Atomic Layer Deposition Reactor Research Articles

Electromagnetically Enhanced Atomic Layer Deposition for Modification of Thin Film Properties through in Situ Substrate Biasing

Atomic layer deposition (ALD) is broadly used for thin, conformal film growth in microelectronics, photovoltaics, and other industries. Enhanced film properties are required for advancement of device performance. Use of electrical current in situ during deposition to drive material properties of films produced by ALD toward a desired state (such as higher conductivity) will increase the utility of thin film materials for device applications. In an ALD reactor bias is applied directly to substrates during ALD precursor pulses. Higher conductivity and growth rates were observed for samples grown with ALD enhanced by in situ substrate biasing. Thicknesses were determined via spectroscopic ellipsometry, uniformity was investigated by atomic force microscopy, and crystallinity by x-ray diffraction. These systems are computationally investigated to probe the mechanism by which the ALD processes are enhanced.

Hollow-cathode plasma deposited vanadium oxide films: Metal precursor influence on growth and material properties

Due to its different polymorphs, including vanadium pentoxide (V2O5) and vanadium dioxide (VO2), the vanadium oxide (VOX) compound is an immensely interesting material with many important applications. While atomic layer deposition (ALD) is among the possible VOX film synthesis methods, literature reports have majorly utilized thermal-ALD, which reveals as-grown amorphous VOX films. Further post-deposition annealing process is needed to crystallize these films. High-temperature crystallization indeed limits the use of low-temperature compatible materials, processes, and substrates. In this work, we report on the low-temperature crystalline VOX film growth in a hollow-cathode plasma-enhanced atomic layer deposition reactor using two different vanadium precursors, tetrakis(ethylmethylamino)vanadium and vanadium(V) oxytriisopropoxide. Oxygen plasmas were used as co-reactants at a substrate temperature of 150 °C. Along with the purpose of investing in the impact of metal precursors on VOX film growth, we also studied Ar-plasma in situ and thermal ex situ annealing to investigate possible structural enhancement and phase transformation. In situ Ar-plasma annealing was performed with 20 s, 20 SCCM Ar-plasma, while post-deposition ex situ annealing was carried out at 500 °C and 0.5 mTorr O2 pressure. In situ ellipsometry was performed to record instant film thickness variation and several ex situ characterizations were performed to extract the optical, structural, and electrical properties of the films. The outcomes of the study confirm that both metal precursors result in as-grown crystalline V2O5 films at 150 °C. On the other hand, post-deposition annealing converted the as-grown crystalline V2O5 film to VO2 film. Finally, we have also successfully confirmed the metal-to-insulator transition property of the annealed VO2 films via temperature-dependent structural and electrical measurements.

Atomic Layer Deposition of NiO Using Different Precursors with Different Oxygen Sources

NiO thin films have attracted tremendous attention owing to their outstanding chemical stability, good magnetic and catalytic properties1. It is also considered an efficient electrocatalyst for water oxidation, and is one of the few p-type semiconductive oxides with optical transparency due to its wide bandgap of 3.6 eV. Furthermore, NiO films can control high and low conductive states under an external electric field, which is very useful for developing resistance-switching random-access memory devices2.NiO films have been prepared by different deposition techniques, including sputtering, sol-gel, pulsed laser deposition, spray pyrolysis, and chemical vapor deposition (CVD), atomic layer deposition (ALD). Among these, ALD is a powerful technique to grow thin conformal films with fine-tuning of both composition and thickness. The commonly used nickel precursors for ALD process are Ni(Cp)2, Ni(MeCp)2, Ni(EtCp)2, Ni(dmamp)2, Ni(dmamb)2, Ni(acac)2, Ni(apo)2, Ni(dmg)2, Ni(thd)2, and Ni(amd)2 in combination with ozone, water, hydrogen peroxide, or oxygen plasma1. However, ALD of NiO is not well developed yet. The major disadvantages associated with these nickel precursors are low volatility, thermal instability, and poor reactivity or narrow ALD window. Some of these precursors are expensive or very difficult to synthesize. The aforementioned situations indicate that developing a new ALD process for the NiO film with new precursors is pertinent.In this work, we explored different Ni-precursors such as Ni ketoiminates, Ni(ipki)2 and Ni(eeki)2 (their synthesis has been proposed earlier3), and Alanis, provided by Air Liquide. These different reactants combined with different oxygen sources (H2O and O3) resulted in high-quality NiO thin films. Surface analyses indicate NiO films are stoichiometric. They are uniform and exhibit a columnar morphology as determined by TEM (Fig.1a). From the growth characteristics, it is found that the Ni precursors have a strong effect on the GPC. Similarly, the oxygen source affects GPC but it also influences the morphology of the layer leading to significant roughness variations (observed by XRR and AFM). The use of H2O can also result in a nucleation delay. Polycrystalline films are observed when the reaction temperature reaches 125°C, and the crystallinity is further enhanced with deposition temperature (Fig.1b). After comprehensive optimization of the ALD process with Alanis precursor, it has been possible to reach one of the highest growth rate of 1.5 Å/cy, which is nearly seven times higher than that obtained with Ni(EtCp)2 based reaction with the same ALD reactor and having a wide temperature window (100 to 175 °C) with ozone. The same precursor also reacted well with water, giving a GPC of 0.9 Å with an initial nucleation delay. With the use of Ni-ketoiminates, the growth rate could be enhanced considerably with a GPC of 0.7 Å in combination with O3 at a higher temperature.The NiO layers were subsequently used as catalysts for water photooxidation in alkaline medium. The electrochemical properties are better when compared to layers grown by PVD. References Y. Zhang, L. Du, X. Liu, and Y. Ding, A high growth rate atomic layer deposition process for nickel oxide film preparation using a combination of nickel (II) diketonate-diamine and ozone, App. Surf. Sci., 2019, 481, 138-143.H.L. Lu, G. Scarel, X.L. Li, M. Fanciulli, Thin MnO and NiO films grown using atomic layer deposition from ethylcyclopentadienyl type of precursors, J. Cryst. Growth, 2008, 310, 5464-5468.3. D. Zywitzki, D.H. Taffa, L. Lamkowski, M. Winter, D. Rogalla, M. Wark, and A. Devi, Tuning coordination geometry of nickel ketoiminates and its influence on thermal characteristics for chemical vapour deposition of nanostructured NiO electrocatalysts, Inorg. Chem., 2020,59, 14, 10059-10070. Figure 1

New Materials for Memory Applications by Atomic Layer Deposition

The architectures of integrated logic and memory devices have shifted from 2D to 3D layouts, enabling a continued scaling of device densities. Atomic layer deposition (ALD) allows the conformal deposition of thin films with sub-nm thickness control on high aspect ratio 3D structures. ALD has become the technique of choice for the synthesis of an increasing number of materials in advanced logic and memory devices. In the first part of this presentation, we briefly review some of the key ALD processes which enable the fabrication of state-of-the-art 3D-NAND and 2D-DRAM. Next, we present the ALD of emerging ferroelectric HfZrO developed at ASM in industrial scale ALD reactors. HfO2 has been established as the preferred gate oxide material for logic devices for almost two decades. The recent discovery of ferroelectricity in HfO2 has opened new applications for HfO2-based materials as active memory elements in DRAM and 3D-NAND. We show that ALD La:HfZrO can achieve high polarization field (2Pr > 60 uC/cm2) and high endurance (> 107 cycle) by applying interface engineering schemes and optimizing deposition processes and film composition (i.e. Hf/Zr ratio, dopant concentration).

Platinum Catalysts via Fluidized Bed Atomic Layer Deposition for PEM Fuel Cells

One of the major challenges for viable and durable proton exchange membrane fuel cells (PEMFC) in heavy duty applications is the development of stable and high-performing catalysts. The highest mass activities and stabilities can be achieved with platinum nanoparticles supported on a porous carbon support. However, these systems still suffer from degradation processes during cell operation, such as carbon corrosion and Ostwald ripening. [1] Atomic layer deposition (ALD) is an attractive alternative to the conventional wet chemical deposition of the platinum catalyst onto the carbon, promising to counteract these problems. [2,3]ALD is a surface coating method, which employs self-limiting half-reactions between gaseous precursor molecules and a solid substrate surface. Usually, two half-reactions separated by purging or vacuuming steps represent one ALD cycle. By performing a defined number of cyclic repetitions, thin films or nanoparticles can be grown on the substrate surface with atomic-layer precision. [4]Typically, ALD is used to coat flat surfaces. To achieve homogeneous coatings on powders, so-called fluidized bed ALD reactors fluidize the nanoparticle-powders in a gas flow, which also contains the gaseous precursor materials. This method ensures a high utilization of the precursor molecules while providing a good mixing of the sample. Thin films and nanoparticles can be deposited in a homogeneous manner onto the powder surface and into its pores. [4]Carbon corrosion of PEMFCs can be suppressed by applying conformal ALD coatings onto the support structure. Additionally, a porous overcoating can fixate the nanoparticles against detachment and dissolution while still leaving access to the reactive sites. [5] Applying ALD to deposit fuel cell catalysts with monodisperse and highly dispersed platinum nanoparticles can counteract Ostwald ripening. [2,6]In this study we show platinum particles deposited via ALD, which are highly dispersed and show a monodisperse size distribution. We further show TiO2-overcoatings to protect the carbon.Literature:[1] J. C. Meier, Beilstein J. Nanotechnol., 2014, 5, 55-67. [2] W-J. Lee, Adv. Mater. Interfaces, 2019, 6 1901210. [3] B. J. O’Neill, ACS Catal., 2015, 5, 1804. [4] W. W. McNeary, Appl. Catal. B, 2019, 254, 587-593. [5] Q. Liu, Chem. Soc. Rev., 2022, 51, 188-236. [6] Z. Xu et al., Appl. Catal. B, 2012, 111-112, 264.

Computational fluid dynamics modeling of a discrete feed atomic layer deposition reactor: Application to reactor design and operation

Novel transistor fabrication methods such as area-selective atomic layer deposition (AS-ALD) are crucial to improving nanopatterning, which is essential for facilitating transistor stacking in semiconducting wafers. However, transistor surfaces are subjected to nonuniformities during the initial AS-ALD adsorption reactions that are attributed to steric hindrance effects. To minimize the role of steric hindrance generated by an oversaturation of reagent on the substrate surface, a discrete feed method is proposed for an ALD reactor configuration where reagent is introduced in short pulses through a perpendicular delivery system. An optimal reactor configuration is developed by modifying the inlet geometries of the reactor to ensure ideal fluid dynamics conditions (e.g., minimal vortices, radial flow distribution) are achieved. Detailed computational fluid dynamics simulations demonstrate the performance of the new reactor configuration and operational strategy.

Low-temperature synthesis of crystalline vanadium oxide films using oxygen plasmas

Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.

Molecular Layer Deposition of Zeolitic Imidazolate Framework-8 Films

Vapor-phase film deposition of metal–organic frameworks (MOFs) would facilitate the integration of these materials into electronic devices. We studied the vapor-phase layer-by-layer deposition of zeolitic imidazolate framework 8 (ZIF-8) by consecutive, self-saturating reactions of diethyl zinc, water, and 2-methylimidazole on a substrate. Two approaches were compared: (1) Direct ZIF-8 “molecular layer deposition” (MLD), which enables a nanometer-resolution thickness control and employs only self-saturating reactions, resulting in smooth films that are crystalline as-deposited, and (2) two-step ZIF-8 MLD, in which crystallization occurs during a postdeposition treatment with additional linker vapor. The latter approach resulted in a reduced deposition time and an improved MOF quality, i.e., increased crystallinity and probe molecule uptake, although the smoothness and thickness control were partially lost. Both approaches were developed in a modified atomic layer deposition reactor to ensure cleanroom compatibility.

In vacuo cluster tool for studying reaction mechanisms in atomic layer deposition and atomic layer etching processes

In this paper, we introduce a vacuum cluster tool designed specifically for studying reaction mechanisms in atomic layer deposition (ALD) and atomic layer etching (ALE) processes. In the tool, a commercial flow-type ALD reactor is in vacuo connected to a set of UHV chambers so that versatile surface characterization is possible without breaking the vacuum environment. This way the surface composition and reaction intermediates formed during the precursor or etchant pulses can be studied in very close to true ALD and ALE processing conditions. Measurements done at each step of the deposition or etching cycle add important insights about the overall reaction mechanisms. Herein, we describe the tool and its working principles in detail and verify the equipment by presenting results on the well-known trimethyl aluminum–water process for depositing Al2O3.

Scaling-Up of Thin-Film Photoelectrodes for Solar Water Splitting Based on Atomic Layer Deposition.

Atomic layer deposition (ALD) is an established method to prepare protective layers for Si-based photoelectrodes for photoelectrochemical (PEC) water splitting. Although ALD has been widely used in microelectronics and photovoltaics, it remains a great challenge to design simple and effective ALD systems to deposit large and uniform protective films for Si-based photoelectrodes with industrial sizes. This paper describes the design and realization of a simple ALD chamber configuration for photoelectrodes with large sizes, in which the influence of a gas redistributor over the gas flow and heat transfer during film growth was revealed by computational fluid dynamics simulations and experimental investigations. A simple circular baffle-type redistributor was proposed to establish a uniform gas flow field throughout the ALD reactor, resulting in a uniform temperature profile. With this simple baffle redistributor, the large-area Al2O3 monitor film (46 nm thickness) reached a good nonuniformity (Nu %) of 0.88% over a large area of 256 cm2. This design enables the fabrication of large-scale photocathodes from standard industrial-grade 166 mm Si(100) wafers (276 cm2) by depositing 50 nm TiO2 protective films with Nu % less than 5%. The obtained photocathode achieves a saturation current of 6.45 A with a hydrogen production rate of 43.2 mL/min under outdoor illumination. This work elucidates how flow pattern and heat transfer may influence the deposition of protective layers over large photoelectrodes, providing guidance for future industrial applications of PEC water splitting.

Machine learning and atomic layer deposition: Predicting saturation times from reactor growth profiles using artificial neural networks

In this work, we explore the application of deep neural networks to the optimization of atomic layer deposition (ALD) processes. In particular, we focus on a one-shot optimization problem, where we try to predict the optimal dose time that leads to saturation everywhere in the reactor based on thickness values measured at different points of an ALD reactor after a single trial growth. In order to tackle this problem, we introduce a dataset designed to train neural networks to predict saturation times based on these inputs for a cross-flow ALD reactor. We then explore the predictive ability of artificial neural networks of different depths and sizes using a separate testing dataset to evaluate their accuracies. The results obtained show that networks trained using stochastic gradient descent methods can accurately predict saturation times without requiring any additional information on the surface kinetics. This provides a viable approach to minimize the number of experiments required to optimize new ALD processes in a known reactor, and it highlights the way machine learning can be leveraged for thin film growth and manufacturing. While the datasets and training procedure depend on the reactor geometry, the trained neural networks provide a general surrogate model connecting thickness values and trial dose times with optimal saturation times that can be reused for different ALD processes within the same reactor.

Seebeck Analysis of ALD Synthesized Thermoelectric Sb2Te3 Thin Films

Thermoelectric materials have attracted significant attention due to their potential application in power generation and refrigeration systems, because they can convert thermal energy into electric energy from waste heat recovery. Thin thermoelectric films can be used to create thermoelectric generators, where the power they create does not result in any additional emissions. PbTe, PbSe and Sb2Te3, Bi2Te3 present excellent choices for thermoelectric energy harvesting devices. Among these, Sb2Te3 and its derivatives have high Seebeck coefficient and good electrical conductivity, which make these the most promising p-type material for low temperature thermoelectric device. The high figure of merit ZT values at relatively low temperature make these exceptional thermoelectric materials. This study investigates Seebeck coefficient measurements of Sb2Te3 thin film synthesized by Atomic Layer Deposition (ALD) technology using a Veeco Nanotech thermal ALD reactor. For the ALD synthesis of Sb2Te3 we used SbCl3 as precursor1 and (Si(CH3)3)2Te Bis(trimethylsilyl)telluride as precursor 2. Antimony trichloride Sb2Te3 is a solid ALD precursor requiring preheating to the melting temperature around 73 ℃ to generate sufficient vapor pressure. A typical film morphology of ALD Sb2Te3 is shown in the SEM micrograph of Fig.1. The efficiency of thermoelectric materials is expressed by the dimensionless thermoelectric figure of merit, ZT = S2σT/ k, where S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. Because the crucial Seebeck coefficient factors directly into the equation by the power of two to determine ZT and the power factor, it is imperative to optimize the thermoelectric thin film material for maximum Seebeck coefficient. Maximizing the figure of merit ZT is challenging because in thermoelectric materials the quantities S, σ and κ are interrelated in such a way as to make independent control of these variables to increase ZT not straightforward. An increase in S typically results in a decrease of σ. Following the Wiedeman- Franz law a decrease in σ produces a decrease in the electronic contribution to κ complicating matters. Figure 1

Atomic layer deposition of ZnO layers on Bi2Te3 powders: Comparison of gas fluidization and rotary reactors

Interface engineering of thermoelectric powder materials via atomic layer deposition (ALD) has attracted significant research interest owing to the dramatic improvement in energy conversion efficiency. Using ALD to uniformly coat ultrathin (a few nanometers) ZnO layers on the microscale irregular shape of bismuth telluride-based powders is a challenge. An ALD reactor that fluidizes or agitates the powders can be adapted for this purpose. In this study, two types of ALD reactors, a gas fluidization reactor and a rotary reactor, were used to coat selenium-doped bismuth telluride powders with ZnO. Uniform and conformal ZnO layers were successfully grown using both the ALD reactors. However, the crystalline structure, particle size distribution, and chemical bonding states of ZnO were affected by reactor type. Pelletization of the ALD-coated powders was performed by spark plasma sintering at a high temperature (500 °C) and pressure (60 MPa). The morphologies of the powders did not change with palletization; however, differences in the chemical states of the ZnO layers on the BTS powders were observed. It was observed that the remnant water molecules and mobile ion species might compensate for the carrier mobility in pellets made of ALD-coated powders.

Hydrogen plasma enhanced oxide removal on GaSb planar and nanowire surfaces

Due to its high hole-mobility, GaSb is a highly promising candidate for high-speed p-channels in electronic devices. However, GaSb exhibits a comparably thick native oxide causing detrimental interface defects, which has been proven difficult to remove. Here we present full oxide removal from GaSb surfaces using effective hydrogen plasma cleaning, studied in-situ by synchrotron-based X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV). GaSb nanowires turn out to be cleaned faster and more efficiently than planar substrates. Since the UHV conditions are not scalable for industrial sample processing, H-plasma cleaning is furthermore used as pre-treatment prior to atomic layer deposition (ALD) of a protective high-k layer to demonstrate the use of the cleaning step in a more realistic fabrication situation. We observe a cleaning effect of the H-plasma even in the ALD environment, but we also find residual Ga- and Sb-oxides at the GaSb-high-k interface, which we attribute to re-oxidation of the cleaned surface. Our results indicate that an improved control of the ALD reactor vacuum environment could realize oxide- and defect-free interfaces in GaSb-based electronics.

Atomic layer deposition of AlN on different SiC surfaces

Thin AlN films were grown using a Picosun R-200 atomic layer deposition (ALD) reactor on SiC surfaces with different crystallographic orientation: on-axis 4H-SiC (0001) and 8° off-axis 4H-SiC. TMA (trimethylaluminium) and NH3 were used as precursors while hydrogen and nitrogen plasma were applied for in-situ substrate cleaning. The substrate temperatures were 400 °C and 450 °C, with 20 ALD cycles. The surface morphology was investigated by scanning electron microscopy (SEM), which revealed nanometer-sized islands in all films. The AlN films deposited on on-axis 4H-SiC at 450 °C substrate temperature exhibited a relatively small roughness of about 0.255 nm. The chemical composition and bonding states were investigated by X-ray photoelectron spectroscopy. For all layers, high-resolution XPS showed Al 2p and N 1s spectra that are characteristic of AlN. These results are a good prerequisite of establishing the growth conditions of AlN films for surface acoustic wave (SAW) devices.

Practical Method for Designing Gas Conditions of Atomic Layer Deposition

In order to effectively develop the atomic layer deposition (ALD) reactor and process, having huge potentials and applications in the advanced technology fields, a practical design method of the gas conditions for the ALD was studied using computational fluid dynamics (CFD). The design method consisting of the following four steps was studied. 1) At a low gas pressure producing no gas recirculation, the maximum difference in the gas phase temperature from the sample stage temperature, ΔT, was obtained at various chamber wall temperatures. 2) The ΔT value was studied at various gas pressures producing the gas recirculation. 3) For determining the applicable process conditions, contour diagrams of the temperature uniformity were obtained utilizing the temperature uniformity equations consisting of various process parameters. 4) The relationships of the maximum gas residence time with the gas flow rate and the gas pressure were obtained. The process in this study is expected to be practical for designing the thermal and gas flow conditions for achieving a fast ALD.

Design and Construction of an ALD Reactor by Growth of Al2O3 Nanostructure Films

Objective: This project focuses on designing, building and commissioning work the atomic layer deposition (ALD) reactor for Al2O3 ultrathin film, which it will be contain specific components and a system's own control unit. Methodology: The ALD reactor was designed under a system to minimize components, flow lines and connections; to reduce manufacturing costs, volume of precursors, among others. Currently, ALD reactors are expensive to sell, maintain and replace parts. The design and manufacture of the ALD reactor manufactured at the University of Sonora (UNISON) is based on the state art with sequential binary reactions of the precursors, for the proposal for the manufacture of solar cells. Contribution: It was possible to build and commission the ALD reactor for the deposition of ultra-thin films, with the characteristics of being reproducible and scalable, which makes it attractive for commercialization. The homemade ALD reactor at UNISON is considered a very interesting equipment for the semiconductor research area, since it is possible to combine different types of materials in the form of films such as oxides and nitrides in the order of Angstroms (Ǻ).

Improved Cycling Stability of LiNi0.8 Co0.1 Mn0.1 O2 Cathode Material via Variable Temperature Atomic Surface Reduction with Diethyl Zinc.

High-Ni-rich layered oxides [e.g., LiNix Coy Mnz O2 ; x> 0.5, x+ y+ z= 1] are considered one of the most promising cathodes for high-energy-density lithium-ion batteries (LIB). However, extreme electrode-electrolyte reactions, several interfacial issues, and structural instability restrict their practical applicability. Here, a shortened unconventional atomic surface reduction (ASR) technique is demonstrated on the cathode surface as a derivative of the conventional atomic layer deposition (ALD) process, which brings superior cell performances. The atomic surface reaction (reduction process) between diethyl-zinc (as a single precursor) and Ni-rich NMC cathode [LiNi0.8 Co0.1 Mn0.1 O2 ; NCM811] material is carried out using the ALD reactor at different temperatures. The temperature dependency of the process through advanced spectroscopy and microscopy studies is demonstrated and it is shown that thin surface film is formed at 100°C, whereas at 200°C a gradual atomic diffusion of Zn ions from the surface to the near-surface regions is taking place. This unique near-surface penetration of Zn ions significantly improves the electrochemical performance of the NCM811 cathode. This approach paves the way for utilizing vapor phase deposition processes to achieve both surface coatings and near-surface doping in a single reactor to stabilize high-energy cathode materials.

Chemical Vapor Deposition of MoS2 for Back-End-of-Line Applications

I.Introduction2D transition metal dichalcogenides (TMDs) exhibit electronic properties from semimetals to wide bandgap semiconductors due to their thickness dependent bandgap. This property opens a wide diversity of applications that can be made from TMDs. For 3D heterogeneous integration of TMDs into the back-end-of-line (BEOL) of Si complementary metal-oxide-semiconductor circuitry, wafer-level direct growth (without film transfer) of TMDs within the BEOL thermal budget limit (550 °C/2 hours or 500 °C/5 hours [1]) is essential. In the literature, the majority of the TMDs (e.g. MoS2) are achieved from high temperature (650 °C - 1100 °C) deposition/anneal processes that are not compatible with BEOL [2]. In this work, MoS2 is successfully grown by chemical vapor deposition (CVD) approximately at/below BEOL thermal budget limit. We explore the potential of the achieved MoS2 films for BEOL logic (i.e. transistors), memory and sensing applications.II. CVD of MoS2 MoS2 was grown by CVD in a commercial 300 mm atomic layer deposition (ALD) reactor fitted with a showerhead. Mo(CO)6 (purity 99.99%, carried by N2) and H2S (purity 99.999%, 1% in Ar) were employed as the precursors. The reactor temperature was restricted at a low temperature range of 350 °C - 550 °C and the chamber pressure was maintained at ∼2.2 Torr. Growth was achieved (confirmed by Raman spectroscopy) on a selection of substrates including: SiO2 (85 nm) on Si (p++), c-plane sapphire, glass, and amorphous alumina (~30 nm by ALD on c-plane sapphire and on glass). It is noted that cm-scale substrates were placed throughout the chamber demonstrating that uniform and continuous film deposition is achievable across a 300 mm diameter with a high level of repeatability.III. Results and DiscussionHigh-resolution cross-sectional transmission electron microscopy (TEM) shows that 2D layered and polycrystalline MoS2 of ~10 nm is formed continuously and uniformly on a SiO2/Si substrate at 550 °C for 2.5 hours. Plan-view TEM also demonstrates the hexagonal structures of the MoS2. A rapid growth rate of ∼6 monolayers/hour on SiO2/Si is remarkable. Growth rate is slightly different depending on the substrates used. A rapid growth time is important in terms of reducing the thermal budget, but this comes at a cost of small grain sizes that are ~5 nm - 20 nm from this work, compared to μm-scale grain size achieved at 550 °C in the CVD of MoS2 by Kang et al., in which 26 hours was needed for 1 monolayer formation [3]. Due to the small grain size, charges are trapped at the high density of grain boundaries restricting lateral charge transport which is detrimental to transistors. This effect is also demonstrated in our work by 4-point resistivity and Hall-effect measurements, which show that the MoS2 films are highly resistive with a very low carrier concentration of 1014 - 1015 cm-3.Furthermore, MoS2 formation is achieved at 450 °C and 350 °C. The reduction in growth temperature from 550 °C, which is at the limit of the maximum temperature of the BEOL thermal budget limit [1], is also a crucial step (apart from reducing growth time) towards achieving 3D heterogeneous integration as this allows room for more manufacturing steps, for example, a dopant incorporation/activation step to functionalize TMDs for transistors.Although MoS2 grain boundaries and stoichiometric defects resulting from the low thermal budget growth are detrimental to lateral charge transport in transistors, these can be leveraged for memory and sensing functions. For memory, vertical transport memristor structures (Au/MoS2/Au) incorporating ~3 nm MoS2 (550 °C, 0.75 hour) show memristive switching and a stable memory window of 105 between the high-low resistive states with a retention time >104 seconds. The switching set and reset voltages are reduced compared to memristors made from single-crystalline MoS2 processed at higher temperatures. For sensing, interdigitated electrode-based gas sensors fabricated on ∼5 nm MoS2 (550 °C, 1.25 hours) show excellent selectivity and sub-ppm sensitivity to NO2 gas, with a notable self-recovery at room temperature without extra energy input.IV. ConclusionsRepeatable, uniform and continuous growth of MoS2 is achieved by CVD approximately at/below BEOL thermal budget limit in a commercial 300 mm reactor. Memristive and gas sensing functionality are achieved from the MoS2 films with an indication of reduced power consumption. This work advances a key enabling technology objective in emerging materials and devices for 3D heterogeneous integration.

Atomic Layer 3D Printing: Influence of Reactor Design and Pattern Geometry

As additive manufacturing in its various forms is shifting the paradigm of traditional manufacturing, the same space opens in the field of thin film deposition. Atomic layer deposition is, due to its inherent separation of reactions, uniquely suitable for adaptation into a 3D printer. In fact, the concept of spatial atomic layer deposition, which can be considered as a precursor for 3D atomic layer printing, goes all the way back to 1974.1 Despite the many challenges of creation and miniaturization of spatial ALD reactors, atomic layer 3D printing was successfully proved as a concept recently.2,3 Confining spatial ALD (atomic layer deposition) laterally to a spot with a size in the micron range allows one to perform ALD cycles by repeated passes of the deposition head above the substrate. The pattern defined by the motions of the deposition head may be arbitrarily complex. This concept allows for the definition of deposits in three dimensions in the manner of classical additive manufacturing (3D printing). However, the vertical resolution of the shapes generated is defined by the surface chemical principles of ALD, and therefore is on the order of single atoms. The lateral resolution depends on the printing head and the gas flows and is currently on the order of hundreds of μm.We have demonstrated the self-limiting behavior of this atomic-layer additive manufacturing (ALAM) procedure for several materials. Under atmospheric conditions, the deposition of TiO2 occurs with the same growth per pass as in conventional ALD. The cross-section of a deposit exhibits a horizontal surface and sharp edges. The self-limiting behavior of the surface chemistry is maintained. As an example of a noble metal, Pt grows in a highly crystalline and even oriented form. Air-sensitive precursors such as the metal alkyls can be handled safely in aerobic conditions, and the growth of Al2O3 and ZnO occurs with familiar characteristics.Thus, ALAM is a novel method allowing for the direct generation of multimaterial structures without the need for preliminary or subsequent patterning. However, for the best performance of atomic layer 3D printing, the influence of geometry of both the reactor and the pattern being printed has to be examined. Generally, due to the necessary spatial separation of precursor and reactant, edge effects are necessarily present. Moreover, deviations from the perfect printing geometry cause additional line edge effects and selectivity defects. Additionally, we created a general theoretical model of effects caused by spatial separation on the printed pattern. The theoretical model was then confronted with experiments performed on the atomic layer 3D printer developed by ATLANT 3D Nanosystems. The theoretical effects and samples analyzed include edges of lines, overlaps of lines including rastering and gradients, multiple paths overlaps during pattern printing and step pattern printing. To prove that these effects are independent of the specific material, the effects are explored for TiO2, ZnO, and Pt.[1] Tuomo Suntola, Jorma Antson. Method for producing compound thin films. US4058430A, United States Patent and Trademark Office, 29 November 1974.[2] Ivan Kundrata, Maksym Plakhotnyuk, Maïssa K. S. Barr, Sarah Tymek, Karol Fröhlich, Julien Bachmann (2020, June 30) An Atomic-Layer 3D Printer [Conference presentation] ALD/ALE 2020[3] Cesar Arturo Masse de la Huerta, Viet H. Nguyen, Abderrahime Sekkat, Chiara Crivello, Fidel Toldra-Reig, Pedro Veiga, Carmen Jimenez, Serge Quessada, David Muñoz-Rojas. Facile patterning of functional materials via gas-phase 3D printing [2020, Cornell University Condensed Matter, Materials Science] Figure 1