capacitors and metal-oxide-semiconductor field-effect transistors (MOSFETs). Plasma-enhanced ALD (PE-ALD) is used for low-temperature thin film growth by alternating exposures of metal precursors and plasma reactants. During the PE-ALD growth of metals, N-plasma, for example, NH3 or a mixture of N2 and H2, has been applied to avoid surface metal oxidation. The PE-ALD of Ru and Co has been experimentally investigated using metal precursors such as RuCp2, Ru(EtCp)2, CoCp2, and CoCp(CO)2. However, the reaction mechanism is not clear and theoretical studies on the reaction mechanism is entirely lacking.In this presentation, we study the PE-ALD growth of Co and Ru by first principle calculations. The (001) surface of both metals, with a hexagonal structure, is the most stable and the (100) surface with a zigzag structure is less stable but has high reactivity. These two surfaces allow the study of the influence of the surface facet. The surface saturation coverage was studied by considering individual adsorption and co-adsorption of NH and NH2 to terminate both surfaces. On the (001) surface, the zero K saturation coverage for NH is 1ML on Ru and 6/9ML on Co. For NH2, the saturation coverages are 6/9ML on Co and Ru surfaces. On the (100) surface, the saturation coverages are 2ML for NH on Ru and Co surfaces, 1.33 ML for NH2 on Ru, and 1ML for NH2 on Co surface. The higher saturation coverage on (100) surface is attributed to the unique trench structure, which provides more available surface sites than that of (001) surface. We also consider co-adsorption of NH and NH2 on (001) and (100) surfaces. The results are then analyzed with ab initio thermodynamics by calculating the Gibbs free energy. Both the ultra-high vacuum (UHV) condition and standard ALD operating condition are used to elucidate the effect of pressure and temperature on the termination of metal surfaces.The adsorption and reactions of metal precursors (CoCp2 and RuCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. Two possible adsorption structures, namely the precursor lying parallel and perpendicular to the NHx-terminated surfaces, were considered at zero K and ALD conditions. Possible reactions include: precursor adsorption, first hydrogen transfer, first Cp ligand dissociation, first Cp ligand desorption, second hydrogen transfer, and second Cp ligand desorption. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method. Our results show that (100) surface has higher activity than (001) surface. In addition, the Cp ligand elimination of CoCp2 has lower barrier than RuCp2, regardless of surface facet. After the metal precursor pulse, the surface is terminated with MCp fragment or M atom depending on surface facet, where M is Ru or Co. The surface N atom and H atom can be eliminated during the following plasma step by forming NH3, N2 or H2. After a full cycle, the surface is an NHx-terminated metal surface and ready for the next cycle. This work will be important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.

Study on fabrication method of composite oxide by room temperature atomic layer deposition

Thin films can be used to improve the surface properties of materials, enhancing elements such as absorption, abrasion resistance and corrosion resistance, for example. These thin films provide the foundation for a variety of applications in various fields and their applications depend on their morphology and stability, which is influenced by how they are deposited. Thin films can be deposited in different ways. One of these is a technology called atomic layer deposition (ALD). Professor Fumihiko Hirose, a scientist based at the Graduate School of Science and Engineering, Yamagata University, Japan, is conducting research on the room temperature ALD of oxide metals. Along with his team, Professor Hirose has developed a new and improved way of performing ALD to create thin films, and the potential applications are endless.