Low-temperature Plasma-enhanced Atomic Layer Deposition Research Articles

Crystalline Tin Disulfide by Low-Temperature Plasma-Enhanced Atomic Layer Deposition as an Electrode Material for Li-Ion Batteries and CO2 Electroreduction

Crystalline Tin Disulfide by Low-Temperature Plasma-Enhanced Atomic Layer Deposition as an Electrode Material for Li-Ion Batteries and CO₂ Electroreduction

Dopamine facilitates Al2O3 film growth on polyethylene terephthalate by low-temperature plasma-enhanced atomic layer deposition

Polymeric materials, including polyethylene terephthalate (PET), are widely used in various fields because of their beneficial properties. Functional films are deposited on these materials through different approaches, such as plasma-enhanced atomic layer deposition (PEALD), to enhance their performance and prolong their life span. However, the inert and thermally fragile nature of most polymers hinders deposition. We developed a strategy for the PEALD of nanoscale Al2O3 films on PET substrates. First, a PET substrate is subjected to alkali treatment, which gives it basic hydrophilicity for the subsequent dopamine modification. After 24 h of dopamine deposition, the substrate shows adequate active sites (phenolic hydroxyl groups), which can chemisorb large amounts of precursor during the initial deposition. The island growth mode was observed during the PEALD processes. We analyzed the detailed chemical components of Al2O3 on alkali-treated PET and dopamine-modified PET. After 100 cycles of deposition, the Al2O3 films on both samples contained much hydrogen. Benefitting from the more active sites, we observed more continuous Al2O3 film on dopamine-modified PET, which exhibited excellent water vapor blocking performance. Our findings suggest that dopamine could act as a ‘bridge’ between polymers and PEALD functional films.

Low-temperature atomic layer deposition of indium oxide thin films using trimethylindium and oxygen plasma

Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80–200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), as well as current-voltage and capacitance-voltage measurements. The SE results yielded a nearly constant growth rate of 0.56 Å per cycle and a thickness inhomogeneity of ≤1.2% across 4-in. substrates in the temperature range of 100–150 °C. The refractive index (at 632.8 nm) was found to be 2.07 for the films deposited at 150 °C. The PEALD-InxOy layers exhibit a direct (3.3 ± 0.2 eV) and an indirect (2.8 ± 0.1 eV) bandgap with an uptrend for both with increasing substrate temperature. Based on XPS characterization, all InxOy samples are free of carbon impurities and show a temperature-dependent off-stoichiometry indicating oxygen vacancies. XRD diffraction patterns demonstrate an onset of crystallization at 150 °C. Consistent with the optical, XPS, and XRD data, the films deposited at ≥150 °C possess higher electrical conductivity. Our findings prove that a low-temperature PEALD process of InxOy is feasible and promising for a high-quality thin-film deposition without chemical impurities on thermally fragile substrates.

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

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

Plasma Enhanced Atomic Layer Deposition of Iridium Oxide for Application in Miniaturized Neural Implants

Abstract High quality recording of neuronal activities and electrical stimulation require neurotechnical implants with appropriate electrode material. Iridium oxide (IrOx) is an excellent choice of material due to its biocompatibility, low electrochemical impedance, superior charge injection capacity, corrosion resistance, longevity, and electrochemical stability. Plasma enhanced atomic layer deposition (PE-ALD) and a suitable precursor, like (Methylcyclopentadienyl) (1,5- cyclooctadiene) iridium, could be a promising technique to produce highly conformal and performant IrOx-films at low temperatures and low costs. Various studies have reported the deposition of iridium oxide, but usually at very high temperatures. These processes are not suitable for polymer substrates and limit the use of such post-processing together with active implants. In this work the (Methylcyclopentadienyl) (1,5-cyclooctadiene) iridium(I) ((MeCp)Ir(COD)) precursor was used as a promising approach for depositing IrOx-films using low temperature PE-ALD. This precursor is normally used for chemical vapour deposition processes. First experiments were carried out on silicon substrates at deposition temperatures of 110 C°. The precursor was heated up to 75 °C and oxygen plasma was used as coreactant. The deposited films were analysed with EDX and AFM, showing a smooth surface and a promising ratio between the elements iridium and oxygen.

A Selective BP/Si Contact Formed by Low-Temperature Plasma-Enhanced Atomic Layer Deposition

It is shown for the first time that thin boron phosphide (BP) layers on silicon substrates can be formed by low-temperature plasma-enhanced atomic layer deposition at 250°C. Experiments demonstrated the possibility of using these BP/Si interfaces as selective hole contacts to silicon in solar cells.

HfO2/SiO2 anti-reflection films for UV lasers via plasma-enhanced atomic layer deposition

The laser-induced damage threshold (LIDT) of HfO2/SiO2 anti-reflection (AR) films for ultraviolet (UV) lasers was improved via low-temperature plasma-enhanced atomic layer deposition (PEALD). Focused on the chemical composition, optical absorption, surface scattering, and laser-resistance, the impact of precursor exposure time on PEALD SiO2 film properties and growth temperature on PEALD SiO2 and HfO2 film properties were investigated respectively. When irradiated by UV laser, PEALD SiO2 film exhibits a higher LIDT than the PEALD HfO2 film, which is consistent with their less impurity content and lower absorption. A bilayer structure HfO2/SiO2 AR film for 355 nm laser was designed and experimentally demonstrated via PEALD growth at a temperature of 150 °C. The prepared PEALD AR film shows a reflectance <0.2% at 355 nm as designed and better laser-damage resistance with a LIDT of 24.4 J/cm2 (355 nm, 7.8 ns) than the conventional e-beam deposition method.

A film-texture driven piezoelectricity of AlN thin films grown at low temperatures by plasma-enhanced atomic layer deposition

Simultaneously inducing preferred crystalline orientation with a strong piezoelectric response in polycrystalline aluminum nitride (AlN) thin films by atomic layer deposition is a technical challenge due to the upscaling of the integration of piezoelectric functionalities, such as sensing and actuation, in micro-devices without any poling process. Utilizing low-temperature plasma-enhanced atomic layer deposition (PE-ALD), highly c-axis-oriented AlN films have been prepared with precise control over the relative composition, purity levels, and chemical states of constituent elements. Tailoring thermodynamic parameters, such as the growth temperature and purging time after the trimethylaluminum precursor pulsing before the N2:H2:Ar plasma reaction, provide the possibility of modulating the texture coefficient and the relative piezoelectric response. The effective transverse piezoelectric e31,f coefficient of 0.37 C/m2 was achieved on the AlN film grown at 250 °C and 30 s with the highest texture coefficient TC(002) of 2.75 along the c-axis orientation. The process proposed, at a low temperature with the highly conformal growth of aluminum nitride thin films by PE-ALD, opens up pathways to design novel piezoelectric functional materials for micro-electro-mechanic system devices with complementary metal oxide semiconductor process temperature compatibility.

PEALD of HfO2 Thin Films: Precursor Tuning and a New Near-Ambient-Pressure XPS Approach to in Situ Examination of Thin-Film Surfaces Exposed to Reactive Gases.

A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO2 followed by in situ thin-film surface characterization of HfO2 upon exposure to reactive gases via near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is reported. The stability of thin films under simulated operational conditions is assessed, and the successful implementation of HfO2 dielectric layers in metal-insulator-semiconductor (MIS) capacitors is demonstrated. Among the series of newly synthesized mono-guanidinato-tris-dialkyl-amido class of Hf precursors, one of them, namely, [Hf{η2-(iPrN)2CNEtMe}(NEtMe)3], was representatively utilized with oxygen plasma, resulting in a highly promising low-temperature PEALD process at 60 °C. The new precursors were synthesized in the multigram scale and thoroughly characterized by thermogravimetric analyses, revealing high and tunable volatility reflected by appreciable vapor pressures and accompanied by thermal stability. Typical ALD growth characteristics in terms of linearity, saturation, and a broad ALD window with constant growth of 1.06 Å cycle-1 in the temperature range of 60-240 °C render this process very promising for fabricating high-purity smooth HfO2 layers. For the first time, NAP-XPS surface studies on selected HfO2 layers are reported upon exposure to reactive H2, O2, and H2O atmospheres at temperatures of up to 500 °C revealing remarkable stability against degradation. This can be attributed to the absence of surface defects and vacancies. On the basis of these promising results, PEALD-grown HfO2 films were used as dielectric layers in the MIS capacitor device fabrication exhibiting leakage current densities less than 10-7 A cm-2 at 2 MV cm-1 and permittivities of up to 13.9 without postannealing.

Edge-Site Nanoengineering of WS2 by Low-Temperature Plasma-Enhanced Atomic Layer Deposition for Electrocatalytic Hydrogen Evolution.

Edge-enriched transition metal dichalcogenides, such as WS2, are promising electrocatalysts for sustainable production of H2 through the electrochemical hydrogen evolution reaction (HER). The reliable and controlled growth of such edge-enriched electrocatalysts at low temperatures has, however, remained elusive. In this work, we demonstrate how plasma-enhanced atomic layer deposition (PEALD) can be used as a new approach to nanoengineer and enhance the HER performance of WS2 by maximizing the density of reactive edge sites at a low temperature of 300 °C. By altering the plasma gas composition from H2S to H2 + H2S during PEALD, we could precisely control the morphology and composition and, consequently, the edge-site density as well as chemistry in our WS2 films. The precise control over edge-site density was verified by evaluating the number of exposed edge sites using electrochemical copper underpotential depositions. Subsequently, we demonstrate the HER performance of the edge-enriched WS2 electrocatalyst, and a clear correlation among plasma conditions, edge-site density, and the HER performance is obtained. Additionally, using density functional theory calculations we provide insights and explain how the addition of H2 to the H2S plasma impacts the PEALD growth behavior and, consequently, the material properties, when compared to only H2S plasma.

Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Tin(IV) Oxide from a Functionalized Alkyl Precursor: Fabrication and Evaluation of SnO2-Based Thin-Film Transistor Devices.

A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle-1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of μFE ≈ 12 cm2 V-1 s-1 for the as-deposited thin films deposited at such low temperatures.

Optimization of a SiOx/SiNxOyCz multilayer structure for a reliable gas diffusion barrier via low-temperature plasma-enhanced atomic layer deposition

Thin film encapsulation (TFE) using a multilayer film composed of SiOx and SiNxOyCz layers deposited by plasma-enhanced atomic layer deposition is demonstrated. To investigate the mechanism of suppressed moisture penetration for the multilayer film, chemical analyses were performed using Fourier transform infrared and Auger electron spectroscopy, and the encapsulation ability was evaluated using the water vapor transmission rate (WVTR). The encapsulation ability of the multilayer film is affected by the nitrogen content in the SiNxOyCz layer and the number of interfaces in the multilayer film. Consequently, TFE using a multilayer film of 20-nm-SiNxOyCz/20-nm-SiOx/20-nm-SiNxOyCz resulted in excellent encapsulation ability, with a WVTR of 7.63 × 10−4 g m−2 day−1.

Study on SiN and SiCN film production using PE-ALD process with high-density multi-ICP source at low temperature

SiN and SiCN film production using plasma-enhanced atomic layer deposition (PE-ALD) is investigated in this study. A developed high-power and high-density multiple inductively coupled plasma (multi-ICP) source is used for a low temperature PE-ALD process. High plasma density and good uniformity are obtained by high power N2 plasma discharge. Silicon nitride films are deposited on a 300-mm wafer using the PE-ALD method at low temperature. To analyze the quality of the SiN and SiCN films, the wet etch rate, refractive index, and growth rate of the thin films are measured. Experiments are performed by changing the applied power and the process temperature (300–500 °C).

Si doped GaP layers grown on Si wafers by low temperature PE-ALD

Low-temperature plasma enhanced atomic layer deposition (PE-ALD) was successfully used to grow silicon (Si) doped amorphous and microcrystalline gallium phosphide (GaP) layers onto p-type Si wafers for the fabrication of n-GaP/p-Si heterojunction solar cells. PE-ALD was realized at 380 °C with continuous H2 plasma discharge and the alternate use of phosphine and trimethylgallium as sources of P and Ga atoms, respectively. The layers were doped with silicon thanks to silane (SiH4) diluted in H2 that was introduced as a separated step. High SiH4 dilution in H2 (0.1%) allows us to deposit stoichiometric GaP layers. Hall measurements performed on the GaP:Si/p-Si structures reveal the presence of an n-type layer with a sheet electron density of 6–10 × 1013 cm−2 and an electron mobility of 13–25 cm2 V−1 s−1 at 300 K. This is associated with the formation of a strong inversion layer in the p-Si substrate due to strong band bending at the GaP/Si interface. GaP:Si/p-Si heterostructures exhibit a clear photovoltaic effect, with the performance being currently limited by the poor quality of the p-Si wafers and reflection losses at the GaP surface. This opens interesting perspectives for Si doped GaP deposited by PE-ALD for the fabrication of p-Si based heterojunction solar cells.

Dielectric barrier layers by low-temperature plasma-enhanced atomic layer deposition of silicon dioxide

Electrothermal measurement techniques often require thin dielectric barriers to isolate active electrical test structures from samples of interest. The combined need for electrical passivation but thermal proximity necessitates the use of an electrically thick but thermally thin barrier layer. Here, we demonstrate a hybrid approach toward constructing sub-300 nm SiO2 multilayer barriers based upon low-temperature plasma-enhanced atomic layer deposition and high density plasma chemical vapor deposition. Using pairs of buried metal test structures, we quantify changes in device resistance and cross-talk after covering the dielectric barrier with thin evaporated gold films and thick electroplated copper films. We show that a hybrid approach to passivating electrothermal measurement devices outperforms individual homogenous barriers formed by either deposition technique.

Low-temperature plasma-enhanced atomic layer deposition of 2-D MoS2: large area, thickness control and tuneable morphology.

Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS2 on SiO2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.

Water Vapor Treatment of Low-Temperature Deposited SnO2 Electron Selective Layers for Efficient Flexible Perovskite Solar Cells

Tin oxide (SnO2) electron selective layers (ESLs) processed by low-temperature plasma-enhanced atomic layer deposition (PEALD) hold promise for fabricating lightweight and efficient flexible lead halide perovskite solar cells (PVSCs). However, the as-synthesized SnO2 ESLs typically lead to flexible PVSCs with lower open-circuit voltage (VOC) and fill factor (FF) as well as a higher degree of current density–voltage (J–V) hysteresis, compared to PVSCs fabricated on rigid substrates. Here, we report that facile water vapor treatment of PEALD-synthesized SnO2 ESLs can effectively improve the VOC and FF while reducing the degree of J–V hysteresis. The improvement in device performance is mainly attributed to the improved conductivity and electrical mobility of SnO2 ESLs enabled by water vapor treatment. With such treatment, our best flexible PVSC fabricated on a commercial substrate shows a power conversion efficiency of 18.36 (17.12)% when measured under a reverse (forward) voltage scan and a stabilized effi...

Highly-impermeable Al2O3/HfO2 moisture barrier films grown by low-temperature plasma-enhanced atomic layer deposition

Polymer substrates are essential components of flexible electronic applications such as OTFTs, OPVs, and OLEDs. However, high water vapor permeability of polymer films can significantly reduce the lifetime of flexible electronic devices. In this study, we examined the water vapor permeation barrier properties of Al2O3/HfO2 mixed oxide films on polymer substrates. Al2O3/HfO2 films deposited by plasma-enhanced atomic layer deposition were transparent, chemically stable in water and densely amorphous. At 60 °C and 90% relative humidity (RH) accelerated condition, 50-nm-thick Al2O3/HfO2 had water vapor transmission rate (WVTR) = 1.44 × 10−4 g m−2 d−1, whereas single layers of Al2O3 had WVTR = 3.26 × 10−4 g m−2 d−1 and of HfO2 had WVTR = 6.75 × 10−2 g m−2 d−1. At 25 °C and 40% RH, 50-nm-thick Al2O3/HfO2 film had WVTR = 2.63 × 10−6 g m−2 d−1, which is comparable to WVTR of conventional glass encapsulation.

Low temperature SiOx thin film deposited by plasma enhanced atomic layer deposition for thin film encapsulation applications

Silicon oxide (SiOx) films were synthesized by plasma enhanced atomic layer deposition (PEALD) using di-isopropylaminosilane [SiH3N(C3H7)2] as the precursor and an oxygen plasma as the reactant. The layers were characterized with respect to different growth temperatures between 60 and 150 °C. The film density and surface roughness values measured by x-ray reflectometry and atomic force microscopy all approached those of thermally grown SiOx. Also, reasonably high breakdown voltages were observed at all deposition temperatures. An interesting phenomenon involves the fact that the SiOx layer deposited at 60 °C is most effective as a moisture barrier, as it exhibits the lowest water vapor transmission rate. X-ray photoelectron spectroscopy analyses indicate that the silicon monoxide bonding characteristic becomes more pronounced as the growth temperature decreases. It is conjectured that such a difference in the bonding state renders the surface of the low temperature SiOx films rather hydrophobic, which suppresses the penetration of moisture. The results indicate that low temperature PEALD SiOx films may be suitable for thin film encapsulation applications in mechanical flexible platforms.

Defect Characterization of PEALD High-k ZrO2Films Fabricated on III–V Materials

To enable scalable MOSFET technology in III–V semiconductor platforms, high quality semiconductor-oxide interfaces are essential. In this paper, a novel low-temperature plasma-enhanced atomic layer deposition (PEALD) technique was applied to deposit nanoscale high-k dielectrics on several III–V substrates, including InP, GaAs, InAs, and GaN. Approximately 7 nm of ZrO2 was grown and patterned to form MOSCAP structures, which were subsequently analyzed through electrical characterization to evaluate dielectric and interface quality. The oxide films fabricated were found to have interface trap densities ranging from 1010 – 1013 eV−1 cm−2, and showed high capacitance densities ( $\boldsymbol {\sim 2.5}~{\boldsymbol \mu }\text{F}$ /cm2). GaN and InP MOSCAPs with ZrO2 dielectric layers were found to have gate currents in line with direct tunneling phenomena and MOS mobilities approaching that of doped bulk semiconductors. Scaled InP MOSFET devices using these experimental values were also simulated using an optimized device structure.