Atomic Layer Deposition Process Research Articles

Research on Interface Properties of Thermally Grown SiO2 and ALD SiO2 Stacked Structures

This study presents a stacked process of thermal and atomic layer deposition (ALD) SiO2 that reduces the interface trap density of 4H-SiC metal-oxide-semiconductor (MOS) capacitors. The channel mobility of metal-oxide-semiconductor field effect transistors (MOSFETS) are reduced due to the high interface trap density as well as coulomb scattering mechanism. Herein, we investigate SiO2/SiC interface properties of a stacked process, which is accomplished via reducing the thickness of thermal oxidation film. Notably, MOS capacitors fabricated with thermal and ALD SiO2 stacked structures can reduce the interface states density (Dit) by twofold at 0.2 eV below the conduction band energy compared with thermally grown SiO2. Additionally, the leakage current increases at a relatively slow rate in the electric field of 5–10 MV cm−1, whereas the leakage current increases sharply when the electric field is higher than 10 MV cm−1. The resultant ALD SiO2 stacked structure provides a new approach to improving interface quality, which allows a reduction in the thermal budget involved in the fabrication of devices.

Microkinetic based growth and property modeling of plasma enhanced atomic layer deposition silicon nitride thin film

We demonstrate a microkinetic modeling framework which is a first principle-based surface reaction thermodynamics modeling methodology to describe the plasma-enhanced atomic layer deposition process of silicon nitride thin film formation. The results illustrating the relationship between silicon nitride growth per cycle (GPC) and quasi self-limiting behavior on both dichlorosilane precursor dose amount and plasma nitridation time are consistent with the experiment. Ultimately, GPC is limited to the equivalent of a half monolayer of a Si3N4 crystalline structure. Importantly, we have observed a strong correlation between subsurface NH terminated Si group concentration and HF wet etch rate by an experiment, which varies with substrate temperature.

Direct growth and interface reactions of ferroelectric Hf0.5Zr0.5O2 films on MoS2

The integration of ferroelectric HfO2 films into two-dimensional layered-material-based devices is expected to provide significant functionality for future electronics. In this study, Hf0.5Zr0.5O2 (HZO) films are directly grown on single-crystalline MoS2 flakes by atomic layer deposition (ALD) with varying deposition temperatures using H2O or O3 oxidants. According to density functional theory calculations and Raman measurements, O3-based ALD oxidizes the MoS2 surface at the atomic layer level, in contrast to the H2O-based ALD process, thus facilitating the conformal deposition of an HZO film (10 nm) without any surface treatment of MoS2 at an elevated ALD temperature of 260 °C. Annealing the O3-based HZO film with a Mo capping layer at 600 °C significantly improves the ferroelectric properties with a symmetrical hysteresis loop on MoS2. However, distinct S out-diffusion accompanied by Hf, Zr, and O diffusion toward the Mo capping layer occurs because of the thermal dissociation of MoS2 at an additional atomic layer level, thereby leading to the subsequent reduction of weak HZO bonds by the released S atoms along the grain boundaries.

Assessing the Environmental Impact of Atomic Layer Deposition (ALD) Processes and Pathways to Lower It

Assessing the Environmental Impact of Atomic Layer Deposition (ALD) Processes and Pathways to Lower It

High-pure AlN crystalline thin films deposited on GaN at low temperature by plasma-enhanced ALD

Aluminum nitride (AlN) thin film has been considered one of the most promising candidates for gate insulating layer of GaN-based HEMT devices. To reach the highest performance of devices, it is still necessary to further reduce the defect density at the interface and within the film of the AlN insulating layer, as well as to develop low temperature processes for good compatibility. In this work, by adopting an extra Ar plasma pulse in the plasma-enhanced atomic layer deposition (PEALD) process cycle, crystalline AlN thin films were deposited on Gallium Nitride (GaN) substrates at a temperature as low as 200 °C and have almost no C and O contaminations detectable in the films. The atomic structure, surface morphology and chemical component of the AlN films have been characterized systemically, showing the great potential of application in GaN devices as gate insulating or passivation layers.

Crystalline ZrO2 films with reduced oxygen vacancy and surface roughness for corrosion protection by atomic layer deposition

ZrO2 coating is widely used for ensuring protection against corrosion and diffusion. Thus, a uniform coating of a high-quality film is key to protecting parts exposed to corrosive gas environments. In this study, we report optimized atomic layer deposition (ALD) process parameters to deposit crystalline ZrO2 films. Cyclopentadienyl tris(dimethylamino) zirconium (CpZr(NMe2)3) was used as the Zr precursor to obtain the crystalline characteristics of ZrO2 films by controlling the deposition temperature. A single phase of cubic ZrO2 was formed at the deposition temperature of 225–250 °C, whereas a mixed phase of monoclinic and tetragonal ZrO2 was obtained at 275–300 °C, suggesting that the tunable properties of the ZrO2 coating layer corresponded to the targeted applications. The mixture of tetragonal and monoclinic phases of the ZrO2 films exhibited the highest corrosion resistance properties due to the reduced oxygen vacancy with minimized surface roughness from the highest crystalline state of stable ZrO2 phases. Achieving superior reproducibility and uniformity enables the present low-temperature ALD to be utilized for commercial coating processes, to be readily applied to many complex-shaped parts, such as diffusers, and to provide a pathway to form a protective layer with improved performance of metals and alloy surfaces.

Control by atomic layer deposition over the chemical composition of nickel cobalt oxide for the oxygen evolution reaction

Anion exchange membrane water electrolysis (AEMWE) is a promising technology for renewable electricity-driven water splitting toward hydrogen production. However, application of AEMWE at industrial scale requires the development of oxygen evolution reaction (OER) electrocatalysts showing long-term stability under mild alkaline conditions. Among these, nickel cobalt oxide thin films are considered promising candidates. The ideal chemical composition of these oxides remains debatable, with recent literature indicating that rock-salt NiCoO2 may exhibit similar OER activity as the traditional spinel NiCo2O4. In this work, we present the development of a plasma-enhanced atomic layer deposition (ALD) process of nickel cobalt oxide thin films (∼20 nm) with focus on the role of their chemical composition and crystal structure on the OER activity. The film composition is tuned using a supercycle approach built upon CoOx cycles with CoCp2 as a precursor and O2 plasma as a co-reactant and NiOx cycles with Ni(MeCp)2 as a precursor and O2 plasma as a co-reactant. The films exhibit a change in the crystallographic phase from the rock-salt to spinel structure for increasing cobalt at. %. This change is accompanied by an increase in the Ni3+-to-Ni2+ ratio. Interestingly, an increase in electrical conductivity is observed for mixed oxides, with an optimum of (2.4 ± 0.2) × 102 S/cm at 64 at. % Co, outperforming both NiO and Co3O4 by several orders of magnitude. An optimal electrocatalytic performance is observed for 80 at. % Co films. Cyclic voltammetry measurements simultaneously show a strong dependence of the OER-catalytic performance on the electrical conductivity. The present study highlights the merit of ALD in controlling the nickel cobalt oxide chemical composition and crystal structure to gain insight into its electrocatalytic performance. Moreover, these results suggest that it is important to disentangle conductivity effects from the electrocatalytic activity in future work.

Ultra-thin gate insulator of atomic-layer-deposited AlO x and HfO x for amorphous InGaZnO thin-film transistors

To strengthen the downscaling potential of top-gate amorphous oxide semiconductor (AOS) thin-film transistors (TFTs), the ultra-thin gate insulator (GI) was comparatively implemented using the atomic-layer-deposited (ALD) AlO x and HfO x . Both kinds of high-k GIs exhibit good insulating properties even with the physical thickness thinning to 4 nm. Compared to the amorphous indium-gallium-zinc oxide (a-IGZO) TFTs with 4 nm AlO x GI, the 4 nm HfO x enables a larger GI capacitance, while the HfO x -gated TFT suffers higher gate leakage current and poorer subthreshold slope, respectively originating from the inherently small band offset and the highly defective interface between a-IGZO and HfO x . Such imperfect a-IGZO/HfO x interface further causes noticeable positive bias stress instability. Both ALD AlO x and HfO x were found to react with the underneath a-IGZO channel to generate the interface defects, such as metal interstitials and oxygen vacancies, while the ALD process of HfO x gives rise to a more severe reduction of a-IGZO. Moreover, when such a defective interface is covered by the top gate, it cannot be readily restored using the conventional oxidizing post-treatments and thus desires the reduction-resistant pre-treatments of AOSs.

Alternative surface reaction route in the atomic layer deposition of NbN thin films for reduced resistivity

Transition metal nitride thin films such as TiN are necessary as conducting electrodes in memory and transistor devices. Due to their high work function (> 4.7 eV), NbN thin films have recently attracted attention to replace TiN thin films as capacitor electrodes using atomic layer deposition (ALD) process in dynamic random access memories (DRAMs) to reduce leakage currents in the capacitors. Unfortunately, the NbN ALD process using NbCl5 and NH3 as the Nb precursor and nitrogen source resulted in a high resistivity (> 500 μΩ·cm) despite its low bulk resistivity (∼10 μΩ·cm at 300 K) because of the residual Cl impurity (> 5%) within the NbN film. In this study, an alternative ALD surface reaction pathway is proposed by introducing H2S gas pulses between the NbCl5 and NH3 pulse steps to lower the Cl impurity concentration. Using the alternative ALD reaction, the residual Cl concentration was reduced below 1.5% (decrease by > 50%) at the specific ALD deposition temperature of 673 K. Finally, this reaction produced a greater decrease in the resistivity (> 30%) of NbN thin films than the conventional NbN ALD at 673 K.

Optimization of optical and structural properties of Al2O3/TiO2 nano-laminates deposited by atomic layer deposition for optical coating.

Optimizing the atomic layer deposition (ALD) process of films is particularly important in preparing multilayer interference films. In this work, a series of Al2O3/TiO2 nano-laminates with a fixed growth cycle ratio of 1:10 were deposited on Si and fused quartz substrates at 300 °C by ALD. The optical properties, crystallization behavior, surface appearance and microstructures of those laminated layers were systematically investigated by spectroscopic ellipsometry, spectrophotometry, X-ray diffraction, atomic force microscope and transmission electron microscopy. By inserting Al2O3 interlayers into TiO2 layers, the crystallization of the TiO2 is reduced and the surface roughness becomes smaller. The TEM images show that excessively dense distribution of Al2O3 intercalation leads to the appearance of TiO2 nodules, which in turn leads to increased roughness. The Al2O3/TiO2 nano-laminate with a cycle ratio 40:400 has relatively small surface roughness. Additionally, oxygen-deficient defects exist at the interface of Al2O3 and TiO2, leading to evident absorption. Using O3 as an oxidant instead of H2O for depositing Al2O3 interlayers was verified to be effective in reducing absorption during broadband antireflective coating experiments.

Mechanistic Insight into Solution-Based Atomic Layer Deposition of CuSCN Provided by In Situ and Ex Situ Methods

Solution-based atomic layer deposition (sALD) processes enable the preparation of thin films on nanostructured surfaces while controlling the film thickness down to a monolayer and preserving the homogeneity of the film. In sALD, a similar operation principle as in gas-phase ALD is used, however, with a broader range of accessible materials and without requiring expensive vacuum equipment. In this work, a sALD process was developed to prepare CuSCN on a Si substrate using the precursors CuOAc and LiSCN. The film growth was studied by ex situ atomic force microscopy (AFM), analyzed by a neural network (NN) approach, ellipsometry, and a newly developed in situ infrared (IR) spectroscopy experiment in combination with density functional theory (DFT). In the self-limiting sALD process, CuSCN grows on top of an initially formed two-dimensional (2D) layer as three-dimensional spherical nanoparticles with an average size of ∼25 nm and a narrow particle size distribution. With increasing cycle number, the particle density increases and larger particles form via Ostwald ripening and coalescence. The film grows preferentially in the β-CuSCN phase. Additionally, a small fraction of the α-CuSCN phase and defect sites form.

Ultra-thin top-gate insulator of atomic-layer-deposited HfOx for amorphous InGaZnO thin-film transistors

In recent years, high-k gate dielectrics have attracted increasing attention in amorphous oxide semiconductor (AOS) thin-film transistors (TFTs), due to the urge for stronger gate controllability in advanced applications of high integration density. In this work, the ultra-thin top-gate insulator of atomic-layer-deposited (ALD) HfOx was developed for the amorphous indium-gallium-zinc oxide (a-IGZO) TFTs. Despite the good electrical characteristics of the 4-nm HfOx-gated a-IGZO transistor, its reliability is considerably poor and ascribed to the abundant interface defects, for example, oxygen vacancies. The interface reaction between HfOx and a-IGZO during the ALD process is clarified to be responsible for such defect generation. To prevent the oxygen-related reaction, the a-IGZO channel is pre-treated using the strong oxidizing plasma, contributing to significantly enhanced electrical stabilities. However, the further thinner HfOx would readily increase the gate leakage current, since the electron can easily tunnel through the ultra-thin HfOx due to the low conduction band offset between HfOx and AOS. The 4-nm ALD HfOx together with the pre-oxidization contributes to the optimal performance and stability of top-gate a-IGZO TFT.

Novel liquid cobalt precursor for metal atomic layer deposition

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

Understanding the multilevel phenomena that enables inorganic atomic layer deposition to provide barrier coatings for highly-porous 3-D printed plastic in vacuums

The potential of 3-D printing polymers to achieve low-weight space flight hardware has seen increasing interest. Additionally, robust 3-D printed polymer vacuum equipment would provide highly attractive low-cost alternatives to conventional industrial-scale tools for the scientific community. Inorganic barrier coatings of plastic parts can be used to minimize outgassing and damage to polymers from extreme vacuum environments. Among different coating technologies in this area of research, atomic layer deposition (ALD) has shown the most promise. Nevertheless, the exact formation morphology of ALD coatings on 3-D printed polymers under vacuum are not yet well understood, which hinders use of 3-D printed polymers in vacuum environments. In this study, the film formation mechanisms of ALD alumina on porous 3-D printed polymers are investigated via SEM, EDS, XRD, ATR-FTIR, and XPS. ALD alumina is deposited on 3-D printed pigmented and un-pigmented acrylonitrile butadiene styrene, polycarbonate, commercially available polypropylene, and pure polypropylene. The results reveal that the formation of the ALD barrier layer, its thickness, and diffusion of ALD precursor materials into the polymer substrate is a multi-scale phenomenon, and that substrate porosity and polymer functionality both dominate film formation behavior. Additionally, this study demonstrates that during ALD processes on 3-D printed polymers a vapor phase infiltration (VPI) growth mode also occurs, especially where porosity is present.

Plasma-assisted fluidized-bed atomic layer deposition of Pd-Cu nanoparticles on porous powder for CO2 hydrogenation

A novel plasma-assisted fluidized-bed atomic layer deposition process to synthesize Pd-Cu bimetallic nanoparticles is reported, using palladium hexafluoroacetylacetonate, copper(I)-N, N′-di-iso-propylacetamidinate and H2 plasma. The process allows us to uniformly deposit Pd-Cu nanoparticles in porous powder, which is a mixture of γ-Al2O3 (30 wt%), amorphous aluminum silicate (50 wt%) and molecular sieve (20 wt%) (ASM). With metal loadings of 13.1 and 2.5 mg g−1 for Pd and Cu, respectively, the afforded 13.1Pd-2.5Cu catalyst shows excellent catalytic performance for the hydrogenation of CO2 in a dielectric barrier discharge reactor with no intentional heating. Under the condition of discharge input power of 24.6 W, H2-to-CO2 ratio of 4 in feed gas, and gas hourly space velocity of 7595 h−1, the conversion of CO2 can reach as high as 38.0%, with the CH4 and CH3OH product selectivities of 6.7% and 12.8%, respectively. Density functional theory calculations are further employed to understand the associated CH3OH formation mechanism.

Recess-Free E-Mode AlGaN/GaN MIS-HFET with Crystalline PEALD AlN Passivation Process

We utilized a plasma-enhanced atomic layer deposition (PEALD) process to deposit an AlN passivation layer on AlGaN/GaN surface to enhance the polarization effects, which enabled the fabrication of an enhancement-mode (E-mode) AlGaN/GaN metal-insulator-semiconductor heterojunction field-effect transistor (MIS-HFET) without the need for a gate recess process. The AlN film deposited by PEALD exhibited a crystalline structure, not an amorphous one. The enhanced polarization effect of introducing the PEALD AlN film on a thin AlGaN barrier was confirmed through electrical analysis. To fabricate the E-mode AlGaN/GaN MIS-HFET, the PEALD AlN film was deposited on a 4.5 nm AlGaN barrier layer and then a damage-free wet etching process was used to open the gate region. The MIS-gate structure was formed by depositing a 15 nm plasma-enhanced chemical vapor deposition (PECVD) silicon dioxide (SiO2) film. The fabricated thin-AlGaN/GaN MIS-HFET demonstrated successful E-mode operation, with a threshold voltage of 0.45 V, an on/off ratio of approximately 109, a specific on-resistance of 7.1 mΩ·cm2, and an off-state breakdown voltage exceeding 1100 V.

In situ analysis of nucleation reactions during TiCl4/H2O atomic layer deposition on SiO2 and H-terminated Si surfaces treated with a silane small molecule inhibitor

Small-molecule inhibitors have recently been introduced for passivation during area-selective deposition (ASD). Small silanes like (N,N-dimethylamino)trimethylsilane (DMATMS) selectively react with −OH sites on SiO2 to form a less reactive –OSi(CH3)3 terminated surface. The –OSi(CH3)3 surface termination can inhibit many atomic layer deposition (ALD) processes, including TiCl4/H2O ALD. However, the mechanisms by which ALD is inhibited and by which selectivity is eventually lost are not well understood. This study uses in situ Fourier-transform infrared spectroscopy to probe the adsorption of DMATMS on SiO2 and the subsequent reactions when the passivated surface is exposed to TiCl4/H2O ALD. The chemisorption of DMATMS on isolated –OH groups on SiO2 is shown to inhibit the reaction with TiCl4. Further, we find that starting with an inherently inhibiting H-terminated Si surface, DMATMS can also react with residual –OH groups and reduce the extent of nucleation. Finally, using Rutherford backscattering spectrometry, the effectiveness of DMATMS passivation on SiO2 and H-terminated Si is quantified during extended ALD cycle numbers. The insight into the mechanisms of passivation by DMATMS and passivation loss can enable the rational design of highly selective ASD processes by carefully matching compatible surfaces, passivating agents, and ALD precursors.

Wide temperature operation of piezoelectric sensors for detecting precursor levels in a canister

Liquid level detection using piezoelectric actuators and sensors is superior to other technologies in accuracy, stability, and durability. In the semiconducting industry, the accurate detection of precursor levels in a canister is directly connected to the quality of the atomic layer growth through chemical vapor deposition and atomic layer deposition processes. However, the sensitivity of the level detection using piezoelectric devices often decreases at a specific temperature range, limiting the wide temperature operation of the canister. We demonstrate reduced sensitivity of the piezoelectric sensors due to a change in detuning frequency by temperature. A model with a simple harmonic oscillator exhibits the fundamental behavior of the actuator amplitude after a finite number of driving pulses. The impedance measurement of a sensor assembly demonstrated a significant shift in the primary resonance frequency due to a change in environmental temperature. By analyzing the simulation data, we established a temperature-dependent number of driving pulses that could extend the operating temperature of the piezoelectric actuators, which can easily be applied to a wide temperature operation for a canister.

Large-area synthesis of high electrical performance MoS2 by a commercially scalable atomic layer deposition process

This work demonstrates a large area process for atomically thin 2D semiconductors to unlock the technological upscale required for their commercial uptake. The new atomic layer deposition (ALD) and conversion technique yields large area performance uniformity and tunability. Like graphene, 2D Transition Metal Dichalcogenides (TMDCs) are prone to upscaling challenges limiting their commercial uptake. They are challenging to grow uniformly on large substrates and to transfer on alternative substrates while they often lack in large area electrical performance uniformity. The scalable ALD process of this work enables uniform growth of 2D TMDCs on large area with independent control of layer thickness, stoichiometry and crystallinity while allowing chemical free transfers to application substrates. Field effect transistors (FETs) fabricated on flexible substrates using the process present a field effect mobility of up to 55 cm2/Vs, subthreshold slope down to 80 mV/dec and on/off ratios of 107. In addition, non-volatile memory transistors using ferroelectric FETs (FeFETs) operating at ±5 V with on/off ratio of 107 and a memory window of 3.25 V are demonstrated. These FeFETs demonstrate state-of-the-art performance with multiple state switching, suitable for one-transistor non-volatile memory and for synaptic transistors revealing the applicability of the process to flexible neuromorphic applications.

Synthesis and characterization of new low molecular weight aluminum organometallic complexes using amino/imine type ligands with possible uses in Atomic Layer Deposition processes

The driver of development and innovation in the microelectronics industry is the miniaturization of the components that make up each technological node. This miniaturization has given rise to new challenges for traditional materials and the need for new fabrication methods with sub-nanometer precision.Based on this, numerous methods have emerged for the fabrication of this type of nanometer materials, among which chemical deposition processes stand out. An industrial example of this type of chemistry is the so-called ALD (Atomic Layer Deposition), which generates deposition of nanometric metal layers, using precursors in a gaseous state.The use of metallic precursors in gaseous state, generates that the process has as a key factor the correct molecular design of the inorganic/organometallic material, since the method imposes strict requirements to the chemical and physical properties of these compounds, such as thermal stability, volatility, synthetic scaling, among others. With this in mind, in this research, new aluminum complexes were synthesized using TMA and imino/amine type ligands with low molecular weight substituents (isopropyl, tertbutyl), to generate stable and volatile liquid complexes in inert atmosphere at room temperature.Finally, four liquid complexes are obtained, stable in nitrogen atmosphere, with yields of 70%. Due to the mentioned characteristics, these complexes could be used as precursors for the deposition of Al and Al2O3 films. In addition, it is demonstrated that the use of TMA in this type of ligands can generate a methylation in the chain of this molecule, which opens a new field in the development of aluminum organometallic complexes.