Density Of Active Species Research Articles

Unveiling the pathways of ethanol decomposition to carbon radicals by nanosecond pulse bubble discharge in liquid: a two-step hybrid model

In recent years, bubble discharge in liquid has become a novel approach for the synthesis of carbon nanomaterials; however, the fundamental discharge process and synthesis mechanism are still not well understood. In this work, we build a two-step simulation model (combining 2D fluid dynamics and zero-dimensional plasma kinetics) to investigate nanosecond pulse discharge in an Ar bubble immersed in liquid ethanol and chemical reaction processes inside. The 2D simulation results show that discharge develops along the gas‒liquid interface where ethanol decomposes, resulting in much higher densities of active species (CH3, C2H5 and OH). The electric field of the selected reference point near the interface obtained by the 2D model is transmitted into the 0D model. The numerical results show that the decomposition of ethanol mainly occurs at the discharge stage, in which electron impact dissociation (e.g. C2H5OH + e → CH2OH + CH3 + e) and Penning dissociation (e.g. C2H5OH + Ar* → CH2OH + CH3 + Ar) dominate. The density of all carbonaceous species rapidly increases during discharge, while that of some carbon radicals (CH and C) continues to increase due to neutral species reactions when discharge ceases. By quantitative analysis of the reaction contributions, the dominant pathways of C2, CH and C are revealed, i.e. C2H5OH → C2H5 → [C2H, C2H2, C2H3] → C2 and C2H5OH → CH3 → CH2 → CH → C. In addition, the formation pathways of H and OH radicals, which are indispensable for the transformation of carbonaceous intermediates, are also analysed.

PLASMA PARAMETRS AND SILICON ETCHING KINETICS IN CF4 + C4F8 + O2 MIXTURE: EFFECT OF CF4/C4F8 MIXING RATIO

In this work, we investigated the influence of fluorocarbon component ratio in the CF4 + C4F8 + O2 gas mixture on electro-physical plasma parameters, steady-state densities of active species and silicon etching kinetics under typical reactive-ion process conditions. The combination of plasma diagnostics (double Langmuir probes, optical emission spectroscopy) and plasma modeling confirmed known peculiarities of plasma chemistry of individual fluorocarbons in the presence of oxygen as well as provided an extended analysis of both fluorine and oxygen atom kinetics in the three-component gas mixture. It was shown that the substitution of CF4 by C4F8 at the constant fraction of O2 a) causes the weak disturbance in electrons- and ions-related plasma parameters (electron temperature, electron density, ion energy flux); b) provides drastically increasing density of polymerizing CFx (x = 1, 2) radicals; and c) results in monotonically decreasing F atoms density. The latter is due to simultaneous changes in both F atom formation rate and their loss frequency, especially in C4F8-rich plasmas. From experiments, it was found that Si etching rate is by more than 85% controlled by its chemical component (in a form of ion-stimulated heterogeneous reaction Si + xF → SiFx) and decreases with increasing C4F8 fraction in a feed gas. The change in effective reaction probability contradicts with the growth of polymer deposition rate and film thickness (as it follows from changes in gas-phase plasma characteristics) but may reflect the weakening of surface passivation by oxygen atoms. For citation: Efremov A.M., Bobylev A.V., Kwon K.-H. Plasma parametrs and silicon etching kinetics in CF4 + C4F8 + O2 mixture: effect of CF4/C4F8 mixing ratio. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 6. P. 29-37. DOI: 10.6060/ivkkt.20246706.6982.

Streamer propagation characteristics of nanosecond pulsed discharge plasma on fluidized particles surface: experimental investigation and numerical simulation

Plasma fluidized-bed contributes to strengthening the interaction between active species in plasma and fluidized powder particles, resulting in higher active species utilization efficiency and superior powders processing/modification performance. However, the plasma streamer dynamics on the fluidized powder particles are still unclear due to the intricacy of plasma fluidized-bed. In this work, the time-resolved evolution behavior of plasma streamers on fluidized powder particles surfaces has been explored in plasma fluidized-bed system based on a simplified pin-cylinder configuration. The results reveal that the entire streamer propagation process includes volume discharge and surface discharge. The maximum electron density generated by surface discharge is one order of magnitude higher than that produced by volume discharge, indicating that surface discharge plays a dominant role in powder particles modification. The presence of fluidized particle will cause streamer branching, and the main streamer splits into two independent sub-streamers for propagation in a ‘parabola-like’ shape. Compared with large-size fluidized particles (1000 µm), streamer wraps a larger area on micron-size fluidized particles (200 µm), with a 78% increase in the coverage area, which is favorable to the modification of powder particles. Furthermore, the evolution of active species on fluidized particle surface is analyzed. The active species (N, O, O2 −) are mainly distributed around the north pole, and N2 + is mainly distributed between 25° and 50° of the particles. With the decrease of fluidized particle size, the polarization effect between particles is significantly enhanced, and the maxima of the number densities of active species increase. These findings help to get a better understanding of the interaction between plasma and fluidized particles in fluidized systems.

ON THE EFFECT OF OXYGEN ON PLASMA CHEMICAL KINETICS IN CF4 + O2 AND C4F8 + O2 GAS MIXTURES

In this work, we performed the comparative study of electro-physical plasma parameters, densities of active species and fluorine atom kinetics in CF4 + O2 and C4F8 + O2 gas mixtures with variable initial compositions at constant gas pressure and input power. The combination of plasma diagnostics by Langmuir probes and plasma modeling confirmed known features of plasma properties in individual fluorocarbon gases as well as allowed one to figure out key chemical processes determining plasma parameters in the presence of oxygen. It was shown that an increase in O2 content with a proportional decrease in the fraction of any fluorocarbon component a) causes relatively weak changes in electrons- and ions-related plasma parameters; b) results in more drastic (compared with the dilution effect) decrease in densities of fluorocarbon radicals due to their oxidation into CFxO, FO and COx compounds; and c) sufficiently influences both formation and decay kinetics of fluorine atoms. The non-monotonic (with a maximum at ~ 40-50% O2) change in the F atom density in the CF4 + O2 plasma repeats behavior of their formation rate after the contribution of processes involving CFxO и FO species. The monotonic increase (with a constancy region up to ~ 40-50% O2) in the F atom density in the C4F8 + O2 plasma contradicts with the change in their formation rate, but results from decreasing decay frequency in gas-phase atom-molecular processes. The predictive analysis of heterogeneous process kinetics was carried out using model-yielded data on fluxes of plasma active species. It was found that a) the addition of oxygen always lowers the plasma polymerizing ability; and b) the C4F8 + O2 plasma keeps the higher polymerizing ability at any feed gas composition.

The enhanced mechanism of microwave-induced discharge between millimeter activated coke

Microwave heating has attracted extensive attention due to its advantages of low energy consumption and rapid heating. Due to its kinetic and thermal effects, microwave-induced activated coke discharge has been widely used in material preparation and reaction enhancement. However, the mechanism of microwave discharge for millimeter activated coke has not been clarified. In this work, we have investigated the mechanism of microwave discharge between millimeter activated cokes by experiments and simulations. Experimental results presented that the increase in particle size caused the inter-particle discharge transferred from weak sparks to large-scale violent discharge. Simulation results indicated that the increase in particle size led to enhanced microwave field resonance between particles, thereby promoting the inter-particle electric field, electron temperature and active species density. Finally, the particle size of cokes affected the deposition of microwave energy in discharge regions between particles. The influence of particle size on microwave-induced activated coke discharge was reflected in two mechanisms, namely, the microwave resonance enhanced mechanism that promoted the microwave energy deposition between particles and the focused microwave energy mechanism that dominated the inter-particle microwave energy deposition distribution concentrated in the center regions of the particle gap.

Contribution of active species generated in plasma to CO2 methanation

CO2 methanation is an effective technology for CO2 reduction. Generally, methanation reactions are accelerated using thermal catalysts. However, the temperature control is difficult because CO2 methanation is an exothermic reaction, and the catalyst is deactivated by overheating. Plasma catalysis can solve this problem by driving this reaction at lower temperatures. Therefore, in this study, we investigated the contribution of the active species generated in the plasma to CO2 methanation. We found that the density of active species is linearly related to the power density, and in particular, the CH4 generation rate is determined by the CO-derived active species, not the H-derived active species. Furthermore, with an increase in the catalyst temperature, a new reaction pathway for CH4 production is added. The results of this study contribute to the understanding of the relationship between the active species produced in plasma and CO2 methanation.

ON THE COMPARISON OF REACTIVE-ION ETCHING MECHANISMS FOR SiO2 AND Si3N4 IN HBr + Ar PLASMA

This work investigated the influence of component ratio in the HBr + Ar gas mixture on electro-physical plasma parameters, steady-state densities of active species and reactive-ion etching (RIE) kinetics for SiO2 and Si3N4 under conditions of inductive RF (13.56 MHz) discharge. The combination of plasma diagnostics by Langmuir probes and plasma modeling indicated that an increase in Ar content at constant gas pressure and input power a) caused an increase in electron temperature and densities of charged species; b) results in increasing ion bombardment intensity; and c) leads to the nearly proportional decrease in Br atoms density and flux. It was found that variations of SiO2 and Si3N4 etching rates vs. mixture composition are qualitatively similar while the maximum difference in corresponding absolute values takes place in pure HBr plasma. The analysis of RIE mechanisms was carried out using model-predicted data on fluxes of ions and bromine atoms. It was found that the dominant SiO2 etching mechanism is the ion-assisted chemical reaction which is characterized by the nearly-constant rate in the range of 0–80% Ar due to an increase in the effective reaction probability. That is why the noticeable intensification of physical sputtering with increasing Ar fraction in a feed gas causes the only weak growth of obtained SiO2 RIE rate. Oppositely, the Si3N4 etching process is mainly contributed by the physical sputtering while the efficiency of ion-stimulated chemical reaction is limited by the low reaction probability. This provides both slower etching process (especially in Ar-poor plasmas) and stronger sensitivity of etching rate to the change in mixture composition. For citation: Efremov A.M., Betelin V.B., Kwon K.-H. On the comparison of reactive-ion etching mechanisms for SiO2 and Si3N4 in HBr + Ar plasma. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 6. P. 37-45. DOI: 10.6060/ivkkt.20236606.6786.

Optical emission spectroscopy study in CO2 methanation with plasma

Methanation of CO2 is a key technology to realize a sustainable society. The reactions should be driven at a lower temperatures from the viewpoint of catalyst stability. Methanation with plasma catalysis can drive reactions at lower temperature than thermal catalysis. However, the reaction mechanism is little understood due to the complexity of the interactions. In this study, we investigated the power and pressure dependence of the methanation efficiency when only plasma is used as a fundamental research. We discuss how these parameters change the vibrational temperature and active species density and affect the methanation efficiency using optical emission spectroscopy.

High-Density NiCu Bimetallic Phosphide Nanosheet Clusters Constructed by Cu-Induced Effect Boost Total Urea Hydrolysis for Hydrogen Production.

The development of urea electrolysis technologies toward energy-saving hydrogen production can alleviate the environmental issues caused by urea-rich wastewater. In the current practices, the development of high-performance electrocatalysts in urea electrolysis remains critical. In this work, the NiCu-P/NF catalyst is prepared by anchoring Ni/Cu bimetallic phosphide nanosheets onto Ni foam (NF). In the experiments, the micron-sized elemental Cu polyhedron is first anchored on the surface of the NF substrate to provide more space for the growth of bimetallic nanosheets. Meanwhile, the Cu element adjusted the electron distribution within the composite and formed Ni/P orbital vacancies, which in turn accelerated the kinetic process. As a result, the optimal NiCu-P/NF sample exhibits excellent catalytic activity and cycling stability in a hybrid electrolysis system for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). Further, the alkaline urea-containing electrolyzer is assembled with NiCu-P/NF as two electrodes reached a current density of 50 mA cm-2 with a low driving potential of 1.422 V, which outperforms the typical commercial noble metal electrolyzer (RuO2||Pt/C). Those findings suggest the feasibility of the substrate regulation strategy to increase the growth density of active species in preparation of an efficient bifunctional electrocatalyst for cracking the urea-containing wastewater.

FEATURES OF PLASMA COMPOSITION AND FLUORINE ATOM KINETICS IN CHF3 + O2 GAS MIXTURE

The effects of initial composition of CHF3 + O2 gas mixture on electro-physical plasma parameters, steady-state densities of active species and fluorine atom kinetics were investigated under the condition of constant gas pressure and input power. The combination of plasma diagnostics by Langmuir probes and model-based analysis of plasma chemistry confirmed known from previous work features of plasma composition in the absence of oxygen (in particular, the domination of HF molecules in a gas phase) as well as demonstrated how the oxygen influences steady-state densities of neutral species through kinetics of both electron-impact and atom-molecular reactions. It was shown that the addition of O2 with a proportional decrease in the content of CHF3 a) results in noticeable changes in electrons- and ions-related plasma parameters (electron temperature, electron density, ion flux); b) provides the effective conversion of fluorocarbon radicals (CHFx, CFx) into such compounds as CFxO, FO and COx; and b) causes an increase in the fluorine atom density up to 50% O2. The last phenomenon contradicts with the change in the total F atom formation rate, but reflects a decrease of their loss frequency in the reaction family of CHFx + F → CFx + HF. The model-based analysis of heterogeneous process kinetics was carried out using the set of tracing parameters based on gas-phase plasma characteristics. It was found that the addition of oxygen lowers the plasma polymerizing ability through both decreasing flux of polymerizing species and accelerating the oxidative destruction of deposited polymer film.

Plasma Scalpels: Devices, Diagnostics, and Applications.

The plasma scalpel is an application of gas discharges in electrosurgery. This paper introduces the device structure and physicochemical parameters of the two types of plasma scalpels, namely, a single-electrode Ar discharge device (argon plasma coagulation) and a two-electrode discharge device in normal saline. The diagnostic methods, including the voltage and current characteristics, optical emission spectroscopy, electron spin resonance, and high-speed imaging, are introduced to determine the critical process parameters, such as the plasma power, the gas temperature, the electron density, and the density of active species, and study the ignition dynamics of the plasma discharges in water. The efficacy of the plasma scalpel is mainly based on the physical effects of the electric current and electric field, in addition to the chemical effects of high-density energetic electrons and reactive species. These two effects can be adjusted separately to increase the treatment efficacy of the plasma scalpel. Specific guidance on further improvements of the plasma scalpel devices is also provided.

Comparative Study of Influence of Experimental Configuration on Densities of Active Species in the Early Afterglows of N2/(0–2.5%)H2 HF Flowing Plasmas

Afterglows of mixed gas of N2 and H2(0–2.5%) flowing microwave discharges in a 5 mm diameter tube connected to a 5 L reactor via a tube of 1.8 cm diameter and 50 cm long, have been studied using optical emission spectroscopy. The obtained results at the entrance of the afterglow tube of 1.8 cm diameter: Short time afterglow (SA), (10–3 s) and inside the 5 L reactor: Long time afterglow (LA), (10–2 s) were then compared. It was found that, in N2 at 2 Torr, 0.5 slpm, the active specie density ratios had a constant value of 10–2 for N/N2, but decreased respectively from 10–3 to 10–4 for N2 (X,v > 13)/N2 and from 10–6 to 10–8 for N+2 /N2. By directly connecting the discharge tube inside the 5 L reactor, the density increases by 10 for N2 (X,v > 13) and by 102 for N2+ by changing the afterglow from LA(10−2 s) to a SA(10–3 s). Moreover and by adding 1% of H2 to N2, the N/N2 and H/H2 ratios had constant values of 1% and 0.2% respectively. The SA(10–3 s) appeared to be more efficient for surface treatments than the LA (10–2 s).

Enhancement of hydrogen radical density in atmospheric pressure plasma jet by a burst of nanosecond pulses at 1 MHz

The generation and enhancement of active species in non-thermal plasmas are always decisive issues with respect to their successful applications. In this work, an atmospheric pressure plasma jet (APPJ) is generated in Ar + 1% CH4 gas flow by a bipolar nanosecond high-voltage (HV) source with a maximum pulse repetition rate up to 1 MHz (i.e. minimum pulse interval ΔT = 1 μs) in burst mode. The absolute density of hydrogen atom at ground state is measured by the two-photon absorption laser-induced fluorescence method. It is observed that with ΔT = 1 μs, the H atom density keeps increasing during the first eight HV pulses and later on, the H atom density is maintained at a quasi-stable value while more HV pulses are applied. When decreasing ΔT from 10 to 1 μs, while keeping the total number of HV pulses the same (with similar coupled energy), the peak H atom density increases by a factor of more than four times, but the decay of H atom density after the pulse burst with ΔT = 1 μs is faster. Another effect of short ΔT is to extend the axial distribution of the H atom outside the APPJ’s nozzle, and the ΔT = 2 μs case has the highest averaged H atom density when taking its temporal evolution and axial distribution into consideration. In this work, we propose that the intensive nanosecond HV burst is an efficient approach to enhance the active species density in non-thermal plasmas when a rapid response is required.

(Invited) Microwave Plasmas Applied for a Large-Scale Production of Graphene & Derivatives

As graphene&derivatives penetrate the industry, it becomes crucial to develop the means to mass produce high quality across all related product segments. Research efforts meet formidable challenges in the marketplace because of the lack of discernible protocols to fabricate customized products at increasingly higher yields while maintaining its quality. Notably, a recent study on the commercial graphene purchased from 60 companies worldwide has found that the majority of companies are selling a product containing less than 10% graphene content and that none of them supplied samples containing more than 50% graphene [1]. To find pathways to broad commercialization of graphene&derivatives the constant efforts must be focused on disruptively improved material synthesis.In the present report we provide substantial evidence that microwave plasma based technologies can be used as a competitive and disruptive alternative to existing chemical methods in the production of free-standing flakes of graphene and N-graphene at a large scale. Prioritizing operational flexibility, microwave plasma induced by surface waves (SW) at atmospheric pressure conditions has been employed [2]. The surface wave sustained plasma produced by the field of a traveling wave has an extended active zone outside the wave launcher because the plasma is sustained by a wave that simultaneously propagates and creates its own propagation structure. In this way, large microwave power densities can be injected into processing area and high population densities of active species of interest can be achieved. The reactor can be virtually separated in two different zones. The first one is the "hot" plasma zone. Here, the wave power is absorbed primarily by plasma electrons, which transfer the power to heavy particles via elastic and inelastic collisions resulting in gas temperatures up to 4000 K. The gas temperature gradually decreases moving away from the SW launcher and drops to about 500 K in the "mild" plasma zone. The carbon precursor mixed with background argon gas is injected in the "hot" high energy density zone where decomposition processes of the injected carbonaceous molecules take place as a result of particles collisions and intensified chemistry. The created radicals become precursors of chain reactions leading to creation of the main building blocks, i.e. carbon atoms and C2 radicals. The transport of plasma generated carbon atoms/molecules into colder zones of the reactor results in formation of solid carbon nuclei that are gradually withdrawn in the outlet plasma stream where kinetic processes of assembly and growth of “flowing” carbon nanostructures take place. Therefore, by tailoring the temperature gradients, the density of carbon molecules and atoms as well as their residence time, selective synthesis of graphene/N-graphene (nitrogen doped graphene) sheets in a narrow range of operational parameters can be achieved. Two different experimental strategies to deliver the nitrogen precursors to the plasma environment, i.e., top-to-down and down-to-top injection, have been applied. The top-to-down approach provides injection of both the carbon and the nitrogen precursors mixed with argon gas into the “hot” plasma zone, while down-to-top approach provides injection of nitrogen precursor into the “mild” plasma zone against the main flux.In this report we demonstrate an operational plasma-based proof-of-concept machine for a controllable, continuous fabrication of graphene and N-graphene at a gram scale ensuring high-degree of single layer selectivity (~50 %), high yield (~30 mg/min) and repeatability while using cheap starting materials (e.g., ethanol, methane, acetonitrile, etc.). The plasma machine provides high level of control and customization via the application of special protocols, resulting in a controllable level of N-doping, oxygen impurities and sheets size (~300 – 400 nm). The fabricated graphene/N-graphene flakes possess high quality as evidenced by comprehensive physicochemical analyses made (for pure graphene C/O ratio: > 50; sp2 carbons~ 70%; for N-graphene C/O ratio: > 40; sp2 carbons >60%).The microwave plasma driven synthesis is controllable at a single step and atmospheric pressure conditions, allows continuous production in an environmentally friendly manner.

Near‐Field Drives Long‐Lived Shallow Trapping of Polymeric C3N4 for Efficient Photocatalytic Hydrogen Evolution

AbstractUndesired photoelectronic dormancy through active species decay is adverse to photoactivity enhancement. An insufficient extrinsic driving force leads to ultrafast deep charge trapping and photoactive species depopulation in carbon nitride (g‐C3N4). Excitation of shallow trapping in g‐C3N4 with long‐lived excited states opens up the possibility of pursuing high‐efficiency photocatalysis. Herein, a near‐field‐assisted model is constructed consisting of an In2O3‐cube/g‐C3N4 heterojunction associated with ultrafast photodynamic coupling. This In2O3‐cube‐induced near‐field assistance system provides catalytic “hot areas”, efficiently enhances the lifetimes of excited states and shallow trapping in g‐C3N4 and this favors an increased active species density. Optical simulations combined with time‐resolved transient absorption spectroscopy shows there is a built‐in charge transfer and the active species lifetimes are longer in the In2O3‐cube/g‐C3N4 hybrid. Besides these properties, the estimated overpotential and interfacial kinetics of the In2O3‐cube/g‐C3N4 hybrid co‐promotes the liquid phase reaction and also helps in boosting the photocatalytic performance. The photocatalytic results exhibit a tremendous improvement (34‐fold) for visible‐light‐driven hydrogen production. Near‐field‐assisted long‐lived active species and the influences of trap states is a novel finding for enhancing (g‐C3N4)‐based photocatalytic performance.

Synergetic effect of packed-bed corona-DBD plasma micro-reactor and photocatalysis for organic pollutant degradation

A packed-bed Corona-DBD hybrid plasma micro-reactor is coupled with a novel heterogeneous photocatalyst-AgFeO2/CNTs/TiO2 for methyl orange degradation. The corona nature of the plasma produces a high concentration of active species, yet maintaining a homogeneous discharge owing to the dielectric barrier nature of the reactor. The submicron channels in the plasma reactor created by glass beads increased dielectric strength and simultaneously produced high energy streamers, which increased the ozone generation by 24%. The AgFeO2/CNTs/TiO2 photocatalyst gave stretched adsorption edge (400–500 nm), reduced bandgap energy (2.3 eV), facilitated degradation under visible light, and provided the additional advantage of the Fenton process, leading to the OH. formation. The three reaction systems were investigated and compared for MO degradation-photocatalysis, plasma reactor and integrated plasma photocatalysis reactor which gave 1.2%, 50.8%, and 98% removal in 2 min respectively. The solution pH, dosage, and loading of AgFeO2 in photocatalyst was investigated. The integrated plasma photocatalysis gave 95% degradation of 250 ml solution of 30 ppm methyl orange at pH 3 with 0.5 g/l of 2.5% AgFeO2/CNTs/TiO2 photocatalysis in 2 min. The integrated plasma photocatalysis reactor increased the number density of active species, particularly, OH. in wastewater which makes it a good choice for azo dyes degradation.

On Relationships between Gas-Phase Chemistry and Reactive Ion Etching Kinetics for Silicon-Based Thin Films (SiC, SiO2 and SixNy) in Multi-Component Fluorocarbon Gas Mixtures.

This work summarizes the results of our previous studies related to investigations of reactive ion etching kinetics and mechanisms for widely used silicon-based materials (SiC, SiO2, and SixNy) as well as for the silicon itself in multi-component fluorocarbon gas mixtures. The main subjects were the three-component systems composed either by one fluorocarbon component (CF4, C4F8, CHF3) with Ar and O2 or by two fluorocarbon components with one additive gas. The investigation scheme included plasma diagnostics by Langmuir probes and model-based analysis of plasma chemistry and heterogeneous reaction kinetics. The combination of these methods allowed one (a) to figure out key processes which determine the steady-state plasma parameters and densities of active species; (b) to understand relationships between processing conditions and basic heterogeneous process kinetics; (c) to analyze etching mechanisms in terms of process-condition-dependent effective reaction probability and etching yield; and (d) to suggest the set gas-phase-related parameters (fluxes and flux-to-flux ratios) to control the thickness of the fluorocarbon polymer film and the change in the etching/polymerization balance. It was shown that non-monotonic etching rates as functions of gas mixing ratios may result from monotonic but opposite changes in F atoms flux and effective reaction probability. The latter depends either on the fluorocarbon film thickness (in high-polymerizing and oxygen-less gas systems) or on heterogeneous processes with a participation of O atoms (in oxygen-containing plasmas). It was suggested that an increase in O2 fraction in a feed gas may suppress the effective reaction probability through decreasing amounts of free adsorption sites and oxidation of surface atoms.

Comparative study of Cl2 + O2 and HBr + O2 plasma chemistries in respect to silicon reactive-ion etching process

The comparative study of Cl2 + O2 and HBr + O2 plasma chemistries in respect to Si reactive-ion etching process was carried out. Both plasma diagnostics and modeling tools were applied to compare effects of gas mixing ratios on steady-state plasma parameters and densities of active species as well as to determine key processes in respect to Cl and Br atom kinetics. It was found that a) the variation of Cl2/O2 and HBr/O2 mixing ratios produces opposite changes in both electrons-related and ions-related plasma parameters in corresponding gas systems; b) the stepwise dissociation pathways, such as Cl2 + O/O(1D) → ClO + Cl and HBr + O/O(1D) → OH + Br, have no principal influence on the halogen atom formation rates; and c) the higher formation rate and the lower recombination frequency for Cl atoms always provide the condition [Cl] > [Br]. The data on fluxes of plasma active species provided the analysis of Si etching kinetics in terms of process-condition-dependent reaction probability and etching yield. It was suggested that both parameters are sensitive to O atom flux through the oxidation of reaction products into the lower volatile SiClxOy and SiBrxOy compounds.

Plasma parameters and densities of active species in mixtures of fluorocarbon gases with argon and oxy-gen

Plasma parameters and densities of active species in mixtures of fluorocarbon gases with argon and oxy-gen

FEATURES OF SiO2 REACTIVE-ION ETCHING KINETICS IN CF4 + Ar + O2 AND C4F8 + Ar + O2 GAS MIXTURES

The effect of Ar/O2 mixing ratio on gas-phase characteristics and SiO2 etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas was studied under conditions of 13.56 MHz inductive RF discharge. The constant processing parameters were fraction of fluorocarbon component in a feed gas (50%) total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). It was found that the full substitution of Ar for O2 in both gas systems results in non-monotonic (with a maximum at ~ 25% Ar + 25% O2) SiO2 etching rates as well as in monotonically increasing photoresist etching rate with higher absolute values for CF4-containing mixture. The steady-state densities of active species were determined using a combination of plasma diagnostics by Langmuir probes and 0-dimensional (global) plasma modeling. Corresponding results indicated that both gas systems are characterized by quite close parameters of electron and ion components while exhibit sufficient differences in the kinetics of neutral species, especially in the presence of O2. The latter produces opposite changes in F atom density as well as in effective probability of ion-assisted chemical reaction vs. Ar/O2 mixing ratio. Relationships between type of fluorocarbon component and heterogeneous process kinetics were analyzed through the set of gas-phase-related parameters (fluxes, flux-to-flux ratios) characterizing chemical etching pathways for SiO2 and formation/destruction balance for the fluorocarbon polymer film. It was suggested that the transition toward O2-rich plasma in the low-polymerizing CF4 + Ar + O2 plasma suppresses the effective probability for SiO2 + F reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. Oppositely, an increase in O2 content in the high-polymerizing C4F8 + Ar + O2 mixture lifts up the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.