Downstream Plasma Research Articles

Performance limits of ion extraction systems with non-circular apertures.

A three-dimensional computer simulation is used to determine the perveance limitations of ion extraction systems with non-circular apertures. The objective of the study is to analyze the possibilities to improve mechanical strength of the ion optics made of carbon-carbon composite materials. Non-circular grid apertures are better suited to the physical structure of carbon-carbon composite materials, than conventionally used circular holes in a hexagonal pattern, because they allow a fewer number of cut fibers. However, the slit-type accelerating systems, usually regarded as the main alternative to the conventional ion optics, have an intolerably narrow range of operating perveance values at which there is no direct ion impingement on the acceleration grid. This paper presents results of comparative analysis of a number of different ion optical systems with non-circular apertures and conventional ion optical systems with circular apertures. It has been revealed that a relatively wide perveance range without direct ion impingement may be obtained with apertures shaped as a square with rounded corners. Numerical simulations show that this geometry may have equivalent perveance range as the traditional geometry with circular apertures while being more mechanically robust. In addition, such important characteristics, as the effective transparency for both the ions and the neutral atoms, the height of the potential barrier reflecting the downstream plasma electrons and the angular divergence of the beamlet also can be very close to these parameters for the optics with circular apertures.

Electrons under the dominant action of shock-electric fields

We consider a fast magnetosonic multifluid shock as a representation of the solar-wind termination shock. We assume the action of the transition happens in a three-step process: In the first step, the upstream supersonic solar-wind plasma is subject to a strong electric field that flashes up on a small distance scale $\Delta z\simeq U_1/ \Omega _{\mathrm e}$ (first part of the transition layer), where $\Omega_{\mathrm e}$ is the electron gyro-frequency and $U_1$ is the upstream speed. This electric field both decelerates the supersonic ion flow and accelerates the electrons up to high velocities. In this part of the transition region, the electric forces connected with the deceleration of the ion flow strongly dominate over the Lorentz forces. We, therefore, call this part the demagnetization region. In the second phase, Lorentz forces due to convected magnetic fields compete with the electric field, and the highly anisotropic and energetic electron distribution function is converted into a shell distribution with energetic shell electrons storing about 3/4 of the upstream ion kinetic energy. In the third phase, the plasma particles thermalize due to the relaxation of free energy by plasma instabilities. The first part of the transition region opens up a new thermodynamic degree of freedom never before taken into account for the electrons, since the electrons are usually considered to be enslaved to follow the behavior of the protons in all velocity moments like density, bulk velocity, and temperature. We show that electrons may be the downstream plasma fluid that dominates the downstream plasma pressure.

A plasma assisted in situ restoration process for sidewall damaged ULK dielectrics

This paper describes a novel in situ restoration process to repair damaged porous low-k materials by a plasma assisted approach. The main advantage is the enhanced repair efficiency due to the formation of small plasma activated multiple repairing fragments as well as the in situ handling to avoid a further k-value increase by water uptake. A liquid repair chemistry was evaporated and inserted into downstream microwave plasma to form activated species with repair character and to prevent the contact of porous low-k materials with UV irradiation and ion bombardment. In this study Octamethylcyclotetrasiloxane (OMCTS) and Bis(dimethylamino)dimethylsilane (DMADMS) were chosen for blanket samples with a k-value of 2.4. Furthermore OMCTS was investigated on patterned ULK trench structures of 62nm width. Besides the OMCTS flow rate, the addition of oxygen, methane or nitrogen was studied with regards to the formation of repair fragments and restoration efficiency.

Hydrogen Plasmas Processing of Graphene Surfaces

To assist the development of plasma processes to pattern graphene in a controlled way, interactions between hydrogen plasma species (H, H+, H2 +) and various types of graphene surfaces (monolayer, nanoribbons, multilayer) are investigated using atomic-scale simulations. It is shown that only “hot” H particles (i.e., with a kinetic energy greater than ~0.4 eV at 300 K) can adsorb on the basal plane of surface-clean graphene while adsorption is barrierless on free edges or vacancies. Surface reaction probabilities (reflection, adsorption, penetration) are found to strongly vary with the incident species energy, which allows to determine specific energy ranges (or process windows) for different types of H2 plasma treatment: lateral etching of graphene nanoribbons (GNRs), cleaning of graphene surfaces or vertical etching of multilayer graphene (MLG) stacks. Molecular dynamics simulations of GNRs trimming in downstream H2 plasmas allow to understand the mechanism which governs the anisotropic etching of ribbons and explains the absence of line-edge roughness on their edges. Interactions between low-energy (5–25 eV) H x + (x = 1, 2) ions with MLG are also investigated. Ion-induced damage (hydrogenation of successive graphene sheets, creation of vacancies) and etching of the MLG stack are found to vary with the ion energy, the ion fluence and the ion composition.

Decontamination of Streptococcus pyogenes and Escherichia coli from Solid Surfaces by Singlet and Triplet Atmospheric Pressure Plasma Jet Arrays

In this work, we studied the manipulation of bactericidal effects of non-thermal atmospheric pressure plasma jet on a gram-positive (Streptococcus pyogenes) and a gram-negative (Escherichia coli) bacteria. Decontamination of S. pyogenes and E. coli was assayed by measurement of the growth inhibition zones on solid surface. Singlet and triplet argon jets were employed to manipulate the bactericidal effect of the jet arrays. The distance effect of the contaminated plates to the plasma nozzle was analyzed, too. It was concluded that the jet distance to the sample could alter the state of downstream plasma jet and in turn changes the growth inhibition zone cross section, significantly. On the other hand, by scaling the inhibition zones, we showed that each plasma jet enabled to disinfect a surface with two orders of magnitude larger than that of the contact jet cross section in 150-s treatment. The results of this work could be used in developing atmospheric pressure plasma jet arrays for extended surface decontamination.

The electron distribution function downstream of the solar-wind termination shock: Where are the hot electrons?

In the majority of the literature on plasma shock waves, electrons play the role of "ghost particles," since their contribution to mass and momentum flows is negligible, and they have been treated as only taking care of the electric plasma neutrality. In some more recent papers, however, electrons play a new important role in the shock dynamics and thermodynamics, especially at the solar-wind termination shock. They react on the shock electric field in a very specific way, leading to suprathermal nonequilibrium distributions of the downstream electrons, which can be represented by a kappa distribution function. In this paper, we discuss why this anticipated hot electron population has not been seen by the plasma detectors of the Voyager spacecraft downstream of the solar-wind termination shock. We show that hot nonequilibrium electrons induce a strong negative electric charge-up of any spacecraft cruising through this downstream plasma environment. This charge reduces electron fluxes at the spacecraft detectors to nondetectable intensities. Furthermore, we show that the Debye length $\lambda _{\mathrm D}^{\kappa}$ grows to values of about $\lambda _{\mathrm D}^{\kappa}/\lambda _{\mathrm D}\simeq 10^{6}$ compared to the classical value $\lambda _{\mathrm D}$ in this hot-electron environment. This unusual condition allows for the propagation of a certain type of electrostatic plasma waves that, at very large wavelengths, allow us to determine the effective temperature of the suprathermal electrons directly by means of the phase velocity of these waves. At moderate wavelengths, the electron-acoustic dispersion relation leads to nonpropagating oscillations with the ion-plasma frequency $\omega _{\mathrm p}$ , instead of the traditional electron plasma frequency.

Localized electron heating and density peaking in downstream helicon plasma

Localized electron temperature and density peaking at different axial locations in the downstream helicon plasma have been observed in a linear helicon device with both geometrical and magnetic expansion. The discharge is produced with an right helical antenna powered by a RF source operating at 13.56 MHz. Axial wave field measurement shows the presence of damped helicon waves with standing wave character folded into it even at low densities ( m). The measured helicon wavelength is just about twice the antenna length and the phase velocity is almost the speed required for electron impact ionization. These experimental observations strongly advocate the Landau damping heating and density production by the helicon waves, particularly in low density plasma such as ours. The electron temperature maximizes at 35–45 cm away from the antenna center in our experiments indicating a local source of heating at those locations. Different mechanisms responsible for this additional heating at a particular spatial location have been discussed for their possible roles. Further downstream from the location of the maximum electron temperature, a density peak located 55–65 cm away from the antenna is observed. This downstream density peaking can be explained through pressure balance in the system.

Anisotropic In-Plane Conductivity and Dichroic Gold Plasmon Resonance in Plasma-Assisted ITO Thin Films e-Beam-Evaporated at Oblique Angles.

ITO thin films have been prepared by electron beam evaporation at oblique angles (OA), directly and while assisting their growth with a downstream plasma. The films microstructure, characterized by scanning electron microscopy, atomic force microscopy, and glancing incidence small-angle X-ray scattering, consisted of tilted and separated nanostructures. In the plasma assisted films, the tilting angle decreased and the nanocolumns became associated in the form of bundles along the direction perpendicular to the flux of evaporated material. The annealed films presented different in-depth and sheet resistivity as confirmed by scanning conductivity measurements taken for the individual nanocolumns. In addition, for the plasma-assisted thin films, two different sheet resistance values were determined by measuring along the nanocolumn bundles or the perpendicular to it. This in-plane anisotropy induces the electrochemical deposition of elongated gold nanostructures. The obtained Au-ITO composite thin films were characterized by anisotropic plasmon resonance absorption and a dichroic behavior when examined with linearly polarized light.

ON COSMIC-RAY PRODUCTION EFFICIENCY AT SU0PERNOVA REMNANT SHOCKS PROPAGATING INTO REALISTIC DIFFUSE INTERSTELLAR MEDIUM

Using three-dimensional magnetohydrodynamics simulations, we show that the efficiency of cosmic-ray (CR) production at supernova remnants (SNRs) is over-predicted if it could be estimated based on proper motion measurements of H$\alpha$ filaments in combination with shock-jump conditions. Density fluctuations of upstream medium make shock waves rippled and oblique almost everywhere. The kinetic energy of the shock wave is transferred into that of downstream turbulence as well as thermal energy which is related to the shock velocity component normal to the shock surface. Our synthetic observation shows that the CR acceleration efficiency as estimated from a lower downstream plasma temperature, is overestimated by 10-40%, because rippled shock does not immediately dissipate all upstream kinetic energy.

Entropy generation at multi-fluid magnetohydrodynamic shocks with emphasis to the solar wind termination shock

In a series of earlier papers, we developed expressions for ion and electron velocity distribution functions and their velocity moments at the passage over the solar wind termination shock. As we have shown there, with the introduction of appropriate particle invariants and the use of Liouville’s theorem one can get explicit solutions for the resulting total downstream pressure by adding up from partial pressure contributions of solar wind protons, solar wind electrons and pick-up protons. These expressions are the first step toward delivering the main contributions to the total plasma pressure in the downstream plasma flow and consistently determine the shock compression ratio. Here we start from these individual fluid pressures downstream of the shock and thereafter evaluate for the first time the shock-induced entropy production of the different fluids, when they are passing over the shock to the downstream side. As shown here, the resulting ion entropy production substantially deviates from earlier calculations using a pseudo-polytropic reaction of the ions to the shock compression, with polytropies selected to describe fluid-specific reactions at the shock passage similar to those seen by the Voyagers. From these latter models, ion entropy jumps are derived that depend on the pick-up ion abundance, while our calculations deliver an abundance-independent ion entropy production that only depends on the shock compression ratio and the tilt angle between the upstream magnetic field and the normal to the shock surface. We also show here that the thermodynamically permitted upper limit in the entropy production is only reached when strongly heated electrons are included in the entropy balance.

Influence of Plasma Chamber Set‐Up on the Surface Modification of Non‐Vulcanized and Pure SBR Rubber Treated at Radio‐Frequencies Air Plasma

Non‐vulcanized styrene‐butadiene rubber thin films were exposed to Radio‐Frequency low‐pressure air plasma. Two different configurations corresponding to direct and downstream RF plasmas were compared. Optical emission spectroscopy provided information about plasma composition and temperature. The plasma treated SBR surfaces were characterized by contact angles and XPS spectroscopy. Two different phenomenological mechanisms are proposed to describe the observed differences according to the plasma configuration used and account for the impact of the activated gas phase on the structure of plasma‐treated non‐vulcanized SBR surface..

Etching mechanisms of graphene nanoribbons in downstream H2 plasmas: insights from molecular dynamics simulations

Lateral etching mechanisms of graphene nanoribbons (GNRs) with zigzag (ZZ) edges in downstream H2 plasmas are investigated using molecular dynamics simulations. A new etching mechanism is found, which occurs in three consecutive phases and requires a continuous exposure of GNRs to H atoms and high substrate temperatures (~800 K). Full hydrogenation of GNR free edges during phase 1 reduces the potential barriers to H chemisorption on near-edge C atoms from the basal plane. Subsequent hydrogenation of near-edge C–C dimers creates mechanical stress between C atoms (due to local sp2-to-sp3 rehybridizations) which leads to the rupture of C–C dimers bonds, unzipping locally the 1st and 2nd edge carbon rows. The unzipping then propagates randomly along the GNR edges and creates suspended linear carbon chains (phase 2). Weakened by their exposure to continuous H bombardment and strong thermal vibrations, the suspended carbon chains may then rupture, leading to the sputtering of their carbon atoms as single C atoms or C2 molecules (phase 3). Thus no formation of volatile hydrocarbon etching products is observed in this three-phase mechanism, which explains why the ribbon edges can be sharp-cut without generation of line-edge roughness, as also observed experimentally. Influence of substrate temperature on ZZ-GNRs etching is investigated and suggests the dominant mechanisms for understanding the temperature dependence of the etch rate observed experimentally (peaks at 800 K and decreases for lower or higher temperatures).

Spectroscopic ellipsometry on Si/SiO2/graphene tri-layer system exposed to downstream hydrogen plasma: Effects of hydrogenation and chemical sputtering

In this work, the optical response of graphene to hydrogen plasma treatment is investigated with spectroscopic ellipsometry measurements. Although the electronic transport properties and Raman spectrum of graphene change after plasma hydrogenation, ellipsometric parameters of the Si/SiO2/graphene tri-layer system do not change. This is attributed to plasma hydrogenated graphene still being electrically conductive, since the light absorption of conducting 2D materials does not depend on the electronic band structure. A change in the light transmission can only be observed when higher energy hydrogen ions (30 eV) are employed, which chemically sputter the graphene layer. An optical contrast is still apparent after sputtering due to the remaining traces of graphene and hydrocarbons on the surface. In brief, plasma treatment does not change the light transmission of graphene; and when it does, this is actually due to plasma damage rather than plasma hydrogenation.

Surface treatment of glass and poly(dimethylsiloxane) using atmospheric-pressure plasma jet and analysis of discharge characteristics

An atmospheric-pressure plasma jet source based on dielectric barrier discharge (DBD) was prepared for the hydrophilic surface treatment of poly(dimethylsiloxane) (PDMS). Discharge characteristics were investigated from intensified charge coupled device (ICCD) images and by optical emission spectroscopy (OES). Different optical emission profiles were observed in plasmas with argon (Ar) and helium (He) feed gases. For Ar plasma, OH and N2 related emission lines are dominant. On the other hand, relatively strong peaks of N2+ and O excited species were observed in He plasma, suggesting that a metastable He atom plays an important role in the downstream plasma region. Ar plasma was further found to be more effective than He plasma for hydrophilic surface modification. The surface reverted to being hydrophobic about 40 h after plasma irradiation.

The effect of voltage waveform and tube diameter on transporting cold plasma strings through a flexible dielectric tube

In this work, we developed transporting atmospheric pressure cold plasma using single electrode configuration through a sub-millimetre flexible dielectric tube beyond 100 cm. It was shown that the waveform of the applied high voltage is essential for controlling upstream and downstream plasma inside the tube. In this regard, sawtooth waveform enabled the transport of plasma with less applied high voltage compared to sinusoidal and pulsed form voltages. A cold plasma string as long as 130 cm was obtained by only 4 kV peak-to-peak sawtooth high voltage waveform. Optical emission spectroscopy revealed that reactive chemical species, such as atomic oxygen and hydroxyl, are generated at the tube exit. The effect of tube diameter on the transported plasma was also examined: the smaller the diameter, the higher the applied voltage. The device is likely to be used for sterilization, decontamination, and therapeutic endoscopy as already suggested by other groups in recent past years.

Single-step plasma synthesis of carbon-coated silicon nanoparticles.

We have developed a novel single-step technique based on nonthermal, radio frequency (rf) plasmas to synthesize sub-10 nm, core-shell, carbon-coated crystalline Si (c-Si) nanoparticles (NPs) for potential application in Li(+) batteries and as fluorescent markers. Hydrogen-terminated c-Si NPs nucleate and grow in a SiH4-containing, low-temperature plasma in the upstream section of a tubular quartz reactor. The c-Si NPs are then transported downstream by gas flow, and are coated with amorphous carbon (a-C) in a second C2H2-containing plasma. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and in situ attenuated total reflection Fourier transform infrared spectroscopy show that a thin, < 1 nm, 3C-SiC layer forms at the c-Si/a-C interface. By varying the downstream C2H2 plasma rf power, we can alter the nature of the a-C coating as well as the thickness of the interfacial 3C-SiC layer. The transmission electron microscopy (TEM) analysis is in agreement with the Si NP core size determined by Raman spectroscopy, photoluminescence spectroscopy, and XRD analysis. The size of the c-Si NP core, and the corresponding light emission from these NPs, was directly controlled by varying the thickness of the interfacial 3C-SiC layer. This size tunable emission thus also demonstrates the versatility of this technique for synthesizing c-Si NPs for potential applications in light emitting diodes, biological markers, and nanocrystal inks.

Effect of Downstream Plasma Treatment on Dissolution of Fluorocarbon Polymer in Organic Solvents

In back-end of line (BEOL), the fluorinated polymer deposited on dielectric sidewalls during patterning [1,2] must be removed prior to subsequent processing steps to achieve good adhesion and coverage of materials (metals) deposited in the etched features. However, it is known that this type of fluorocarbon polymer is chemically inert to many existing wet clean solutions, including aqueous solutions such as fluoride ion-containing or highly alkaline solutions, and solvent mixtures. Exposure of the polymer to UV irradiation of selective wavelengths and doses significantly modifies the polymer film, which results in substantial removal ability in a subsequent wet clean process [2-3].

Downstream plasma transport and metal ionization in a high-powered pulsed-plasma magnetron

Downstream plasma transport and ionization processes in a high-powered pulsed-plasma magnetron were studied. The temporal evolution and spatial distribution of electron density (ne) and temperature (Te) were characterized with a 3D scanning triple Langmuir probe. Plasma expanded from the racetrack region into the downstream region, where a high ne peak was formed some time into the pulse-off period. The expansion speed and directionality towards the substrate increased with a stronger magnetic field (B), largely as a consequence of a larger potential drop in the bulk plasma region during a relatively slower sheath formation. The fraction of Cu ions in the deposition flux was measured on the substrate using a gridded energy analyzer. It increased with higher pulse voltage. With increased B field from 200 to 800 Gauss above racetrack, ne increased but the Cu ion fraction decreased from 42% to 16%. A comprehensive model was built, including the diffusion of as-sputtered Cu flux, the Cu ionization in the entire plasma region using the mapped ne and Te data, and ion extraction efficiency based on the measured plasma potential (Vp) distribution. The calculations matched the measurements and indicated the main causes of lower Cu ion fractions in stronger B fields to be the lower Te and inefficient ion extraction in a larger pre-sheath potential.

Insights into platelet-based control of coagulation

The coagulation process is activated by tight control mechanisms, in which platelets play prominent and unique roles. In thrombosis and hemostasis, activated platelets regulate the coagulation system in various ways: by exposing a phosphatidylserine surface for thrombin formation, by supporting fibrin formation, and by regulating the retraction of a fibrin clot. In this review we discuss the involvement of platelet receptors, other membrane proteins, downstream signaling proteins, cytoskeleton-linked proteins and plasma proteins in these procoagulant functions. Studies with both genetically modified mice and pharmacological inhibitors indicate that, for collagen-adhered platelets, in part common signaling pathways lead to phosphatidylserine exposure, generation of thrombin and fibrin, and retraction of the fibrin clot. However, prolonged Ca(2+) elevation leads to thrombin generation, whereas integrin-dependent signaling stimulates fibrin clot retraction. Contact-dependent signaling pathways, triggered by homotypic platelet-platelet interactions, act in particular via the integrin route.

On the maximum energy of shock-accelerated cosmic rays at ultra-relativistic shocks

The maximum energy to which cosmic rays can be accelerated at weakly magnetised ultra-relativistic shocks is investigated. We demonstrate that for such shocks, in which the scattering of energetic particles is mediated exclusively by ion skin-depth scale structures, as might be expected for a Weibel-mediated shock, there is an intrinsic limit on the maximum energy to which particles can be accelerated. This maximum energy is determined from the requirement that particles must be isotropized in the downstream plasma frame before the mean field transports them far downstream, and falls considerably short of what is required to produce ultra-high-energy cosmic rays. To circumvent this limit, a highly disorganized field is required on larger scales. The growth of cosmic ray-induced instabilities on wavelengths much longer than the ion-plasma skin depth, both upstream and downstream of the shock, is considered. While these instabilities may play an important role in magnetic field amplification at relativistic shocks, on scales comparable to the gyroradius of the most energetic particles, the calculated growth rates have insufficient time to modify the scattering. Since strong modification is a necessary condition for particles in the downstream region to re-cross the shock, in the absence of an alternative scattering mechanism, these results imply that acceleration to higher energies is ruled out. If weakly magnetized ultra-relativistic shocks are disfavoured as high-energy particle accelerators in general, the search for potential sources of ultra-high-energy cosmic rays can be narrowed.