Downstream Plasma Research Articles

Modeling of atmospheric gas-stream processing using a microwave excited all-dielectric resonant plasma discharge

Computational modeling of an all-dielectric resonant plasma discharge for the processing of atmospheric air streams is presented. A single dielectric resonator (DR) pair separated by a small gap is considered. Governing equations for plasma, flow and electromagnetics are coupled to resolve the entire process characterized by fast timescale plasma breakdown followed by slow timescale transport of radicals catalyzed in the discharge. The resonant frequencies of the DR system are determined by solving for the electromagnetic wave across a range of microwave frequencies in the absence of the plasma. The DR operating at the resonant frequency (on-resonance mode) results in 12 times larger electric field amplification in the dielectric gap as compared to its operation in an off-resonance mode. Plasma breakdown occurs in the DR gap where the electric field amplification is the highest, due to constructive interference of electromagnetic wave modes from the adjoining cylindrical dielectrics. The plasma frequency in on-resonance mode is smaller than the resonant frequency of the DR. This inhibits the formation of surface wave modes and results in a uniform electromagnetic power deposition and volumetric plasma generation in the dielectric gap. It is seen that oxygen radicals (O) and oxygen metastable states O2 a1 and O2 b1 are dominant products of the plasma catalysis process. The active radical species are transported with the flow where they can be used in downstream plasma processing applications.

Effects of Downstream Plasma Exposure on β-Ga2O3 Rectifiers

The effects of downstream plasma exposure with O2, N2 or CF4 discharges on Si-doped Ga2O3 Schottky diode forward and reverse current-voltage characteristics were investigated. The samples were exposed to discharges with rf power of 50 W plasma at a pressure of 400 mTorr and a fixed treatment time of 1 min to simulate dielectric layer removal, photoresist ashing or surface cleaning steps. Schottky contacts were deposited through a shadow mask after exposure to avoid any changes to the surface. A Schottky barrier height of 1.1 eV was obtained for the reference sample without plasma treatment, with an ideality factor of 1.0. The diodes exposed to CF4 showed a 0.25 V shift from the I–V of the reference sample due to a Schottky barrier height lowering around 14%. The diodes showed a decrease of Schottky barrier height of 2.5 and 6.5% with O2 or N2 treatments, respectively. The effect of plasma exposure on the ideality factor of diodes treated with these plasmas was minimal; 0.2% for O2 and N2, 0.3% for CF4, respectively. The reverse leakage currents were 1.2, 2.2 and 4.8 μA cm−2 for the diodes treated with O2, and CF4, and N2 respectively. The effect of downstream plasma treatment on diode on-resistance and on-off ratio were also minimal. The changes observed are much less than caused by exposure to hydrogen-containing plasmas and indicate that downstream plasma stripping of films from Ga2O3 during device processing is a relatively benign approach.

Charged particle transport across an obstacle in a non-flowing partially magnetized plasma column

A wake is created in a plasma when a macroscopic body blocks the flow of charged particles from entering in to a downstream plasma region. The phenomena leads to a strong depletion in charged particle density behind the obstacle. In this paper, charged particle transport inside an ionization free region behind a macroscopic obstacle has been investigated for the case of a non-flowing, partially magnetized plasma column. Surprisingly, it is found that the transport of hotter electron population inside the void region is enhanced due to the application of axial magnetic field. Furthermore, the radial plasma density and potential variation inside the obstructed region show an opposing trend than the region outside the obstacle. A phenomenological model is given to explain the mechanism behind observing these trends.

Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers

The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology.

In situ XPS analysis of the electronic structure of silicon and titanium thin films exposed to low-pressure inductively-coupled RF plasma

Carbon contamination of synchrotron and free-electron lasers beamline optics continues to be a major nuisance due to the interaction of the intense photon beams with the surfaces of the optical elements in the presence of residual gases even in ultrahigh vacuum (UHV) conditions. Among the available in situ cleaning strategies, low-pressure radio frequency (RF) plasma treatment has emerged as a useful and relatively simple approach to remove such carbon contamination. However, the irreversible damage that the plasma may induce in such critical surfaces has to be carefully characterized before its general application. In this study, we focus on reducing the amount of carbon from UHV chamber inside surfaces via silicon and titanium coatings using a low-pressure inductively-coupled downstream plasma source and we characterize the surface alterations by in situ X-ray photoemission spectroscopy (XPS). The in situ mirror cleaning is simulated by means of silicon wafers. We observe upward band bending, which translates into lower binding energies of the photoemission lines, that we attribute to the generation of vacancies and trapped charges in the oxide layers.

Kinetic Simulations of Cosmic-Ray-modified Shocks. I. Hydrodynamics

Abstract Collisionless plasma shocks are efficient sources of nonthermal particle acceleration in space and astrophysical systems. We use hybrid (kinetic ion—fluid electron) simulations to examine the nonlinear feedback of the self-generated energetic particles (cosmic rays, CRs) on the shock hydrodynamics. When CR acceleration is efficient, we find evidence of both an upstream precursor, where the inflowing plasma is compressed and heated, and a downstream postcursor, where the energy flux in CRs and amplified magnetic fields play a dynamical role. For the first time, we assess how nonlinear magnetic fluctuations in the postcursor preferentially travel away from the shock at roughly the local Alfvén speed with respect to the downstream plasma. The drift of both magnetic and CR energy with respect to the thermal plasma substantially increases the shock compression ratio with respect to the standard prediction, in particular exceeding 4 for strong shocks. Such modifications also have implications for the spectrum of the particles accelerated via diffusive shock acceleration, a significant result detailed in a companion paper.

Heat Fluxes in Collisionless Magnetohydrodynamic Shock Waves

Properties of solutions for parallel magnetohydrodynamic (MHD) shock waves in collisionless plasma with heat fluxes obtained in the 8-moment MHD approximation are discussed. The domain of upstream shock-wave plasma parameters in which there are solutions that preserve physical meaning of the downstream plasma parameters is determined. New results have also been obtained for the heat fluxes behind the front of parallel shock waves in collisionless plasma.

Detached helium plasma simulation by a one-dimensional fluid code with detailed collisional-radiative model

To increase the accuracy of a particle, momentum, and energy source terms in the detached helium plasma simulation, rate coefficients with the collisional-radiative model were introduced into the fluid code LINear Divertor Analysis (LINDA). Obtained effective rate coefficients and related source terms were compared with those from the conventional empirical databases. It is shown that a high-density condition in future fusion devices causes larger deviation between the effective and the empirical source terms. One-dimensional detached plasma simulation indicated that the peak amplitude of the plasma density during the rollover is sensitive to the source term difference related to the recombination. This study additionally revealed that the heating effect in the three-body recombination process strongly affects the detached plasma formation and downstream plasma parameters.

Probing the thermodynamic conditions of the heliosheath plasma by shock wave propagation

Context. The pressure equilibrium between the inner heliosheath and the outer heliosheath (referred to as the local interstellar medium) is an eminent theoretical and practical problem; theoretical, because the relevant pressure carriers have to be identified, and practical, because data must be gathered in order to confirm such a pressure equilibrium. The problem is closely connected with the stability of the heliopause, that is, of the tangential discontinuity between these two counterflowing media, and is of utmost importance for understanding the stability of the whole circumsolar plasma structure. Aims. In this paper we analyze the thermodynamic conditions of the multi-fluid plasma between the solar wind termination shock and the heliopause determining the total heliosheath pressure. We look into this problem from a theoretical standpoint and revisit theoretical descriptions of the solar wind plasma after its passage over the solar wind termination shock, thereafter forming the subsonic heliosheath region. Methods. Hereby we take into account the 3D magnetohydrodynamics shock conditions and the resulting 3D temperature structure of the downstream plasma flow. We use a kind of seismological procedure to probe the heliosheath plasma by inquiring into the propagation conditions of traveling shock wave perturbations in this predetermined 3D heliosheath plasma structure. We discuss the fact that the front geometry of such a traveling shock wave most probably does not remain spherical, if it was to begin with, due to asymmetric shock propagation conditions. In contrast, the wave front is likely to become strongly deformed into an upwind bulge. Results. Concerning the plasma pressure, in addition to solar wind and pick-up proton pressures, we have to take into account the solar wind electron pressure which as a surprise turns out to be of comparable magnitude. As a consequence, the characteristic propagation speed of the traveling shock wave in the weakly magnetized heliosheath plasma is given as a mixed speed expressed by the sound speeds of the protons and the electrons. We describe local low-energy proton density signatures that can be found in Voyager-2 proton data as a consequence of traveling shock wave passages and show that the total local plasma pressure can be directly derived from them.

The Compression of the Heliospheric Magnetic Structure by Interplanetary Shocks: Is the Structure at 1AU a Manifestation of Solar-Wind Turbulence or Is It Fossil Structure From the Sun?

As a test of the action of MHD turbulence in the solar wind, the compression of the heliospheric magnetic structure at 1AU by 109 interplanetary shocks is examined. In the magnetic structure the orientations of solar-wind strong current sheets are statistically examined versus time in the downstream plasmas after shock compression. If the current sheets of the solar wind are features of an active MHD turbulence, they should be destroyed and remade with isotropic orientations on the timescale of an eddy-turnover time. If the current-sheet orientations remain anisotropic after the shock compression, it is an indication that the current sheets of the solar wind are not created by MHD turbulence. This statistical analysis finds no evolution of the current-sheet orientations after the solar wind is compressed by the shocks, implying a non-turbulent origin of the current sheets. A possibility is that the heliospheric magnetic structure at 1 AU is fossil structure from the solar corona.

Observation of the Large‐Amplitude and Fast‐Damped Plasma Sheet Flapping Triggered by Reconnection‐Induced Ballooning Instability

AbstractIn this study, we reported the large‐amplitude and fast‐damped flapping of the plasma sheet, which co‐occurred with magnetic reconnection. Data from the Double Star TC‐1 and Cluster satellites were used to analyze the features of the plasma sheet flapping 1.4 RE earthward of an ongoing magnetic reconnection event. The flapping was rapidly damped, and its amplitude decreased from the magnetohydrodynamics scale to the subion scale in 5 min. The variation in the flapping period (from 224 to 20 s) indicated that the source of the flapping had highly dynamic temporal characteristics. The plasma sheet flapping propagated duskward through a kink‐like wave with a velocity of 100 km/s, which was in agreement with the group velocity of the ballooning perturbation. A correlation analysis between the magnetic reconnection and plasma sheet flapping indicated that the magnetic reconnection likely facilitated the occurrence of ballooning instability by altering the state of plasma in the downstream plasma sheet. In this regard, the reconnection‐induced ballooning instability could be a potential mechanism to generate the flapping motion of the plasma sheet.

Slow-, Tight-Binding Inhibition of CYP17A1 by Abiraterone Redefines Its Kinetic Selectivity and Dosing Regimen.

Substantial evidence underscores the clinical efficacy of inhibiting CYP17A1-mediated androgen biosynthesis by abiraterone for treatment of prostate oncology. Previous structural analysis and in vitro assays revealed inconsistencies surrounding the nature and potency of CYP17A1 inhibition by abiraterone. Here, we establish that abiraterone is a slow-, tight-binding inhibitor of CYP17A1, with initial weak binding preceding the subsequent slow isomerization to a high-affinity CYP17A1-abiraterone complex. The in vitro inhibition constant of the final high-affinity CYP17A1-abiraterone complex ( ( K i * = 0.39 nM )yielded a binding free energy of -12.8 kcal/mol that was quantitatively consistent with the in silico prediction of -14.5 kcal/mol. Prolonged suppression of dehydroepiandrosterone (DHEA) concentrations observed in VCaP cells after abiraterone washout corroborated its protracted CYP17A1 engagement. Molecular dynamics simulations illuminated potential structural determinants underlying the rapid reversible binding characterizing the two-step induced-fit model. Given the extended residence time (42 hours) of abiraterone within the CYP17A1 active site, in silico simulations demonstrated sustained target engagement even when most abiraterone has been eliminated systemically. Subsequent pharmacokinetic-pharmacodynamic (PK-PD) modeling linking time-dependent CYP17A1 occupancy to in vitro steroidogenic dynamics predicted comparable suppression of downstream DHEA-sulfate at both 1000- and 500-mg doses of abiraterone acetate. This enabled mechanistic rationalization of a clinically reported PK-PD disconnect, in which equipotent reduction of downstream plasma DHEA-sulfate levels was achieved despite a lower systemic exposure of abiraterone. Our novel findings provide the impetus for re-evaluating the current dosing paradigm of abiraterone with the aim of preserving PD efficacy while mitigating its dose-dependent adverse effects and financial burden. SIGNIFICANCE STATEMENT: With the advent of novel molecularly targeted anticancer modalities, it is becoming increasingly evident that optimal dose selection must necessarily be predicated on mechanistic characterization of the relationships between target exposure, drug-target interactions, and pharmacodynamic endpoints. Nevertheless, efficacy has always been perceived as being exclusively synonymous with affinity-based measurements of drug-target binding. This work demonstrates how elucidating the slow-, tight-binding inhibition of CYP17A1 by abiraterone via in vitro and in silico analyses was pivotal in establishing the role of kinetic selectivity in mediating time-dependent CYP17A1 engagement and eventually downstream efficacy outcomes.

First EMC3-EIRENE simulation of edge plasma behavior and heat flux deposition on divertor target on HL-2M

Abstract The EMC3-EIRENE code has been applied for the first time to simulate the edge plasma characteristics of the new tokamak device HL-2M. Impacts of the upstream electron density on the downstream plasma parameters and heat flux deposition on divertor targets are studied based on a parameter scan of upstream electron density from 1.0×1019 to 3.5×1019 m−3. The downstream plasma on HL-2M exhibits a good performance easily accessible to high-recycling regime. A prior detachment regime is obtained at the divertor region with the upstream electron density higher than 2.5×1019 m−3, which results from the stronger momentum loss for the high density scenarios. The heat flux depositions at the outboard divertor targets are higher than that at the inboard one for the same upstream electron density. Further, the enhanced upstream electron density can lead to an effective suppression in the peak value of heat loads on divertor targets.

Vacuum ultraviolet-absorption spectroscopy and delocalized plasma-induced emission used for the species detection in a down-stream soft-etch plasma reactor

Vacuum ultraviolet-absorption spectroscopy (AS) and emission spectroscopy (ES) from delocalized probe plasma are implemented in the downstream chamber of a soft-etch industrial plasma reactor. A capacitively coupled plasma plasma, running in the upper compartment in He/NF3/NH3/H2 mixtures at about 1 Torr, produces reactive species which flow through a shower head into a downstream chamber, where they can etch different μ-electronic materials: Si, SiO2, SiN, etc. The ES reveals the presence of F atoms, while the dissociation rates of NF3 and NH3 are deduced from the AS, as well as the density of HF molecules, produced by chemical chain-reactions between dissociation products of NF3, NH3, and H2. The variations of HF density as a function of the NH3 flow rate suggest the possible formation of NH4F molecules in the plasma.

A Statistical Analysis of the Fluctuations in the Upstream and Downstream Plasmas of 109 Strong‐Compression Interplanetary Shocks at 1 AU

AbstractThe upstream and downstream plasmas of 109 strong‐compression forward interplanetary shocks are statistically analyzed using 3‐s measurements from the WIND spacecraft. The goal is a comparison of the fluctuation properties of downstream plasmas in comparison with the fluctuation properties of upstream plasmas in the inertial range of frequencies and the magnetic‐structure range of spatial scales. The shocks all have density compression rations of ~2 or more. When possible, each shock is categorized according to the type of solar wind plasma it propagates through: 15 shocks are in coronal‐hole‐origin plasma, 42 shocks are in streamer‐belt‐origin plasma, 36 shocks are in sector‐reversal‐region plasmas, and 11 shocks are in ejecta plasma. The statistical study examines magnetic field and velocity spectral indices, the Alfvénicity, the fluctuation amplitudes, Alfvén ratios, the degree of plasma inhomogeneity, and Taylor microscales, looking in particular at (1) fluctuation values downstream that are related to fluctuation values upstream and (2) systematic differences in fluctuation values associated with the type of plasma. It is argued that inhomogeneity of the downstream plasma can be caused by spatial variations in the shock normal angle θBn caused by field direction variations in the upstream magnetic structure. The importance of determining the type of plasma that the shock propagates through is established.

ICP argon discharge simulation: The role of ion inertia and additional RF bias

Inductively coupled plasma (ICP) argon discharge with additional RF bias on the electrode is studied numerically and compared with experimental data. The role of ion inertia is shown by comparing the two numerical approaches: drift-diffusion approximation and a separate equation for ion momentum in the model. Two different discharge geometries are studied: simple cylindrical geometry and two chambers geometry with a downstream plasma. The difference in the calculated plasma density is shown for lower pressure values. Two approaches give similar results at a pressure of 100 mTorr. The downstream ICP discharge with an additional 12 MHz bias was calculated for the range of voltage amplitude from 20 to 215 V. The ion energy distribution function at the RF-biased electrode is calculated by the kinetic model. The results are compared with experimentally measured plasma density and ion energy spectra in the two-chamber discharge geometry. A good agreement was obtained between the calculated and experimental data.

Control of ion-flux and ion-energy in direct inductively coupled plasma reactor for interfacial-mixing plasma-enhanced atomic layer deposition

The effects of low-energy (<15 eV) high-flux O2+ ion bombardment on the properties of Al2O3 films deposited on 3D nanostructures by plasma-enhanced atomic layer deposition (PE-ALD) were investigated. High-dose O2+ ion bombardment (>1017 cm−2 cycle−1) during the oxidation steps caused interfacial mixing, and AlSiOx films with abrupt interfaces were formed on Si surfaces. Interfacially mixed AlSiOx films were selectively formed on single-crystal Si, amorphous Si, and degraded SiO2 surfaces, whereas normal ALD Al2O3 films were formed on thermally grown SiO2 surfaces. At the same time, the interfacially mixed AlSiOx films were selectively formed on the horizontal top and bottom faces of the 3D nanostructures, whereas normal ALD Al2O3 films were formed on the vertical sidewalls. The morphology and thickness of the film deposited on the amorphous Si surface were the same as those on the single-crystal Si surface. The interfacially mixed AlSiOx film possessed rough surface morphology and a layered structure of Al-/Si-/Al-rich AlSiOx layers. The low-energy high-flux O2+ ion bombardment condition required for the interfacial-mixing ALD was realized in a direct inductively coupled plasma (ICP) reactor with a self-resonant planar coil, in which high-density plasma was excited near the substrate. The O2+ ion flux was found to be controllable over a wide range through variation in the O2 pressure. The ratio of O2+ ion flux at 0.01 Torr to that at 1 Torr was 289. The steep decrease of the ion flux with increasing pressure was attributed to the decrease of electron density in the upstream plasma for intensifying electron energy loss and the decrease of the ambipolar diffusion coefficient in the downstream plasma. A comparison of electron densities near the substrate and those at the presheath edge calculated from measured positive ion fluxes using the Bohm criterion revealed that negative ions, which significantly affect the positive ion flux, scarcely exist near the substrate. The interfacial-mixing PE-ALD has the potential to realize area-selective and topographically selective depositions, which are key technologies for fabricating next-generation electronic devices with 3D nanostructures. The direct ICP reactor is suitable for realizing selective deposition using the interfacial-mixing ALD.

Isotropic dry etching of Si selectively to Si0.7Ge0.3 for CMOS sub-10 nm applications

The fabrication of Si0.7Ge0.3 sub-10 nm nanochannels in gate-all-around devices requires a highly selective Si isotropic etching process. The etching of Si selectively to Si0.7Ge0.3 with CF4/N2/O2 downstream plasma has been investigated using various morphological and surface characterization techniques. Conditions such as 400 W microwave power, 700 mTorr pressure, 25 °C chuck temperature, and 22% CF4:22% N2:56% O2 feed gas mixture were found to be optimum for selectivity and etch rates. X-ray photoelectron spectroscopy showed that, during the etching process, a highly reactive 8 nm thick SiOxFy layer is formed on Si. Meanwhile, a 2 nm thick passivation layer is observed on SiGe. The latter is a mixture of SiOxFy and GeOxFy species that protected the alloy from etching. The process selectivity was improved by investigating different wet and dry oxidant treatments prior to etching. The dry oxidant treatment gives the best results in terms of selectivity. These results obtained on blanket wafers have been validated on pattern wafers. Scanning electron microscopy demonstrated that SiGe nanowires were fully released with a high selectivity after dry oxidation followed by the etching process.

Damage to porous SiCOH low-k dielectrics by O, N and F atoms at lowered temperatures

Information on the degradation of porous organosilicate glasses (OSGs) by active radicals and atoms is of high importance for their integration as low-k dielectrics in the next generation of ultra large scale integration (ULSI) production. The films’ degradation is caused by depletion of coverage of the pore surface methyl. OSG samples with differing porosities and pore sizes were treated by O, N, and F atoms at lowered temperatures (down to −45 °С) downstream of O2, N2, and SF6 inductively coupled plasma discharges, respectively. It has been shown that lowering the temperature reduces film degradation. In the case of O atoms, this reduction is insignificant, while the effect is much more noticeable for F atoms. In addition, an accumulation of F atoms forms a fluorocarbon layer during F atom treatment. The accumulated fluorine can interact with surface Si atoms, giving rise to film etching. The film degradation under O and F atoms increases with pore size and porosity, due to deeper atom penetration. In the case of N atoms, even a small temperature reduction essentially decreases OSG degradation, while –CH3 modification is less clear in the cases of O and F atoms. Density functional theory and ab initio molecular dynamics simulations of reaction mechanisms show that atom reactions with surface Si–CH3 groups are the initial stages of OSG degradation. Subsequent mechanisms include branched reaction pathways with the formation, modification, and destruction of different surface groups. In the cases of O and F atoms, these reactions lead to the formation of HxCО and CFx bonds. For the direct reaction of N(4S) atoms with –СН3 groups, significant activation energy is required. Under downstream plasma conditions, –СН3 groups are destroyed in reactions with metastable particles, and first of all, with metastable atoms N(2D), N(2P) and molecules , which are also produced inside the pores in the surface recombination of nitrogen atoms.

Influence of cold hollow cathode geometry on the radial characteristics of downstream magnetized plasma column

In this paper, the effect of cathode geometry on the radial characteristics of a downstream plasma column is presented for a cold cathode type of hollow cathode (HC) system. It is observed that when an axial magnetic field is applied to a cylindrical cathode, the downstream plasma exhibits an off-centered peak in the plasma density. However, as the magnetic field increases, the discharge extinguishes rapidly above a critical value, due to the suppression in secondary electron emission from the cylindrical cathode surface. On the other hand, by replacing the hollow cylinder with a cone-shaped cathode an oblique sheath is formed with respect to the axial magnetic field. This configuration helps in sustaining the discharge at twice the magnetic field than in the cylindrical case. It is also found that the downstream plasma exhibits a peak plasma density at the center for the case of the conical cathode. The above experimental observations for each HC setup has been qualitatively explained with the help of a phenomenological model.