Thermal Atomic Layer Deposition Research Articles

(Invited) Role of Oxidant Gas for Atomic Layer Deposition of HfxZr1−XO2 Thin Films on Ferroelectricity of Metal-Ferroelectric-Metal Capacitors

We fabricated 400°C-annealed TiN/HfxZr1−xO2 (HZO)/TiN metal–ferroelectric–metal (MFM) capacitors using H2O and O2 plasma as oxidant gases of thermal (TH) and plasma-enhanced atomic layer deposition (PE-ALD), respectively, for HZO films. The PE-ALD film formed a more ferroelectric orthorhombic phase compared with the TH-ALD case. Therefore, the MFM capacitor with the PE-ALD film showed higher remanent polarization and dielectric constant. For the PE-ALD case, moreover, an oxygen-rich interfacial layer (O-rich-IL) was formed between the HZO film and TiN-bottom electrode during the ALD process. Thus, the MFM capacitor with the PE-ALD film showed less degradation of switching polarization during field cycling compared with the TH-ALD case, because an O-rich-IL should prevent the interface reaction and formation of additional oxygen vacancies in the PE-ALD film during field cycling. Based on these results, it is important to consider the selection of an ALD oxidant gas for the fabrication of HZO-based MFM capacitors.

Enhancing Carbon Fiber Fabrics with ALD AlxOy Coatings: An Investigation of Thickness Effects on Weight, Morphology, Coloration, and Thermal Properties

This study explores the impact of non-stoichiometric aluminum oxide (AlxOy) coatings applied via thermal atomic layer deposition (ALD) on carbon fiber fabrics (CFFs), emphasizing volume per cycle, FESEM analyses, color transitions, and thermal stability enhancements. Using trimethylaluminum and water at 100 °C, AlxOy was deposited across a range of 1000 to 5000 ALD cycles, with film thicknesses extending up to 500 nm. This notable increase in the volume of material deposited per cycle was observed for the 3D CFFs, highlighting ALD’s capability to coat complex structures effectively. FESEM analyses revealed the morphological evolution of CFF surfaces post-coating, showing a transition from individual grains to a dense, continuous layer as ALD cycles increased. This morphological transformation led to significant color shifts from green to red to blue, attributed to structural coloration effects arising from variations in film thickness and surface morphology. Thermogravimetric analyses (TGA and dTG) indicated that the AlxOy coatings enhanced the thermal stability of CFFs, with a postponement in degradation onset observed in samples subjected to more ALD cycles. In essence, this research highlights the nuanced relationship between ALD processing parameters and their collective influence on both the aesthetic and functional properties of CFFs. This study illustrates ALD’s potential in customizing CFFs for applications requiring specific color and thermal resilience, balancing the discussion between the surface morphological changes and their implications for color and thermal behavior.

Exploring TMA and H2O Flow Rate Effects on Al2O3 Thin Film Deposition by Thermal ALD: Insights from Zero-Dimensional Modeling

This study investigates the impact of vapour-phase precursor flow rates—specifically those of trimethylaluminum (TMA) and deionized water (H2O)—on the deposition of aluminum oxide (Al2O3) thin films through atomic layer deposition (ALD). It explores how these flow rates influence film growth kinetics and surface reactions, which are critical components of the ALD process. The research combines experimental techniques with a zero-dimensional theoretical model, designed specifically to simulate the deposition dynamics. This model integrates factors such as surface reactions and gas partial pressures within the ALD chamber. Experimentally, Al2O3 films were deposited at varied TMA and H2O flow rates, with system conductance guiding these rates across different temperature settings. Film properties were rigorously assessed using optical reflectance methods and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. The experimental findings revealed a pronounced correlation between precursor flow rates and film growth. Specifically, at 150 °C, film thickness reached saturation at a TMA flow rate of 60 sccm, while at 200 °C, thickness peaked and then declined with increasing TMA flow above this rate. Notably, higher temperatures generally resulted in thinner films due to increased desorption rates, whereas higher water flow rates consistently produced thicker films, emphasizing the critical role of water vapour in facilitating surface reactions. This integrative approach not only deepens the understanding of deposition mechanics, particularly highlighting how variations in precursor flow rates distinctly affect the process, but also significantly advances operational parameters for ALD. These insights are invaluable for enhancing the application of ALD technologies across diverse sectors, including microelectronics, photovoltaics, and biomedical coatings, effectively bridging the gap between theoretical predictions and empirical results.

Properties of low-resistivity molybdenum metal thin film deposited by atomic layer deposition using MoO2Cl2 as precursor

Challenges have arisen in selecting suitable candidates for interconnects and metal contacts due to the exponential increase in metal resistivity at scaled pitches. Molybdenum (Mo) has emerged as a promising alternative to the traditional metals such as copper or tungsten owing to its low electrical resistivity and electron mean free path. In this study, we investigated the formation of a molybdenum film grown by thermal atomic layer deposition (ALD) using a MoO2Cl2 solid precursor and H2 and NH3 gases as the reducing agents. A molybdenum nitride film served as the seed layer on a SiO2 substrate before molybdenum film deposition. The analysis focused on the film's phase, morphology, chemical bonding states, and resistivity across various thicknesses. X-ray diffraction (XRD) confirmed the presence of polycrystalline BCC planes. Our analyses confirmed the successful growth of the molybdenum metal thin film, which, at a thickness of 10 nm, exhibited a record-low resistivity of approximately 13 μΩ cm.

C-Axis Aligned Composite InZnO via Thermal Atomic Layer Deposition for 3D Nanostructured Semiconductor.

Amorphous oxide semiconductors have been widely studied for various applications, including thin-film transistors (TFTs) for display backplanes and semiconductor memories. However, the inherent instability, limited mobility, and complexity of multicomponent oxide semiconductors for achieving high aspect ratios and conformality of cation distribution remain challenging. Indium-zinc oxide (IZO), known for its high mobility, also faces obstacles in instability resulting from high carrier doping density and low ionization energy. To address these issues and attain a balance between mobility and stability, adopting a highly aligned structure such as a c-axis aligned crystalline IGZO could be advantageous. However, limited studies have reported enhanced electrical performance using crystalline IZO, likely attributed to the high thermal stability of the individual components (In2O3 and ZnO). Here, we first propose a c-axis aligned composite (CAAC) IZO with superior TFT properties, including a remarkable performance of field-effect mobility (μFE) of 55.8 cm2/(V s) and positive-bias-temperature-stress stability of +0.16 V (2 MV/cm, 60 °C, 1 h), as well as a low subthreshold swing of 0.18 V/decade and hysteresis as 0.01 V, which could be obtained through optimization of growth temperature and composition using thermal atomic layer deposition. These results surpass those of TFTs based on nanocrystalline/polycrystalline/amorphous-IZO. We conducted a thorough investigation of CAAC-IZO and revealed that the growth temperature and cation distribution profoundly influence the crystal structure and device properties. Finally, we observed excellent compositional conformality and 97% step coverage of IZO on a high-aspect-ratio (HAR) structure with an aspect ratio reaching 40:1, which is highly promising for future applications. Our results include a detailed investigation of the influence of the crystal structure of IZO on the film and TFT performance and suggest an approach for future applications.

Unfolding an Elusive Area-Selective Deposition Process: Atomic Layer Deposition of TiO2 and TiON on SiN vs SiO2.

Area-selective atomic layer deposition (AS-ALD) processes for TiO2 and TiON on SiN as the growth area vs SiO2 as the nongrowth area are demonstrated on patterns created by state-of-the-art 300 mm semiconductor wafer fabrication. The processes consist of an in situ CF4/N2 plasma etching step that has the dual role of removing the SiN native oxide and passivating the SiO2 surface with fluorinated species, thus rendering the latter surface less reactive toward titanium tetrachloride (TiCl4) precursor. Additionally, (dimethylamino)trimethylsilane was employed as a small molecule inhibitor (SMI) to further enhance the selectivity. Virtually perfect selectivity was obtained when combining the deposition process with intermittent CF4/N2 plasma-based back-etching steps, as demonstrated by scanning and transmission electron microscopy inspections. Application-compatible thicknesses of ∼8 and ∼5 nm were obtained for thermal ALD of TiO2 and plasma ALD of TiON.

Reliable ferroelectricity down to cryogenic temperature in wake-up free Hf0.5Zr0.5O2 thin films by thermal atomic layer deposition

The performance and reliability of ferroelectric thin films at temperatures around a few Kelvin are critical for their application in cryo-electronics. In this work, TiN/Hf0.5Zr0.5O2/TiN capacitors that are free from the wake-up effect are investigated systematically from room temperature (300 K) to cryogenic temperature (30 K). We observe a consistent decrease in permittivity (ε r) and a progressive increase in coercive electric field (E c) as temperatures decrease. Our investigation reveals exceptional stability in the double remnant polarization (2P r) of our ferroelectric thin films across a wide temperature range. Specifically, at 30 K, a 2P r of 36 µC/cm2 under an applied electric field of 3.0 MV/cm is achieved. Moreover, we observed a reduced fatigue effect at 30 K in comparison to 300 K. The stable ferroelectric properties and endurance characteristics demonstrate the feasibility of utilizing HfO2 based ferroelectric thin films for cryo-electronics applications.

Growth of conformal TiN thin film with low resistivity and impurity via hollow cathode plasma atomic layer deposition

Copper has been used as an interconnect material in integrated semiconductor devices because of its excellent conductivity, mechanical strength, and electromigration resistance. Introducing a diffusion barrier layer using transition metals such as Ti, Ta, W, Mo, and their nitrides can effectively prevent copper diffusion into the transistor region. TiN is widely used as the diffusion barrier. Plasma-enhanced atomic layer deposition (PEALD), which uses plasma to activate molecular reactions, can be used to fabricate high-quality thin films at lower temperatures than thermal atomic layer deposition. However, its high electrical resistivity and poor step coverage are disadvantageous for its adoption in highly scaled three-dimensional structures. In this study, TiN thin films were fabricated using PEALD with a hollow cathode plasma (HCP) source. The fabricated TiN exhibited a high density (5.29 g/cm3), which was very close to the theoretical density of TiN. Moreover, it has low electrical resistivity (132 μΩ cm) and excellent step coverage (>98%) in a trench pattern with a high aspect ratio of 32:1. These results suggest the possible application of the PEALD of TiN films using HCP sources in semiconductor device manufacturing.

Study of silicon surface passivation by ZnOx/AlOx stack prepared using super-cycle approach in thermal ALD process

High quality surface passivating films are essential for high efficiency solar cells. In this work, we present our study about the silicon surface passivation performance of thermal atomic layer deposited (T-ALD) ZnOx/AlOx stack deposited using super-cycle approach. Super-cycle approach is composed of three cycles of DEZ-DI water system followed by one cycle of TMA-DI water system. The number of super-cycles is varied to change the film thickness. Excellent passivation with surface recombination velocity ∼ 7 cm/s was achieved under this study for hydrogen annealed films. The passivation mechanism is related to the saturation of defects by hydrogen after annealing and further improvement in fixed oxide charges by one order of magnitude (1010 to 1011 cm−2). Hydrogenation at the optimum temperature (450⁰C) involves the transport of hydrogen atoms towards the interface and their interaction with dangling bonds at the Si surface leading to the effective passivation. The optical transmittance of the film in the visible region of the spectrum is found to be >95 % for all the deposited films. XPS data reveals that deposited films are Al rich which is possible due to ligand exchange reactions during the growth process. Annealing of ZnOx/AlOx stacks in hydrogen ambient and their Al-rich behavior helps in the realization of good passivation performance. XPS study also shows that Al-O bond improves with film thickness. According to valence band spectra, a relative shift in Fermi level position with regard to the valence band improves band bending, which improves field effect passivation. Such high-quality ZnOx/AlOx passivating films may improve the device efficiency.

Investigation of atomic layer deposition methods of Al2O3 on n-GaN

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states.

Suppressing substrate oxidation during plasma-enhanced atomic layer deposition on semiconductor surfaces

Plasma-enhanced atomic layer deposition (PE-ALD) is widely employed in microelectronics, energy, and sensing applications. Typically, PE-ALD processes for metal oxides utilize remote inductively coupled plasmas operated at powers of >200 W, ensuring a sufficient flux of oxygen radicals to the growth surface. However, this approach often leads to significant oxidation of chemically sensitive substrates, including most technological semiconductors. Here, we demonstrate that plasma powers as low as 5 W can effectively suppress substrate oxidation while maintaining the structural, optical, and electronic quality of the films. Specifically, we investigate the growth of titanium oxide (TiOx) using two commonly used metalorganic precursors, titanium isopropoxide and tetrakis(dimethylamino)titanium. Films deposited with 5 and 300 W oxygen plasma power are nearly indiscernible from one another, exhibiting significantly lower defect concentrations than those obtained from thermal ALD with H2O. The low plasma power process preserves desired physical characteristics of PE-ALD films, including large optical constants (n > 2.45 at 589 nm), negligible defect-induced sub-bandgap optical absorption (α < 102 cm−1), and high electrical resistivity (>105 Ω cm). Similar behavior, including suppressed interface oxidation and low defect content, is observed on both Si and InP substrates. As an example application of this approach, the assessment of InP/TiOx photocathodes and Si/TiOx photoanodes reveals a significant improvement in the photocurrent onset potential in both cases, enabled by suppressed substrate oxidation during low power PE-ALD. Overall, low power PE-ALD represents a generally applicable strategy for producing high quality metal oxide thin films while minimizing detrimental substrate reactions.

Development of Schottky barrier field-effect transistors (SB-MOSFET) with ultra-low thermal budget

This work presents the development of SB-MOSFET transistors incorporating thin HfO2 as a thin gate dielectric deposited by thermal atomic layer deposition (ALD) at low temperature. Aluminum thin films were used as the source/drain (S/D) and gate (G) electrodes, using e-beam evaporation at high-vacuum conditions. Even though low subthreshold slope (SS) values of 111 mV/dec and threshold voltage (Vth) of 0.58 V were obtained using a low thermal budget of 150 °C for transistor fabrication, scanning electron microscopy (SEM) analysis highlights the importance of proper mask alignment in order to enhance electrical performance and thus, reproducibility of the electrical characteristics of this device. This device can be fully integrated into silicon after standard Complementary Metal-Oxide-Semiconductor (CMOS)-compatible processing, so that it could be easily adopted into Front-End-Of-Line or even in Back-End-Of-Line stages of an integrated circuit, where low thermal budget is required and where its functionality could be increased by developing additional and fast logic.

Area-selective atomic layer deposition of palladium

We report area-selective deposition of palladium using sequential area-blocking and area-activation ALD processes. Thermal atomic layer deposition (ALD) of palladium is investigated at 100 °C using palladium (II) hexafluoroacetylacetonate and hydrogen. Palladium deposition does not proceed at such a low temperature unless catalytic sites are present to dissociate hydrogen during Pd film nucleation and before sufficient Pd has accumulated to serve as the catalyst for hydrogen dissociation. Ultrathin (<2 nm) nickel metal surfaces served as the initial catalytic sites and are prepared via low temperature (100 °C) reduction of NiO ALD films using a H-atom doser operating at 3 × 10−5 Torr. The Ni0 films are shown to seed the ALD of Pd, demonstrating a route to Pd ALD by area activation. Blanket NiO films are used to study Pd ALD growth and patterned NiO films, formed by area-blocking ALD, are used to demonstrate a bottom-up approach to patterned Pd films.

Practical and Thermal Atomic Layer Deposition of NiO as Hole‐Transporting Layers for Inverted Perovskite Solar Cells

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has significantly improved through advancements in fabrication methods, which have primarily focused on the perovskite absorber layer. The significance of improving the charge transport layer as the next crucial step toward achieving highly stable and efficient PSCs has also been emphasized. In inverted PSCs (i‐PSCs), the selection of a suitable p‐type hole‐transporting layer (HTL) has been restricted to mainly organic materials due to the rarity of p‐type inorganics. The instability and inherent disadvantages of organics necessitate the use of stable p‐type oxides as HTLs for i‐PSCs. Herein, uniform, conformal, and practical, yet thermal atomic layer deposition (ALD) for NiO is demonstrated by employing two different oxidant, ozone (O3) and hydrogen peroxide (H2O2). Both ALD‐NiO films are characterized by X‐ray diffraction and X‐ray reflection. By conducting X‐ray photoelectron spectroscopy analysis of the ALD‐NiO surfaces, a correlation between the oxidation power of the oxidant during ALD and the surface oxidation state of the ALD‐NiO films is established. Finally, the relationship between the oxidation state of the surfaces with different oxidant and the i‐PSC performance is verified. The fabricated i‐PSCs exhibit a PCE exceeding 19%.

Crystallinity-Controlled Atomic Layer Deposition of Ti-Doped ZrO2 Thin Films

We investigated Ti-doped ZrO2 deposition using a cyclopentadienyl tris(dimethylamino) zirconium (CpZr(NMe2)3) precursor and a titanium tetraisopropoxide (TTIP) precursor using an O3 thermal atomic layer deposition process. In addition, the effect of Ti doping concentration on the chemical bonding and electrical properties of the Ti-doped ZrO2 thin films was studied. O3 was used at a high concentration of 400 g m−3. We varied the Ti doping concentration by controlling the rate of the supercycle process in the ZrO2 process window of 200 °C–300 °C. As a result, the highest dielectric constant was observed at a Ti doping concentration of 2.5% because it enhances the crystallinity of ZrO. Excessive Ti doping hinders crystal formation.

Effect of hydrogen on the chemical state, stoichiometry and density of amorphous Al2O3 films grown by thermal atomic layer deposition

Amorphous oxide thin films grown by thermal atomic layer deposition (ALD) typically contain high impurity concentrations of hydrogen, which affects both chemistry and structure and thereby the functional properties, such as the barrier properties in, for example, microelectronic and photovoltaic devices. This study discloses the effect of H incorporation in amorphous Al2O3 ALD oxide films on the local chemical binding states of Al, O and H, as well as the oxide density and stoichiometry, by a combined analytical approach using elastic recoil detection analysis, Rutherford backscattering spectroscopy and full chemical state analysis by dual‐source X‐ray photoelectron spectroscopy (XPS)/hard X‐ray photoelectron spectroscopy (HAXPES). The experimental findings are compared with crystalline anhydrous α‐Al2O3 and hydroxide α‐Al (OH)3 reference phases and further supported by molecular dynamic simulations. It is shown that H preferably forms covalent –OH hydroxyl bonds with O in the nearest‐neighbour coordination spheres of interstitial Al cations, which affects both the ligand electronic polarizability and the bond length of the randomly interconnected [AlOn] polyhedral building blocks in the amorphous ALD oxide films.

Thermal Atomic Layer Deposition of Silicon Carbonitride: A Density Functional Theory Study

Silicon carbonitride (SiCN) films have received considerable attention due to their lower dielectric constant and better etch selectivity than silicon nitride (SiN) films [1]. Therefore, SiCN is replacing SiN in a variety of applications, such as dielectric barriers for copper interconnects or sidewall spacers for self-aligned contacts. SiCN films have been mainly prepared by plasma-enhanced chemical vapor deposition (PECVD) or plasma-enhanced atomic layer deposition (PEALD). However, plasma-based deposition techniques suffer from poor film quality and lack of conformability on high-aspect-ratio patterns. Thermal atomic layer deposition at high process temperatures can deposit high-quality thin films with excellent conformality. However, no studies have been published on thermal ALD in SiCN, while there are a few reports on PEALD in SiCN. In this work, we studied the thermal ALD process of SiCN by alternating exposures to a carbon-containing silicon precursor and NH3 and compared it with the thermal ALD of SiN processes using carbon-free precursors. Density functional theory (DFT) calculations were used to model and simulate the continuous ligand exchange reactions between the silicon precursor and the substrate surface. The carbon incorporation in the growing film when using carbon-containing precursors was predicted by DFT, which is in good agreement with the experimental observations. Acknowledgments This work was supported by Hansol Chemical Co., Ltd.

Plasma Anneal As a Stress Control Method in Low Temperature Al2O3 ALD Coatings

The control of mechanical physical stress is an important requirement in a variety of applications, such as flexible electronics or thick optical coatings where film stress can cause delamination. Traditionally, in thermal ALD it has been difficult to control the stress. Al2O3 deposited at low temperature is consistently found to have a tensile stress level in the range of a 300 to 450 MPa ( refs 1-3). It is well established that In magnetron sputtering , film stress can be controlled straightforwardly using the Argon pressure during deposition, as this affects the level of bombardment of the growing film with energetic particles.Paranjpe et al ( ref 1) have demonstrated that the use of a plasma anneal leads to densification and transition of the stress from tensile to compressive. The process consists of a sequence of deposition of a thin layer ( e.g. 5 nm) by thermal ALD, followed by exposure to a low energy plasma. The atomic displacements caused by low energy ions from the plasma tend to lead to filling of nanoscopic voids and point defects, thus releasing tensile stress.In this presentation, we will review an in depth study of the effect of various process parameters on the ability of a capacitively coupled in-situ plasma anneal to control stress. The following parameters will be reviewed: Gas composition: Argon plasmas, Oxygen plasmas and Argon oxygen mixtures are compared. Gas Pressure: The effect of gas pressure on plasma characteristics and the resulting modification of the coating is studied RF Power: RF Power was varied between 0 and 300 W ( 1.32 W/cm2) Plasma duration : The duration of the plasma treatment was varied from 0 to 60 seconds.A process window was identified where the plasma anneal can be used as an effective, controllable tool to reduce stress in the coatings. This is illustrated in fig. 1, which shows the substrate warpage (“bow”) observed in 75 mm thick PET foils with a 50 nm Al2O3 coating deposited at 90 °C. It can be seen that the untreated coating (0 sec plasma) has a bow corresponding to a tensile stress of 370 MPa, in accordance with literature values. As the plasma anneal time increases, the stress value decreases. Applying a 60 sec plasma every 5 nm allows to deposit a virtually stress free coating. Simultaneously with the stress reduction, an increase of the optical index and a slight reduction of the thickness are observed, consistent with a densification of the coating.

(Invited) Atomic Layer Processing of MoS2

Molybdenum disulfide (MoS2) is one of several transition metal dichalcogenides consisting of a layer of transition atoms sandwiched between layers of chalcogens. Interest in MoS2 has been driven by its 1.8 eV direct electronic band gap in monolayer form and its moderate electron mobility (10-100 cm2/Vs) even when only three atoms (~6.5 Å) thick. These properties offer potential for retaining or improving speed and efficiency in scaled electronic devices.Advancing MoS2 and other 2D materials into high volume manufacturing of semiconductor devices requires scalable deposition and etching processes. The process of atomic layer deposition (ALD) has been used to deposit 2D materials and can deposit films at lower temperatures for back-end-of-line (BEOL) compatibility. However, ALD relies on covalent surface reactions. When these reactions occur at high density during film nucleation, the resulting film is typically amorphous or nanocrystalline and requires thermal annealing to form a more well-ordered layered crystal structure. A key challenge is to establish a low temperature ALD process that achieves a layered crystal structure with low defectivity. In terms of material removal, MoS2 films have been etched by plasma-based atomic layer etching (ALE). ALE is analogous to ALD except that cyclic surface reactions promote volatility and lead to film removal rather than deposition. Together, atomic layer deposition and atomic layer etching constitute complementary facets of atomic layer processing and enable new pathways for nanomanufacturing.Here, we describe our recent progress in thermal ALD and thermal ALE of MoS2 films. For ALD of MoS2, we use cycles of MoF6 and H2S between 150-350 °C. Nucleation on metal oxide surfaces shows a strong temperature dependence, which is attributed in part due to the temperature dependence of surface hydroxyl groups. Density functional theory (DFT) calculations support the role of hydroxyls in promoting nucleation and reveal that MoF6 dissociates on the surface rather than participating in a ligand exchange reaction. The dissociation reaction leads to the formation of a metal fluoride interface between the oxide surface and the MoS2 film. Surface species following MoF6 include MoOF4 and MoO3, both of which convert to MoS2 during the H2S pulse, which releases H2O and HF byproducts. Films deposited at 200 °C and below are amorphous but convert to a layered structure upon annealing in H2S. Deposition at 250 °C yields films with a layered crystal structure without post-deposition annealing.Thermal ALE of MoS2 films deposited by ALD can be achieved using alternating exposures of MoF6 and H2O. It was found that MoS2 films are fluorinated by the MoF6 and then oxygenated by H2O. Volatile byproducts are MoF2O2, H2S, and HF, which are primarily released during the H2O half-cycle. Thermal ALE of MoS2 is temperature dependent and follows Arrhenius behavior. At 200 °C, the mass loss per cycle is 6 ng/cm2 (~0.5 Å/cycle) and reaches 271 ng/cm2 at 300 °C. Etch stop behavior was observed once the etched film approached the metal fluoride interface of the growth surface. Layered MoS2 deposited by ALD with post-deposition annealing exhibited layer-by-layer etching at 0.2 Å/cycle at 250 °C.The ALD results provide key insights into the nucleation and growth of MoS2 films using low temperature ALD. Further work is required to control basal plane orientation but achieving crystalline films well within BEOL thermal budgets is promising. The results from the ALE study provide insights into the etching reactions for MoS2. Combining the two processes offers greater control over MoS2 films. Using ALD followed by ALE and post-deposition annealing, we achieved few-layer MoS2 films. These combined thermal processes represent a pathway for integration of MoS2 films into device manufacturing.

Multi-Scale Simulations of Gas-Phase Particles Generated in Plasma Enhanced Atomic Layer Deposition Processes

Recently, Plasma Enhanced Atomic Layer Deposition (PEALD) has been investigated as an advanced semiconductor deposition method to expand the number and variety of materials other than those available in thermal ALD. The high reactivity of the plasma species at the deposition surface during PEALD has the advantage of increasing the degree of freedom of processing conditions and enabling a wider range of material properties. PEALD consists of four steps in the order of feed, 1st purge, reaction, and 2nd purge. In this reaction step, the secondary gas-phase reaction is important because the properties of the thin film vary greatly depending on the particles that arrive on the growing surface. Therefore, the particle distribution in the gas phase before arrival on the surface should be accurately predicted to estimate the film quality. Generally, the DSMC (Direct Simulation Monte Carlo) method is a commonly used method for analyzing particle distribution in the gas phase. The DSMC method requires reaction and scattering models that simulate the behavior between molecules. Variable Hard Sphere and Variable Soft Sphere are often used as the model in DSMC. These models are about molecular transport, such as molecular diffusion, and cannot deal with molecular reactions. The total collision energy model is determined for molecular reactions, but it may not accurately describe molecular reactions in non-equilibrium flow. The purpose of the study is to develop a new reaction-scattering model in non-equilibrium flow in the DSMC method for determining the particle distribution in the gas phase in the PEALD processes. Here, we focus on ammonia plasma used in the reaction step of boron nitride nanomaterial films, which are used in various applications, such as semiconductors and fine ceramics with two-dimensional and three-dimensional structures. This study focused on neutral and ionic molecules, NH3 and NH4 +, which can be the main collisional species in secondary gas-phase reactions.A molecular dynamics simulation with a reactive force field (ReaxFF) was performed. The left figure shows the simulation system of NH3 and NH4 + collisions. The periodic boundary conditions were applied to the simulation box for all directions in the size of 100 Å × 100 Å × 100 Å. Randomly oriented NH3 and NH4 + were placed in a line at an initial distance of 15 Å. NH4 + was given an electric charge of –0.22892 on N and 0.30723 on H. The initial translational energy of NH4 + was set at 5-60 eV, and the difference in the vertical coordinate at 0.1-5.0 Å. The collisions were sampled 2000 times.The right figure shows the calculated collision cross section, which determines whether a collision occurs or not. The collision coefficient bcoll represents the difference in the maximum vertical coordinate at which the absolute value of the scattering angle is greater than 1 degree. The collision cross section decreases as the translational energy increases because the negligible attraction-force act on NH3 and NH4 + at a long distance. When the translational energy exceeds 40 eV, the collisional cross-section no longer decreases and becomes asymptotic. The reason can be considered as follows. As the cross-section approaches 40 Å2, the collision cross section asymptotically approaches 40 Å2, regardless of the translational energy, because the attractive and repulsive forces are balanced. We compared our results with a reference model in grey color. In the reference model, Etr, 0 represents the minimum translational energy, σT,0 is the collision cross section at the minimum, Etr is the translational energy, and ω is a fitting parameter. Here, ω was set at –3.1. The reference model shows a good agreement with our results. However, the collision cross section is asymptotic above 40 eV, which may affect the results when the DSMC simulations are performed. We will apply the reaction scattering model to the DSMC method and analyze the gas-phase reaction of PEALD, including a comparison of the results of existing models. Figure 1