Plasma-enhanced Atomic Layer Deposition Process Research Articles

Atomic layer deposited Mo2N thin films using Mo(CO)6 and NH3 plasma as a Cu diffusion barrier

Thin films of molybdenum nitride (Mo2N) are prepared using sequential exposure of molybdenum hexacarbonyl [Mo(CO)6] and NH3 plasma in a plasma-enhanced atomic layer deposition (PEALD) reactor. Several process parameters such as the deposition temperature, plasma power, and post-annealing conditions are systematically investigated to achieve the best quality films. The superior growth kinetics is evident with a significantly higher growth per cycle (GPC) value with lower incubation period for this PEALD process (~1.1 Å, ~36 cycles) when compared to thermal ALD (~0.3 Å, ~63 cycles), both carried out at 200 °C. The growth rate of the Mo2N film reveals a significant jump above 215 °C, indicating a severe decomposition of Mo(CO)6, however, polycrystalline γ-Mo2N films with face-centered-cubic structure are evident within the deposition temperature range of 200–230 °C. The sharp decrease in the resistivity of the as-grown Mo2N films is observed with increasing deposition temperature, film thickness, and plasma power. The resistivity could be further lowered by a post-annealing process and the lowest resistivity of ~395 μΩ cm is achieved for the thin film deposited with 300 watt plasma power and annealed at 700 °C. Finally, the Cu-diffusion barrier capability of an extremely thin film (~7 nm) of as-deposited Mo2N is evaluated in Cu/PEALD-Mo2N/Si structure. The X-ray diffractometry analysis confirms that such a thin layer can successfully prevent the diffusion of Cu up to 500 °C and significant Cu3Si formation was observed only at 600 °C and above. The gradual failure of the diffusion barrier upon annealing is further investigated using electrical impedance (EI) analyses in two different modes. In-plane EI measurements directly indicate the change in the Cu layer at the top, and the formation of Cu3Si can be inferred from the through-plane mode. A comparison with PEALD and thermal-ALD-grown MoNx from EI analyses further reflects the superiority of plasma-enhanced process towards fabricating diffusion barrier layer.

Characteristics of silicon nitride deposited by very high frequency (162 MHz)-plasma enhanced atomic layer deposition using bis(diethylamino)silane

Silicon nitrides, deposited by capacitively coupled plasma (CCP)-type plasma enhanced atomic layer deposition (PEALD), are generally applied to today’s nanoscale semiconductor devices, and are currently being investigated in terms of their potential applications in the context of flexible displays, etc. During the PEALD process, 13.56 MHz rf power is generally employed for the generation of reactive gas plasma. In this study, the effects of a higher plasma generation frequency of 162 MHz on both plasma and silicon nitride film characteristics are investigated for the purpose of silicon nitride PEALD, using bis(diethylamino)silane (BDEAS) as the silicon precursor, and N2 plasma as the reactant gas. The PEALD silicon nitride film deposited using the 162 MHz CCP exhibited improved film characteristics, such as reduced surface roughness, a lower carbon percentage, a higher N/Si ratio, a lower wet etch rate in a diluted HF solution, lower leakage current, and higher electric breakdown field, and more uniform step coverage of the silicon nitride film deposited in a high aspect ratio trench, as compared to silicon nitride PEALD using 13.56 MHz CCP. These improved PEALD silicon nitride film characteristics are believed to be related to the higher ion density, higher reactive gas dissociation, and lower ion bombardment energy to the substrate observed in N2 plasma with a 162 MHz CCP.

Ultrahigh purity conditions for nitride growth with low oxygen content by plasma-enhanced atomic layer deposition

Ultrahigh purity (UHP) reactor conditions provide a process environment for growth of nitride thin films with low oxygen content by plasma-enhanced atomic layer deposition (PEALD). In particular, UHP conditions correspond to partial pressures below 10−8 Torr for impurities within the PEALD process environment to limit incorporation before, during, and after film growth. In this article, we identify the various sources of background oxygen species and describe the measures taken to obtain UHP reactor conditions. For example, in situ ellipsometry results are presented that reveal the impact of oxygen incorporation on film resistivity during and after titanium nitride PEALD due to elevated levels of oxygen impurities in the argon process gas. A model is also developed that shows the significance of water permeation through elastomer vacuum seals. These examples demonstrate the importance of process gas purification and elimination of elastomer permeation toward achieving a UHP environment. X-ray photoelectron spectroscopy (XPS) depth profile data for titanium, aluminum, and silicon nitride by PEALD reveal bulk oxygen levels below 1 at. %, thereby demonstrating the effectiveness of UHP reactor conditions at reducing oxygen incorporation. Consistent with XPS, depth profile secondary ion mass spectroscopy results for titanium nitride PEALD confirm bulk oxygen content less than 1 at. %, further establishing the effectiveness of a UHP background for high purity nitride film growth.

Resolving self-limiting growth in silicon nitride plasma enhanced atomic layer deposition with tris-dimethylamino silane precursor

Self-limiting character of the involved surface reactions is essential for highly uniform and conformal growth in atomic layer deposition (ALD). However, the poor conformal coverage (<75%) that is often reported with silicon nitride (SiNx) plasma enhanced ALD (PEALD) processes using metalorganic Si-precursors is confounding. In this article, we report our study of the SiNx PEALD process using the tris-dimethylamino silane (3DMAS) precursor and forming gas (5% H2–95% N2) reactant plasma. For the substrate temperature (Tsub) range of 50 °C ≤ Tsub ≤ 150 °C, growth per cycle (GPC) for SiNx deposition was found to approach saturation at 0.034 ± 0.001 nm/cycle though higher Tsub required longer 3DMAS exposures (tA) for saturation. However, for Tsub > 150 °C, SiNx GPC was seen to increase with tA, indicating nonself-limiting growth from potential chemical vapor deposition-like side reactions emerging at higher temperatures. The refractive index (n) of 2.097 ± 0.003 at 2 eV with an optical bandgap of ∼1.7 eV determined from in situ spectroscopic ellipsometry measurements, and peaks s1 and n1 with ΔBE = 295.42 eV in Si2p and N1s XPS spectra measured on the capped SiNx sample were found to agree with the optical constants and chemical characteristics reported for the silicon nitride material. SiNx films deposited at Tsub = 250 °C (nonself-limiting) were found to be more resistant to ambient oxidation as compared to SiNx PEALD films grown at Tsub = 100 °C. Although an entire 30 nm thick SiNx PEALD film was oxidized after an unavoidable long ambient exposure, a cross-sectional transmission electron microscope image showed a conformal coverage of 95%–98% in a 3D trench structure with an aspect ratio of 4.5. Furthermore, higher resistance to ambient oxidation in plasma treated of SiNx PEALD films demonstrates a potential of postgrowth treatments to improve desirable material properties without resorting to high-temperature processes.

Machine learning-based modeling and operation of plasma-enhanced atomic layer deposition of hafnium oxide thin films

Plasma-enhanced atomic layer deposition (PEALD) has demonstrated its superiority at coating ultra-conformal high dielectric thin-films, which are essential to the fin field-effect transistors (FinFETs) as well as the advanced 3D V-NAND (vertical Not-AND) flash memory cells. Despite the growing research interest, the exploration of the optimal operation policies for PEALD remains a complicated and expensive task. Our previous work has constructed a comprehensive 3D multiscale computational fluid dynamics (CFD) model for the PEALD process and demonstrated its potential to enhance the understanding of the process. Nevertheless, the limitation of computational resources and the relatively long computation time restrict the efficient exploration of the operating space and the optimal operating strategy. Thus, in this work, we apply a 2D axisymmetric reduction of the previous 3D model of PEALD reactors with and without the showerhead design. Furthermore, a data-driven model is derived based on a recurrent neural network (RNN) for process characterization. The developed integrated data-driven model is demonstrated to accurately characterize the key aspects of the deposition process as well as the gas-phase transport profile while maintaining computational efficiency. The derived data-driven model is further validated with the results from a full 3D multiscale CFD model to evaluate model discrepancy. Using the data-driven model, an operational strategy database is generated, from which the optimal operating conditions can be determined for the deposition of Hafnium Oxide (HfO2) thin-film based on an elementary cost analysis.

Reactivity of different nitriding agents with chlorine-terminated surface during atomic layer deposition of silicon nitride

The atomic layer deposition (ALD) of silicon nitride using silicon chloride was simulated by density functional theory (DFT) calculation to compare different nitriding agents. We chose the NSiCl2*/SiCl*-terminated Si3N4 substrate as the adequate chlorine-terminated substrate. We confirmed that the thermal ALD reaction using NH3 or N2H4 is energetically favorable and found that the reaction with N2H4 shows an activation energy of 1.77 eV, which is lower than 3.30 eV of the reaction with NH3. In the plasma-enhanced ALD (PEALD) process using NH3 plasma, NH2− ion and H⋅ radical remove the Cl atoms of the Si3N4 surface to form active sites for the following reaction, –SiNH2* group or –Si*. N⋅ radicals in N2 plasma may not remove the Cl atoms because the recombination reaction is more favored. The PEALD process using NH3 plasma showed a growth rate of 0.09 nm/cycle at 300 °C, but the growth rate of the process using N2 plasma is negligible, which is in good agreement with our DFT calculations.

Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition.

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga2O3 films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., α-Ga2O3 (006) and β-Ga2O3 (4̅02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures ≥ 190 °C by using an optimized plasma-enhanced ALD process such that α-Ga2O3 (006)∥α-Al2O3 (006) and β-Ga2O3 (2̅01)∥α-Al2O3 (006). Both α-Ga2O3 (006) and β-Ga2O3 (2̅01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 °C was determined to have superior properties among as-deposited films. Using TEM to locate α-Ga2O3 and β-Ga2O3 domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of α-Ga2O3 domains in the film and produce pure β-Ga2O3 films via an energy-efficient process. A pure β-Ga2O3 phase on sapphire with β-Ga2O3 (2̅01)∥α-Al2O3 (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 °C preceded by the low deposition temperature of 190 °C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energy-efficient fabrication process.

Multiscale computational fluid dynamics modeling and reactor design of plasma-enhanced atomic layer deposition

Plasma-enhanced atomic layer deposition (PEALD) is one of the most widely adopted deposition methods in the semiconductor industry. It is chosen largely due to its superior ability to deliver ultra-conformal dielectric thin-films with high aspect-ratio surface structures, which are encountered more and more often in the novel design of metal-oxide-semiconductor field-effect transistors (MOSFETs) in the NAND (Not-And)-type flash memory devices. Compared with the traditional thermal ALD method, PEALD allows for lower operating temperature and speeds up the deposition process with the involvement of plasma species. Despite its popularity, the development of PEALD operation policies remains a complicated and expensive task, which motivates the construction of an accurate and comprehensive simulation model. While existing models have described the individual or partially coupled domains, none of these models has captured all three domains in the PEALD process: surface reaction, macroscopic gas transport, and plasma generation. In this work, a comprehensive multiscale computational fluid dynamics (CFD) model is developed for a remote PEALD reactor used in the deposition of HfO2 thin-films. First, a previously developed kinetic Monte-Carlo (kMC) model is adapted for the multiscale simulation to describe the surface reactions. Then, two macroscopic models, specifically tailored for the remote plasma reactor, are formulated to describe the dynamic behaviors of the plasma generation and bulk species transport domains, respectively. Additionally, an integrated message passing interface (MPI) scheme is built to couple and resolve the communication between different scales in the model. The model results are then validated with experimental data and an automated workflow is established for model calculations without human intervention. Finally, a set of baseline operating conditions derived from the model simulation results is proposed to guarantee the production of high-quality thin-films.

Effect of Oxygen Source on the Various Properties of SnO2 Thin Films Deposited by Plasma-Enhanced Atomic Layer Deposition

Herein, we performed a comparative study of plasma-enhanced atomic layer deposition (PEALD) of SnO2 films using Sn(dmamp)2 as the Sn source and either H2O plasma or O2 plasma as the oxygen source in a wide temperature range of 100–300 °C. Since the type of oxygen source employed in PEALD determines the growth behavior and resultant film properties, we investigated the growth feature of both SnO2 PEALD processes and the various chemical, structural, morphological, optical, and electrical properties of SnO2 films, depending on the oxygen source. SnO2 films from Sn(dmamp)2/H2O plasma (SH-SnO2) and Sn(dmamp)2/O2 plasma (SO-SnO2) showed self-limiting atomic layer deposition (ALD) growth behavior with growth rates of ~0.21 and 0.07–0.13 nm/cycle, respectively. SO-SnO2 films showed relatively larger grain structures than SH-SnO2 films at all temperatures. Interestingly, SH-SnO2 films grown at high temperatures of 250 and 300 °C presented porous rod-shaped surface morphology. SO-SnO2 films showed good electrical properties, such as high mobility up to 27 cm2 V−1·s−1 and high carrier concentration of ~1019 cm−3, whereas SH-SnO2 films exhibited poor Hall mobility of 0.3–1.4 cm2 V−1·s−1 and moderate carrier concentration of 1 × 1017–30 × 1017 cm−3. This may be attributed to the significant grain boundary and hydrogen impurity scattering.

Low temperature epitaxial growth of GaP on Si by atomic-layer deposition with plasma activation

An approach for epitaxial growth of GaP layers on Si substrates at low temperature (380 °C) by plasma-enhanced atomic layer deposition (PEALD) is explored. A significant improvement of the crystalline properties of the GaP layers is obtained using additional in-situ Ar plasma treatment. The epitaxial growth for the first 20–30 nm of GaP on Si is demonstrated from transmission electron microscopy. Moreover, the use of in-situ Ar plasma treatment during the PEALD process allows one to increase the growth rate per cycle from 0.9 ± 0.1 Å/cycle to 1.9 ± 0.1 Å/cycle and reduce the RMS roughness from 3.76 nm to 1.88 nm. The effect of Ar plasma treatment on the electronic properties of the GaP/Si interface is studied by deep level transient spectroscopy (DLTS). A defect level at (0.33 ± 0.03) eV below the conduction band is observed in the subsurface layer of Si for the GaP/Si structure grown under Ar plasma treatment. However, the defect response observed by DLTS vanishes after rapid thermal annealing at 500 ºC in nitrogen ambient.

Microscopic modeling and optimal operation of plasma enhanced atomic layer deposition

Plasma enhanced atomic layer deposition (PEALD) is one of the most widely adopted deposition methods used in the semiconductor industry. It is chosen largely due to its superior ability to produce ultra-thin high-k dielectric films, which are needed for the further miniaturization of microelectronic devices with the pace of Moore's Law. In contrast to the traditional thermal atomic layer deposition (ALD) method, PEALD allows for high deposition growth per cycle (GPC) under low operating temperature with the help of high energy plasma species. Despite the experimental effort in finding new precursors and plasmas, the detailed surface structures and reaction mechanisms in various PEALD processes remain hard to understand because of the limitation of current in-situ monitoring techniques and the deficiency of the first-principles based analysis. Therefore, in this work, an accurate, yet efficient kinetic Monte Carlo (kMC) model and an associated machine learning (ML) analysis are proposed to capture the surface deposition mechanism and to propose optimal operating conditions of the HfO2 thin-film PEALD using tetrakis-dimethylamino-Hafnium (TDMAHf) and oxygen plasma. Density Functional Theory (DFT) calculations are performed to obtain the key kinetic parameters and the structural details, subsequently employed in the kMC model. After the kMC model is validated by experimental data, a database is generated to explore a variety of precursor partial pressure and substrate temperature combinations using the kMC algorithm. A feed-forward Bayesian regularized artificial neural network (BRANN) is then constructed to characterize the input–output relationship and to investigate the optimal operating conditions.

The role of plasma in plasma-enhanced atomic layer deposition of crystalline films

The inclusion of plasma in atomic layer deposition processes generally offers the benefit of substantially reduced growth temperatures and greater flexibility in tailoring the gas-phase chemistry to produce specific film characteristics. The benefits plasmas provide, however, come at the cost of a complex array of process variables that often challenge the ability to predict, a priori, the influence of any one input parameter. In this work, the authors attempt to provide some clarity as to how plasmas are formed and controlled and how they can most optimally be employed within the framework of atomic layer deposition. To begin, the authors cover some of the fundamentals of plasma generation along with the production of energetic and reactive species and their transport within the plasma. They then focus on how different plasma generation schemes and geometries, often employed in plasma-enhanced atomic layer deposition (PEALD), differ in their production of energetic and reactive species. They also address the plasma-surface interactions that are critical for film growth and control of crystallinity. Throughout this work, the authors use both current experimental data and a review of previously published works to describe how variations in the approach to plasma generation and the interactions between plasma-produced species and the growth surface influence the plasma reactant step in PEALD processes. The authors highlight two case studies to demonstrate how these relationships can be used to control the phase purity of crystalline titanium dioxide (TiO2) films and grow crystalline growth of semiconducting indium nitride (InN).

Experimental and numerical analysis of the effects of ion bombardment in silicon oxide (SiO2) plasma enhanced atomic layer deposition (PEALD) processes

Reactions between a precursor-absorbed SiO2 surface and energetic ion species, i.e. Ar+ or O+ ions, in a plasma enhanced atomic layer deposition (PEALD) process were investigated using an in situ X-ray photoelectron spectrometer system and a molecular dynamics (MD) simulation. Both the experiment and simulation results showed that N and/or C atoms originating from the precursor molecules remained on the surface as amorphous carbon and/or cyanide after 50 eV ion bombardment. The precursor-originated atoms as well as the SiO2 film were removed by the incident ions when its energy increased to 100 eV, and exceedingly small amount remained on the surface. The MD simulation results showed that chemical effects had a more obvious effect on the removal of C atoms at lower incident energies, while purely physical effects dominated at high O+ ion incident energies. These results indicated the importance of ion energy in PEALD processes in terms of film quality.

Direct plasma-enhanced atomic layer deposition of aluminum nitride for water permeation barriers

Oxygen-free, transparent, and insulating thin films at fabrication temperatures below 100 °C are important for sensitive materials and interfaces, like organic electronics. In this work, a capacitive coupled plasma-enhanced atomic layer deposition process of aluminum nitride with trimethylaluminum as a precursor and a mixture of hydrogen/nitrogen as the reactive gas was studied at 80 and 200 °C. The film properties were characterized by spectroscopic ellipsometry, x-ray photoelectron spectroscopy, scanning electron microscopy, and electrical measurements. The growth per cycle stayed constant at around 1 Å, and the refractive index decreased slightly from 1.97 at 200 °C to 1.93 at 80 °C. While the AFM surface roughness was below 0.5 nm at 80 °C deposition temperature, scanning electron microscopy images reveal blister generation at 200 °C on silicon. The x-ray photoemission spectroscopy measurements show a layer composition of Al:N of 1.0:0.9 with a few percent of oxygen and carbon, indicating good air stability and reasonable stoichiometry. The metal-insulator-metal capacitance measurements showed a dielectric constant of 11 at both temperatures. The water vapor transmission rate for a 20 nm thick film on polyethylene naphthalate films was lower than 0.5 mg/(m2⋅day) at 38 °C/90%.

A liquid alkoxide precursor for the atomic layer deposition of aluminum oxide films

For large-scale atomic layer deposition (ALD) of alumina, the most commonly used alkyl precursor trimethylaluminum poses safety issues due to its pyrophoric nature. In this work, the authors have investigated a liquid alkoxide, aluminum tri-sec-butoxide (ATSB), as a precursor for ALD deposition of alumina. ATSB is thermally stable and the liquid nature facilitates handling in a bubbler and potentially enables liquid injection toward upscaling. Both thermal and plasma enhanced ALD processes are investigated in a vacuum type reactor by using water, oxygen plasma, and water plasma as coreactants. All processes achieved ALD deposition at a growth rate of 1–1.4 Å/cycle for substrate temperatures ranging from 100 to 200 °C. Film morphology, surface roughness, and composition have been studied with different characterization techniques.

MOS Capacitance Measurements for PEALD TiO2 Dielectric Films Grown under Different Conditions and the Impact of Al2O3 Partial-Monolayer Insertion.

In this paper, we report the plasma-enhanced atomic layer deposition (PEALD) of TiO2 and TiO2/Al2O3 nanolaminate films on p-Si(100) to fabricate metal-oxide-semiconductor (MOS) capacitors. In the PEALD process, we used titanium tetraisopropoxide (TTIP) as a titanium precursor, trimethyl aluminum (TMA) as an aluminum precursor and O2 plasma as an oxidant, keeping the process temperature at 250 °C. The effects of PEALD process parameters, such as RF power, substrate exposure mode (direct or remote plasma exposure) and Al2O3 partial-monolayer insertion (generating a nanolaminate structure) on the physical and chemical properties of the TiO2 films were investigated by Rutherford backscattering spectroscopy (RBS), Raman spectroscopy, grazing incidence X-ray diffraction (GIXRD), and field emission scanning electron microscopy (FESEM) techniques. The MOS capacitor structures were fabricated by evaporation of Al gates through mechanical mask on PEALD TiO2 thin film, followed by evaporation of an Al layer on the back side of the Si substrate. The capacitors were characterized by current density-voltage (J-V), capacitance-voltage (C-V) and conductance-voltage (G-V) measurements. Our results indicate that RF power and exposure mode promoted significant modifications on the characteristics of the PEALD TiO2 films, while the insertion of Al2O3 partial monolayers allows the synthesis of TiO2/Al2O3 nanolaminate with well-spaced crystalline TiO2 grains in an amorphous structure. The electrical characterization of the MOS structures evidenced a significant leakage current in the accumulation region in the PEALD TiO2 films, which could be reduced by the addition of partial-monolayers of Al2O3 in the bulk of TiO2 films or by reducing RF power.

Low-temperature growth of gallium oxide thin films by plasma-enhanced atomic layer deposition

Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts) in range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4 × 1012 and 1–2 × 1012 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.

Growth characteristics and film properties of plasma-enhanced and thermal atomic-layer-deposited magnesium oxide thin films prepared using bis(ethylcyclopentadienyl)magnesium precursor

The continuous device scaling of dynamic random access memories has been increasing demands for the development of dielectric materials with high dielectric constants and low leakage currents. In this study, we developed MgO thin films through atomic layer deposition (ALD) using bis(ethylcyclopentadienyl)magnesium combined with two different reactants, H2O or O2 plasma, and compared the physical and chemical characteristics of MgO thin films produced by thermal ALD (Th-ALD) and plasma-enhanced ALD (PEALD). The films were deposited in a temperature range of 200–400 °C, and self-limited surface reactions were observed for both ALD processes. Th-ALD MgO films showed the oxygen deficient composition, while more stoichiometric MgO films were achieved by PEALD process. To evaluate the electrical characteristics of the MgO films, the metal–insulator–metal capacitors were fabricated. The electrical characteristics of the MgO film, such as the dielectric constant and leakage current, were compared according to the reactant type. The bulk-limited leakage current conduction mechanisms of the Th-ALD and PEALD MgO thin films were also investigated.

Probing the Origin and Suppression of Vertically Oriented Nanostructures of 2D WS2 Layers.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) such as WS2 are promising materials for nanoelectronic applications. However, growth of the desired horizontal basal-plane oriented 2D TMD layers is often accompanied by the growth of vertical nanostructures that can hinder charge transport and, consequently, hamper device application. In this work, we discuss both the formation and suppression of vertical nanostructures during plasma-enhanced atomic layer deposition (PEALD) of WS2. Using scanning transmission electron microscopy studies, formation pathways of vertical nanostructures are established for a two-step (AB-type) PEALD process. Grain boundaries are identified as the principal formation centers of vertical nanostructures. Based on the obtained insights, we introduce an approach to suppress the growth of vertical nanostructures, wherein an additional step (C)—a chemically inert Ar plasma or a reactive H2 plasma—is added to the original two-step (AB-type) PEALD process. This approach reduces the vertical nanostructure density by 80%. It was confirmed that suppression of vertical nanostructures goes hand in hand with grain size enhancement. The vertical nanostructure density reduction consequently lowers film resistivity by an order of magnitude. Insights obtained in this work can contribute toward devising additional pathways, besides plasma treatments, for suppressing the growth of vertical nanostructures and improving the material properties of 2D TMDs that are relevant for nanoelectronic device applications.

In vacuostudies on plasma-enhanced atomic layer deposition of cobalt thin films

Plasma-enhanced atomic layer deposition (PE-ALD) of cobalt (Co) using cyclopentadienylcobalt dicarbonyl [CpCo(CO)2] combined with hydrogen, nitrogen, ammonia, and argon based plasma gases was investigated. The utilized ALD tool was clustered to an ultrahigh vacuum analytic system for direct surface analyses including X-ray photoelectron spectroscopy (XPS). The combination with a nondestructive surface analysis system enabled a sample transfer without vacuum break and thereby a direct qualification and quantification of the chemical surface composition under quasi in situ conditions. The authors studied the influence of process parameters (e.g., pulse times, plasma power, and substrate temperature) on film compositions and film properties. The occurrence and prevention of sputtering effects due to ion bombardment at high plasma powers were discussed. Beyond those results, precise information about the impact of different plasma gas compositions on the resulting film properties was obtained. Cobalt films grown using a hydrogen/nitrogen (H2/N2) plasma as a coreactant showed a stable film composition (CoNx) with a high Co content of 75 at. %. Using scanning electron microscopy and four point probe measurements, a moderate electrical resistivity of about 56 μΩ cm was calculated for a 20 nm film. The high sensitivity of in vacuo XPS measurements allowed investigations of interface reactions for a single PE-ALD pulse as well as investigations of the initial film growth mechanisms. The nucleation of CoNx films during PE-ALD using H2/N2 plasma as a coreactant was investigated on several substrate materials by XPS. After the very first cycle of the PE-ALD process, no Co could be detected on all the investigated substrates. XPS revealed that the plasma pulse was needed to provide active binding sites for the adsorption reaction of precursor molecules due to the formation of Si-Nx or Si-NxOy surfaces. Therefore, the plasma pulse plays an important role in the PE-ALD process of Co on silicon surfaces. The early cycles were characterized by the onset of Co—O bonds. The homogeneous film body on all substrates consisted of Co-nitride compounds.