Hydrogen Plasma Research Articles

The Etching Mechanisms of Diamond, Graphite, and Amorphous Carbon by Hydrogen Plasma: A Reactive Molecular Dynamics Study

AbstractUnderstanding how hydrogen plasma etches various potential products during the diamond growth process can contribute to improving the knowledge of diamond growth. However, due to the absence of an in situ characterization technique during the etching process, the complex chemical reactions involved in the process obscure the atomic‐scale etching mechanisms. In this paper, the etching mechanisms of diamond (001), graphite (0001), and amorphous carbon substrates irradiated by hydrogen plasmas are investigated and compared using molecular dynamics simulations based on ReaxFF. When the incident energy of H atoms is 1 eV, the rate of carbon loss from graphite and amorphous carbon are far higher than that from diamond. As the incident energy of H atoms increases, the etching rate of diamond shows a slow increase, while the etching rates of amorphous carbon and graphite exhibit more significant increases. It can be concluded that the etching rate of diamond is significantly lower than that of graphite and amorphous carbon under H plasma. In the Chemical Vapor Deposition (CVD) process of diamond growth, the generated graphite and amorphous carbon are rapidly etched, leaving only diamond. This offers a plausible explanation for the growth mechanism of diamond through CVD.

In Situ Monitoring of Etching Characteristic and Surface Reactions in Atomic Layer Etching of SiN Using Cyclic CF4/H2 and H2 Plasmas.

Cyclic atomic layer etching (ALE) of SiN with high selectivity to SiO2, utilizing a hydrofluorocarbon deposition followed by exposure to hydrogen plasma, is presented. The surface reaction mechanism and etching behavior were investigated with in situ attenuated total reflectance Fourier transformation infrared spectroscopy (ATR-FTIR) and spectroscopic ellipsometry. In the deposition step, the hydrofluorocarbon film was deposited on top of the SiN films using the CF4/H2 plasmas with varying H2 contents (33 to 85%). Subsequently, the surface-modified SiN film was exposed to a hydrogen plasma for etching. The self-limiting SiN etching was observed, where the etch depth solely depended on the F concentration of the deposited hydrofluorocarbon layer once its thickness exceeded a critical value. A high selectivity of approximately 8.6 for SiN over SiO2 was achieved. The in situ ATR-FTIR spectra revealed that during the deposition step, besides the formation of the C-H peak associated with hydrofluorocarbon deposition, the appearance of the N-H4 absorbance band indicated the formation of an ammonium fluorosilicate layer on top of SiN. In the subsequent H2 plasma etching step, both the surface modification layer and the pre-deposited hydrofluorocarbon layer were removed. The removal of the surface-modified layer and hydrofluorocarbon layer was associated with the etch rate during H2 plasma exposure. These findings indicate the importance of the formation and removal of the surface modification layer for achieving ALE of SiN. The dissociation of the hydrofluorocarbon layer by the H2 plasma released reactants that interacted with SiN, leading to the formation of a new surface modification layer. The etching process significantly slowed down once the hydrofluorocarbon deposition and surface modification layer were completely removed.

Evolution of Cu-In Catalyst Nanoparticles under Hydrogen Plasma Treatment and Silicon Nanowire Growth Conditions.

We report silicon nanowire (SiNW) growth with a novel Cu-In bimetallic catalyst using a plasma-enhanced chemical vapor deposition (PECVD) method. We study the structure of the catalyst nanoparticles (NPs) throughout a two-step process that includes a hydrogen plasma pre-treatment at 200 °C and the SiNW growth itself in a hydrogen-silane plasma at 420 °C. We show that the H2-plasma induces a coalescence of the Cu-rich cores of as-deposited thermally evaporated NPs that does not occur when the same annealing is applied without plasma. The SiNW growth process at 420 °C induces a phase transformation of the catalyst cores to Cu7In3; while a hydrogen plasma treatment at 420 °C without silane can lead to the formation of the Cu11In9 phase. In situ transmission electron microscopy experiments show that the SiNWs synthesis with Cu-In bimetallic catalyst NPs follows an essentially vapor-solid-solid process. By adjusting the catalyst composition, we manage to obtain small-diameter SiNWs-below 10 nm-among which we observe the metastable hexagonal diamond phase of Si, which is predicted to have a direct bandgap.

Effect of pulsed hydrogen and helium plasma irradiation on the formation of solid solutions in the thin-film CdTe/CdS heterosystem

The interphase interaction of cadmium sulfide and cadmium telluride thin film layers under irradiation with pulses of hydrogen and helium plasma was investigated by the X-ray diffraction method. CdS/CdTe heterosystem samples were obtained by the hot wall method on glass substrates with a transparent FTO electrode. During deposition of CdS layers 0.3–0.32 µm thick, the temperature of the evaporator zone was 590 °C, and the substrate temperature was 395 °C. CdTe layers 3.8–4.0 µm thick were deposited at an evaporation zone temperature of 520 °C and a substrate temperature of 497 °C. After deposition, the “chloride” treatment of the heterosystem was carried out. The “chloride” treatment consisted in the deposition of a cadmium chloride film 0.7 μm thick on the surface of a CdTe by thermal vacuum evaporation without heating the substrate and by annealing the entire structure in air at a temperature of 410-415 °C for 20 min. The composition of the solid solutions was determined from the lattice period according to Vegard's rule. In the initial state, along with the cubic phase of cadmium telluride, the presence of CdTe1-xSx solid solutions with a sulfur concentration of 3 % and 8.2 % is observed. After irradiation of the CdS/CdTe heterosystems with hydrogen plasma pulses, the entire base layer turned into a solid solution with a sulfur concentration of 3 %. At the same time, two more phases of CdTe1-xSx solid solutions with sulfur concentrations of 6 % and 11.5 % were observed. The relative concentration of the cadmium telluride phase in the initial state was 84 %, after irradiation with hydrogen plasma pulses it was 82 %. In the initial state, the phases of solid solutions with sulfur content of 3 % and 8.2 % had relative concentrations of 7 % and 9 %, respectively; after irradiation, their relative concentrations became 15 % and 3 %, respectively. After irradiation of the CdS/CdTe heterosystem with helium plasma pulses, the entire base layer turned into a solid solution with a sulfur concentration of 1.5 %. In this case, two more phases with a sulfur concentration of 3.7 % and 7.9 % were observed. The relative concentration of cadmium phase telluride after irradiation with hydrogen plasma pulses decreases to 79 %. After irradiation, the relative concentration of the phase of solid solutions with sulfur contents of 3 % and 8.2 % increases to 17 % and 12 %, respectively. The experimentally observed difference in the evolution of the phase composition of the CdS/CdTe film heterosystem under irradiation with hydrogen and helium plasma pulses may be due to the fact that irradiation with more massive helium atoms leads to a greater thermal effect, which causes melting.

Early-stage structure and evolution mechanism of hydrogen supersaturated surface layers on tungsten under low-energy plasma exposure

Trapping at irradiation-induced defects significantly enhances hydrogen-isotope retention in metals with low hydrogen solubility. Here, we present the first microscopic observation of nanocavity formation on tungsten surfaces exposed to low-dose hydrogen plasma at room temperature. The nanocavities exhibit very high aspect ratios between their lateral size L and thickness ω (L/ω: 60-90) and represent the predominant microstructural defects in the early development stage of the hydrogen supersaturated surface layer on hydrogen plasma-irradiated tungsten (see Nucl Fusion57 (2017) 016026, Acta Mater201 (2020) 55). We also quantify the vacancy yield in tungsten under hydrogen irradiation with ion energies below the threshold to create stable Frenkel pairs. Such defect production at sub-threshold energy is conventionally deemed impossible, but we consider that the exposed surface acts as a defect sink with strong absorption bias for mobile self-interstitial atoms from temporarily created Frenkel pairs. This results in a high excess of immobile vacancies in the nanometer-thin surface layer and their agglomeration causes the evolution of large, disc-like nanocavities.

The role of charge-exchange processes in probing hydrogen plasma with a heavy ion beam

Charge-changing processes of low-charged ions, used in hydrogen plasma probing by the heavy ion beam probe method, are considered. Along with the ionization of beam ions by plasma electrons and protons, the charge-exchange processes of ions on H atoms and protons are also studied. It is shown that charge exchange of beam ions on plasma protons and H atoms, which is rarely taken into account, plays an important role in beam–plasma interaction. New data on the cross sections and rates of ionization and charge-exchange processes are presented for Tl+ and Tl2+ ions, which are frequently used for plasma diagnostics. Calculations are performed for hydrogen plasma temperatures Te = 1 eV–10 keV and densities Ne = 1012–1014 cm−3 at relatively low and high ion-beam velocities vb = 0.2 and 1.0 a.u., respectively. Special attention is paid to the determination of the electron temperatures at which the charge-exchange processes on H atoms and protons are important. Multiple ionization of beam ions by plasma electrons and protons is briefly discussed.

Production of H− ions by thermal desorption process

H− or D− ion current extracted from a hydrogen or deuterium discharge plasma can be substantially enhanced by introducing a small quantity of an alkaline metal, such as cesium, into the extraction region of the ion source. The increase in the extractable H− or D− current is accompanied by a large reduction in the electron output current. This phenomenon of H− or D− enhancement only occurs when the temperature of the plasma electrode is above 50 °C and reaches a maximum around 300 °C. New experimental results here demonstrate that thermal desorption of H atoms from the surface of an electrode together with a reduction of its surface work-function can account for the increase in H− output current observed in the cesium-seeded volume H− ion sources.

Molecular data needs for advanced collisional-radiative modelling for hydrogen plasmas

Population models for molecular hydrogen (H2) are applicable in various fields of plasma physics and particularly in fusion research: they are necessary for the evaluation of plasma diagnostics (e.g. optical emission spectroscopy) or can be used to evaluate effective reaction rates for molecular processes (e.g. molecular-assisted recombination in divertor plasmas). The accuracy and completeness of population models for molecular hydrogen is strongly linked to the accuracy and availability of molecular reaction data. While there are recently huge improvements in the field of electron impact cross sections, the shortfalls regarding input data availability persist in the area of collisions between heavy particles and H2. An overview of the status of population models for H and H2 based on the Yacora solver is given. The data needs for collisional-radiative modelling are demonstrated by means of three examples comprising different detail levels, namely a purely electronic collisional-radiative model for the singlet system of H2, a corona model for the Fulcher-α band and a vibrationally resolved collisional-radiative model for the electronic ground state X1 of H2.Graphical abstractElectronic energy level diagram of the hydrogen molecule delimiting the population models discussed in this work

Atmospheric Non-thermal Plasma Reduction of Natively Oxidized Iron Surfaces

Plasma in hydrogen-containing atmospheres is an efficient method for the reduction of iron oxides. Although a vast number of approaches were performed for the reduction of bulk Fe oxides with thermal hydrogen plasmas, there is almost no information about the non-thermal plasma reduction efficiency in the atmospheric pressure range. In the current article we present the reduction of natively oxidized iron surfaces applying a dielectric barrier discharge plasma in an Ar/H2 atmosphere at 1000 hPa. By varying the surface temperature from 25 to 300 °C, we studied the plasma reduction efficiency, which was then compared with a thermal method. Whereas plasma treatments at 25 °C and 100 °C did not result in the significant reduction of iron oxidized species, experiments at 200 °C and 300 °C yielded a reduction of approximately 88% and 91% of initial oxidized components already after 10 s, respectively. Moreover, we observed an increase in the efficiency with a plasma-thermal reduction in comparison to a thermal method, which was attributed to the presence of atomic hydrogen in the plasma phase. Analysis of morphology revealed the formation of Fe–C structures on surfaces after thermal and plasma-thermal treatments at 200 °C and 300 °C that may be connected with the diffusion of bulk contaminations to the deoxidized surface and reactions between the reduced Fe with plasma-activated adventitious carbon. Conclusively, the plasma was characterized by analyzing the reactive species and the electron temperatures.

Effect of Impurity Ions on Ion Current Flowing into an Ion Sensitive Probe during N<sub>2 </sub>and H<sub>2 </sub>Seeding in Hydrogen Plasma

Effect of Impurity Ions on Ion Current Flowing into an Ion Sensitive Probe during N<sub>2 </sub>and H<sub>2 </sub>Seeding in Hydrogen Plasma

Plasma sheath with multi‐species of positive ions and surface produced negative ions

AbstractThe structure of a plasma sheath has been investigated in front of a caesium‐coated metallic plate, using a simple theoretical model. Along with the electrons, the plasma is composed of multi‐species of positive ions, and surface and volume produced negative ions. While the volume produced negative ions are common in many plasma processing chambers, the surface produced negative ions are of critical importance, especially to the Neutral Beam Injection (NBI) systems. The surface negative ions are produced via the coated metallic plate. With a single species of positive ion, the Bohm criterion, which determines the sheath edge potential, is multi‐valued for a specific range of electronegativity (). The width of the multi‐valued region has been reported to increase with the surface production yield (). Calculations are done for hydrogen plasma with a slight admixture of a second gas such as argon. Besides discussing the differences brought in by the mere presence of a second ion, the paper highlights the importance of the positive ion current density in determining the Bohm criterion. In addition, the effect of three species of positive ions in the surface production process has been discussed.

Engineered Surface Chemistry and Enhanced Energetic Performance of Aluminum Nanoparticles by Nonthermal Hydrogen Plasma Treatment.

Extracting the maximum chemical energy from aluminum nanoparticles (Al NPs) during oxidation is essential for their use in energetic applications. However, the shell of native Al2O3 limits the release of chemical energy by acting as a diffusion barrier and dead weight. Engineering the surface properties of Al NPs by modifying their shell chemistry can reduce the inhibiting effects of the oxide shell on the rate and heat release of oxidation. Here, we employ nonthermal hydrogen plasma at high power and a short time to alter the shell chemistry by doping it with Al-H, as examined and confirmed by HRTEM, FTIR, and XPS. Thermal analysis (TGA/DSC) shows that Al NPs with modified surfaces exhibit augmented oxidation and heat release (33% higher than those of untreated Al NPs). The results demonstrate the promising effect of nonthermal hydrogen plasma in engineering the shell chemistry of Al NPs to improve their overall energetic performance during oxidation.

Effect of Heat and Plasma Treatment on Carboranethiol Self-Assembled Monolayers on Copper

Carborane (C2B10H12) molecules are unique precursors for self-assembled monolayer (SAM) applications. These 3D, icosahedral (12-vertex) molecules allow for the formation of well-ordered layers, potentially with fewer defects than traditional linear alkyl SAMs, and are amenable to cross-linking via labile H atoms with a variety of mechanisms including heat, plasma, and radiation (e.g., UV, e-beam). We have investigated the deposition of SAMs of 1,2-dithiol-ortho-carborane (O) and 9-thiol-meta-carborane (M) on copper from the vapor phase in ultrahigh-vacuum conditions, along with the effect of thermal (150–400 °C) and plasma (N2, H2, Ar, O2) postgrowth treatment. Both films show rapid deposition in the vapor phase with approximate monolayer or few layer coverage, as well as a mixture of physisorption (thiol adsorption) and chemisorption (thiolate binding). Both films additionally show notable stability to thermal treatment up to 400 °C, with only gradual (not abrupt) decrease in boron coverage and minor changes in chemical composition/makeup; however, with the monothiol derivative M showing greater loss of boron and greater oxidation. Nitrogen (N2) plasma treatment leads to partial nitrogenation of boron, while hydrogen (H2) and argon (Ar) plasma treatments lead to partial oxidation of boron, with some parasitic growth (increase in B coverage) especially for the dithiol derivative O. Oxygen (O2) plasma treatment shows an aggressive oxidation (of boron, carbon, sulfur, and copper), but appears to passivate the layers (i.e., the boron oxide based layer formed remains stable and does not change with further plasma exposure). Both heat and plasma treatments reduce copper oxide at the interface and increase thiolate binding (thus conceivably stabilizing the films). Indeed, exposure to N2 plasma appears to further stabilize the films toward heat treatment. These findings highlight that these carboranethiol SAMs represent a robust option for various functional and protective layer applications, and that heat and plasma may be used to further stabilize these films, to modify their properties, or as part of a more complex fabrication scheme.

Conducting surface layers formed by hydrogenation of O-implanted β-Ga2O3

Lightly n-type β-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) on heavily n-type doped β-Ga2O3 substrate was implanted with 1 MeV O ions to a fluence of 1016 cm−2. The film remained β-polymorph and showed no broadening of the x-ray rocking curve width after irradiation even though the calculated number of primary defects was very high. The implanted region was characterized by a strong compensation, likely due to the presence of a high density of split Ga vacancy acceptors. Treatment of the irradiated film in dense hydrogen plasma at 330 °C for 0.5 h led to the formation of a conducting surface layer about 0.5 µm-thick with carrier density 1017 cm−3, a suppression of the signal due to Fe acceptors in Deep Level transient Spectroscopy (DLTS) and a strong enhancement of DLTS peak caused by centers at Ec-0.74 eV (so called E2 * traps). The mechanism appears to be that hydrogen plasma treatment leads to creation of a high number of donor states due complexing of hydrogen with Ga vacancies and to passivation of Fe acceptors with hydrogen donors.

Hydrogen Plasma under Conditions of Electron-Cyclotron Resonance in Microelectronics Technology

Hydrogen Plasma under Conditions of Electron-Cyclotron Resonance in Microelectronics Technology

The influence of hydrogen plasma etching on the surface conductivity of the polycrystalline diamond surface and the temperature-dependent properties

With the development of carbon-based electronics, the polycrystalline diamond has drawn much attention in diamond electronic devices; as compared to single crystalline diamond, the scale of the polycrystalline diamond can be fabricated over 4 in. much more easily. However, until now, the influence factors on the surface conductivity of hydrogen-terminated polycrystalline diamond (H-PCD) are still unclear. In this work, the carrier concentration and mobility of H-PCD were investigated through Hall effect measurement. It was found that besides surface roughness, the temperature at which the equilibrium states of adsorption and desorption of H-PCD surface adsorbates are established also plays an important role in carrier concentration and mobility. With the increase of the establishment temperature, both carrier concentration and mobility show a decreasing tendency, which may be determined by the coefficient of the surface ionic scattering, phonon scattering, and the grain boundaries on the H-PCD surface. The investigation of this work will give some insight into the control of the surface conductivity on H-PCD from the aspect of the equilibrium state established temperature and surface roughness.

Studying the efficacy of hydrogen plasma treatment for enabling the etching of thermally annealed ruthenium in chemical solutions

Ruthenium (Ru) is a noble metal and is known to be resistant to many common chemicals and mixtures. We report in this study a controlled etching/recessing of Ru via wet processing and a combination of dry and wet process using metal-free chemical mixtures. The wet etching study was performed using both as-deposited and annealed atomic layer deposited ruthenium (Ru) films. The etching rate of Ru film in periodic acid mixtures was found to decrease substantially after being annealed at elevated temperatures (300 and 420 °C). X-ray photoelectron spectroscopy results indicated that this behavior could be explained by the formation of a thin surface ruthenium oxide layer induced by the thermal anneal. A short treatment in H2-based atmosphere at elevated temperature or in H2 plasma rendered the annealed Ru film etchable, explained by the reduction of RuOx film at the surface. The increase in surface roughness after wet etching was most likely caused by the gradual formation of oxidized Ru at the surface. Similarly, non-uniform etching of Ru was also observed for damascene Ru structure with the critical dimension ranging from ∼12 to 35 nm. An alternative approach using O2-based plasma achieved a good control of the etching (recessing) depth but also generated a thin layer of “residues” close to the interface between Ru and the liner (TiN). The results from this study have demonstrated that a short H2 plasma treatment prior to the wet process using the same chemical mixtures was able to etch the modified Ru surface layer together with the interfacial residues selectively to bulk Ru.

Plasma for Crewed Transit and Planetary Habitation: Research Efforts at Kennedy Space Center

Plasma-based space applications offer alternative means to Earth-based manufacturing for advanced chemical processing in low Earth orbit (LEO) and beyond. The deployment of plasma systems drives a number of interdisciplinary systems that support human life for crewed transit and planetary habitation. The main obstacle to terrestrially implementing plasma-based applications is economics. This article describes the economics for Earth and Space, the current state of the art, and future considerations from plasma science, economic, and engineering standpoints. The overarching goal of this article is to introduce the usefulness of plasma-space applications to achieve logistics reduction and Earth independence for crewed transit and planetary habitation. The discussion herein includes a subset of applications specifically being investigated at NASA’s Kennedy Space Center (KSC): plasma waste gasification, plasma-assisted nutrient recovery of inedible biomass via ash leaching, hydrogen plasma Lunar regolith reduction for oxygen liberation, plasma for space agriculture, the versatility of plasma-activated water (PAW) for space living, and plasma cleaning and processing. The pathway of plasma research from university laboratories and government funding to technology development for supporting human life is in its infancy and has the potential to influence future planetary colonization plans.

Tailoring Black TiO2 Thin Films: Insights from Hollow Cathode Hydrogen Plasma Treatment Duration

In this study, we report the use of a radiofrequency plasma-assisted chemical vapor deposition (RF-CVD) system with a hollow cathode geometry to hydrogenate anatase TiO2 thin films. The goal was to create black TiO2 films with improved light absorption capabilities. The initial TiO2 was developed through magnetron sputtering, and this study specifically investigated the impact of hollow cathode hydrogen plasma (HCHP) treatment duration on the crucial characteristics of the resulting black TiO2 films. The HCHP treatment effectively created in-bandgap states in the TiO2 structure, leading to enhanced light absorption and improved conductivity. Morphological analysis showed a 24% surface area increase after 15 min of treatment. Wettability and surface energy results displayed nonlinear behavior, highlighting the influence of morphology on hydrophilicity improvement. The anatase TiO2 phase remained consistent, as confirmed by diffractograms. Raman analysis revealed structural alterations and induced lattice defects. Treated samples exhibited outstanding photodegradation performance, removing over 45% of methylene blue dye compared to ~25% by the pristine TiO2 film. The study emphasized the significant impact of 15-min hydrogenation on the HCHP treatment. The research provided valuable insights into the role of hydrogenation time using the HCHP treatment route on anatase TiO2 thin films and demonstrated the potential of the produced black TiO2 thin films for photocatalytic applications.

Hydrogen plasma characteristics in a microwave chemical vapor deposition chamber

Hydrogen plasma characteristics in a microwave chemical vapor deposition chamber