Noble Gas Plasma Research Articles

Lithography-Free Nanofabrication on Various Semiconductors by the Codeposition Etching Technique.

Nano-/microstructures can be formed with the aid of small amounts of impurities during deposition with noble gas plasma irradiation, which is referred to as codeposition etching. This can be a new method for lithography-free semiconductor nanofabrication. Here, the codeposition etching method was employed with argon plasma and molybdenum (Mo) impurities on various semiconductors. Structures can be formed only on substrates that have a lower sputtering yield than the seed impurity. The density, area, and height of structures are related to both the impurity deposition rate and the substrate material. Moreover, two mechanisms of impurity nucleation are proposed according to time dependence results for the formation of the structures.

Investigation of the interaction of dense noble gas plasmas with cold cathodes: II Arc spot ignition on Au, Pd and Pt cathodes

AbstractInvestigations of the ignition of cathode spots by arcs operated in atmospheric pressure noble gas atmosphere on electrodes made of Al, Cu, Ti and graphite are extended to electrodes made of Au, Pd and Pt. Rods with a diameter of 2 mm are inserted in a UHV tight stainless steel vessel filled with Ar 5.6 or Kr 4.8. The negatively biased electrodes are brought into interaction with arc plasma by a magnetic blast field. Their end faces are mostly polished with diamond grinding powder, some Pd electrodes are additionally glowed, and other Pd electrodes are pasted with Pd powder. The arc spot ignition is observed by short time photography, streak camera records and temporally highly resolved optical spectroscopy for a lower and higher voltage applied between the arc plasma and the commutation electrode. Measurements of the commutation time tc elapsing between a signal generated by the arc plasma in front of the commutation electrode and the start of the commutation current show that at low voltages tc is in parts drastically increased by oxide layers. The measurements of tc for different electrode materials demonstrate that tc decreases with decreasing boiling temperature of the material. It confirms that not only ions of the filling gas but also of vaporized electrode material in front of the commutation electrode initiate and promote the arc spot ignition. SEM records show that different electrode treatments cause primarily different arc traces on the electrode surface.

Investigation of the interaction of dense noble gas plasmas with cold cathodes:III—Arc spot ignition on pure and doped W cathodes

AbstractThe study of the commutation of atmospheric pressure noble gas arcs on cold cathodes made of Al, Cu, Ti, graphite, Au, Pd, and Pt is extended to W cathodes. Rods with a diameter of 2 mm are inserted in a UHV tight stainless steel vessel filled with Ar 5.6 or Kr 4.8. The negatively biased electrodes are brought into interaction with arc plasma by a magnetic blast field. Their end faces are mostly polished with diamond grinding powder; some are electrolytic polished, others additionally covered with a thick oxide layer or pasted with W powder. Moreover, electrodes are investigated being doped with Al, K, and Si or doped with ThO2. The arc commutation is observed by short time photography, streak camera records, and temporally highly resolved optical spectroscopy for a lower and higher voltage applied between the arc plasma and the commutation electrode (CE). Varying the electrode properties revealed that basically ionized vapour of electrode material and less distinctly lowering of the work function by doping accelerate the arc commutation. It is initiated by plasma ions, which initially generate secondary electrons across the whole end face of the electrode. Since the electron emission varies locally, the multiplication of ions within the plasma layer by emitted electrons varies too. The positive feedback between both provokes a constriction of the commutation current and the power input into an arc spot. The electrode vapour promotes at first the development of an arc spot but finally quenches it by cooling the plasma layer in front of it.

Investigation of the interaction of dense noble gas plasmas with cold cathodes: I—Experimental setup and application to Al, Cu, Ti, and graphite cathodes

AbstractThe formation of arc spots on cold cathodes, called arcing, is an interference factor in all kinds of plasma technical devices. It is investigated by igniting an arc in a clean atmospheric pressure Ar or Kr filling of a stainless steel vessel. It is brought into interaction by a magnetic blast field with a negatively biased commutation electrode (CE), formed by a rod with a diameter of 2 mm consisting of pure Al, Cu, Ti, or graphite; its end face is polished with diamond grinding powder. The arc commutation is observed by short‐time photography, streak camera records, and temporally highly resolved optical spectroscopy for different applied voltages. The emission of ion lines of the filling gas and of vaporized electrode material by the plasma in front of the cathode is indicating the formation of a positive space charge layer in front of its surface. It provides the ions, which induce secondary electron emission of the cathode surface and with it, the arc commutation. The commutation time tc elapsing between a signal generated by the arc plasma in front of the CE and the beginning of a current flow through the electrode increases for low voltages with increasing permittivity of a surface layer formed by electrically insulating metal oxide and decreases with increasing electrical conductivity of the layer. It is lowest for graphite without an oxide layer. On scanning electron microscope pictures, taken of the end face of the electrode after arc commutation, craters with diameters of less than 1 μm are visible.

Physical vapor deposition using a coaxial ion acceleration method.

A novel physical vapor deposition method involving electromagnetic acceleration using a set of coaxial electrodes has been developed. In this study, the coaxial ion acceleration method is applied for a diamond-like carbon (DLC) thin film formation. In the developed method, the central electrode made of the deposition material is sputtered by the noble gas plasma current and accelerated toward the deposition chamber. Because the sputtered ions are accelerated by the Lorentz self-force, the ion injection energy can be controlled separately from the plasma temperature. In addition, the gaseous hydrocarbon, which is commonly used for DLC formation, is not required since a noble gas is used as the discharge gas.

(Invited) Electrochemical Characterization of the Interface between Atmospheric Noble Gas Plasma Jet and Aqueous Solution

Elucidation of general features of the interface between atmospheric pressure plasma jets and aqueous solutions would be beneficial to environmental, agricultural and biomedical applications. Such fundamental descriptions are critically important to design process chemistries and scale-up production. In this presentation, our recent efforts towards that end will be discussed. The focus of the talk will be on free jets of low temperature radiofrequency (RF) plasma generated using noble gas in contact with water, in contrast to the often used two-electrode configuration with e.g. micro-plasmas. Experiments will be presented in support of a perspective that views the plasma-liquid interface as a boundary condition that locally constrains the reduction potential, which can be controlled using parameters such as the plasma gas composition, applied RF power, and jet stand-off distance. A unique spatial distribution of the reduction potential that develops to maintain charge neutrality in the aqueous solution will be presented. The consequences for process design and scale-up will be expanded upon.

GAS PRODUCTS OF ARGON PLASMA INTERACTION WITH POLYARAMID AND POLY(ETHYLENE TEREPHTHALATE)

Non-equilibrium plasma is widely used for surface modification of polymer materials. Reactions of plasma active species with polymers lead not only to the surface modification, but to the formation of gaseous products, which change the plasma composition and internal plasma parameters. It results in the dependence of surface etching and modification kinetics on the quantity of material been treated (on sample sizes in a reactor). These phenomena are known as so called “loading effect” and have been studied for the treatment of polypropylene and polyimide films, poly(ethylene terephthalate) films and fabrics in oxygen and air plasma. It can be expected that gas products of noble gas plasma action on polymers will change strongly plasma parameters and modification results. In this paper, experimental data are represented on composition of gas products and their evolution rates at the treatment of polyaramid films, fibers and poly(ethylene terephthalate) fabric in low-pressure argon plasma. Poly(ethylene terephthalate) textile fabric (PET) made of monofilament yarns (SAATI, S.p.A., Italy) and polyaramide (PA) films and complex yarns “Rusar®” (Termotex, Russia) were used in experiments. Direct current discharge was excited in a flow of argon (technical grade) in a glass tube reactor with 3 cm inner diameter. Fabric samples were placed as cylinders on the reactor wall in the discharge positive column. Square of PET samples was varied from 18 to 111 cm2. The PA yarns with the total length of 550 or 1020 cm were placed in the reactor on special holder with the length of 20 cm. Total gas pressure in reactor was varied from 30 to 300 Pa at discharge current of 20 – 110 mA. Gas flow rate in the reactor was kept constant and equal to 30 cm´s-1. Gas phase analysis was carried out by the methods of plasma emission spectroscopy and mass-spectrometry. Plasma emission spectra and mass-spectra show evolution of H2, CO and H2O molecules at the treatment of PA and PET in argon plasma. Evolution rates for different gas products and their mole fractions have been obtained as functions of total gas pressure. Increase in polymer sample square has been shown to result in changing the ratios between evolution rates for different gas products. Sum of CO, H2 and H2O mole fractions increases with polymer sample square and decreases with gas pressure. Dissociation of molecular gas products results in the changing plasma active species. Oxygen and hydrogen atom lines and OH emission bands are observed in plasma emission spectra. Alteration of intensity ratio for О(3p3P ® 3s3S0) and Ar (4p3D3 ® 4s3P2) lines (IO/IAr) versus time after discharge starting shows the possible participation of oxygen atoms in heterogeneous reactions with polymers. Evolution of molecular gas products of heterogeneous reactions influences plasma properties and rates of plasma chemical reactions. The data obtained will be used for the further analysis of mechanisms of heterogeneous reactions of plasma active species with polymers.

Improved Ignition Efficiency in Method for Laser-induced Noble Gas Plasma Light Source

Improved Ignition Efficiency in Method for Laser-induced Noble Gas Plasma Light Source

Terahertz Wave Generation From Noble Gas Plasmas Induced by a Wavelength-Tunable Femtosecond Laser

We investigated the effects of pump wavelength on terahertz wave generation via two-color laser-induced plasma in various gas targets including nitrogen and noble gases. The terahertz energy was measured as functions of pump wavelength, input pulse energy, gas species, and pressure. The results suggest that the plasmas in the heavier gases induced by relatively longer wavelength lasers are more likely to generate higher energy terahertz waves. However, the existence of phase slippage in the neutral gases causes distinct fluctuations in the terahertz energy as the gas pressure increases. Meanwhile, the terahertz waves produced in the heavier gas are more likely to saturate at relatively higher pressures or incident optical powers. We propose an ionization rate formula associated with the gas species and pump wavelength to theoretically explain the validity of the experimental results. As a result, the terahertz pulse energy approaches a maximum value of 0.099 μJ per pulse in xenon with a 1500-nm excitation laser, and the terahertz wave to optical pulse energy conversion efficiency reaches 5.6 × 10–3, which is an order of magnitude higher than that of a conventional 800-nm pump laser.

Microwrinkle structures on refractory metal surfaces irradiated with noble gas plasma species

Microwrinkle structures with a pitch of less than 100 nm and up to 600 nm on refractory metals like tungsten (W) and molybdenum, which are irradiated by noble gas ions such as neon and helium, have been studied systematically. The wrinkle formation mechanism is thought to be a buckling of the hard surface layer supported by the soft elastic substrate, which is induced by a penetration of noble gas species from the irradiated surface. Microwrinkle forms on this structure under lateral compressive strain/stress fields coming from thermal constriction on the way to substrate cooling. Such a process should be anticipated when walls in a fusion reactor are attacked by heat pulses such as edge localized modes and/or vertical displacement events, and therefore might be an initial stage of W surface damage.

Contribution of electron-atom collisions to the plasma conductivity of noble gases.

We present an approach which allows the consistent treatment of bound states in the context of dc conductivity in dense partially ionized noble gas plasmas. Besides electron-ion and electron-electron collisions, further collision mechanisms owing to neutral constituents are taken into account. Especially at low temperatures of 10^{4}to10^{5}K, electron-atom collisions give a substantial contribution to the relevant correlation functions. We suggest an optical potential for the description of the electron-atom scattering which is applicable for all noble gases. The electron-atom momentum-transfer cross section is in agreement with experimental scattering data. In addition, the influence of the medium is analyzed, the optical potential is advanced including screening effects. The position of the Ramsauer minimum is influenced by the plasma. Alternative approaches for the electron-atom potential are discussed. Good agreement of calculated conductivity with experimental data for noble gas plasmas is obtained.

Comparison of induced damage, range, reflection, and sputtering yield between amorphous, bcc crystalline, and bubble-containing tungsten materials under hydrogen isotope and noble gas plasma irradiations

Binary-collision-approximation simulation of hydrogen isotope (i.e., hydrogen, deuterium, and tritium) and noble gas (i.e., helium, neon, and argon) injections into tungsten materials is performed. Three tungsten structures (i.e., amorphous, bcc crystalline, and helium bubble-containing structures) are prepared as target materials. Then, the trajectories of incident atoms, the distribution of recoil atoms, the penetration depth range of incident atoms, the sputtering yield, and the reflection rate are carefully investigated for these target materials.

Effect of feed-gas humidity on nitrogen atmospheric-pressure plasma jet for biological applications.

We investigate the effect of feed-gas humidity on the oxidative properties of an atmospheric-pressure plasma jet using nitrogen gas. Plasma jets operating at atmospheric pressure are finding uses in medical and biological settings for sterilization and other applications involving oxidative stress applied to organisms. Most jets use noble gases, but some researchers use less expensive nitrogen gas. The feed-gas water content (humidity) has been found to influence the performance of noble-gas plasma jets, but has not yet been systematically investigated for jets using nitrogen gas. Low-humidity and high-humidity feed gases were used in a nitrogen plasma jet, and the oxidation effect of the jet was measured quantitatively using a chemical dosimeter known as FBX (ferrous sulfate-benzoic acid-xylenol orange). The plasma jet using high humidity was found to have about ten times the oxidation effect of the low-humidity jet, as measured by comparison with the addition of measured amounts of hydrogen peroxide to the FBX dosimeter. Atmospheric-pressure plasma jets using nitrogen as a feed gas have a greater oxidizing effect with a high level of humidity added to the feed gas.

Plasma Behavior in a High‐Temperature Noble Gas Plasma Disk‐Shaped MHD Power Generator

SUMMARYR‐θ two‐dimensional numerical simulations have been carried out to clarify the plasma behavior in a high‐temperature noble gas plasma disk‐shaped magnetohydrodynamic (MHD) generator. At low inlet total temperature and high load resistance, the plasma has spiral structure which is similar to the nonuniform structure under the weak noble gas ionization condition in a seed‐plasma MHD generator. As seen in a linear‐shaped Faraday‐type MHD generator, the plasma becomes stable with increase in the inlet total temperature because the coulomb collision of electrons becomes dominant. Even at low inlet total temperature, the ionization instability can be suppressed for low load resistance, because the relatively low electron temperature due to less Joule heating makes the ionization relaxation time longer than plasma residential time.

Influence of dynamic screening on the scattering cross sections of the particles in the dense nonideal plasmas of noble gases

Within the dynamic screening potential model, elastic scattering processes between electrons and atoms in partially ionized plasmas were investigated using the method of phase functions. The phase shifts were calculated by solving the Calogero equation. Differential and total cross sections for the scattering of electrons on noble gas atoms were calculated and compared with experimental and other theoretical data. It was shown that the polarization potential is adequate for description the interaction between charged and neutral particles in partially ionized plasma. Analysis of the results showed that the phase shifts of electron atom scattering obtained with taking into account the dynamic screening are larger than the data obtained with consideration of static screening. The results can be used to calculate the various transport coefficients of the semiclassical dense plasma.

The hot hELicon eXperiment (HELIX) and the large experiment on instabilities and anisotropy (LEIA)

The West Virginia University Hot hELIcon eXperiment (HELIX) provides variable density and ion temperature plasmas, with controllable levels of thermal anisotropy, for space relevant laboratory experiments in the Large Experiment on Instabilities and Anisotropy (LEIA) as well as fundamental studies of helicon source physics in HELIX. Through auxiliary ion heating, the ion temperature anisotropy (T⊥/T∥) is variable from 1 to 20 for parallel plasma beta (β = 8πnkTi∥/B2) values that span the range of 0.0001 to 0.01 in LEIA. The ion velocity distribution function is measured throughout the discharge volume in steady-state and pulsed plasmas with laser induced fluorescence (LIF). The wavelengths of very short wavelength electrostatic fluctuations are measured with a coherent microwave scattering system. Operating at low neutral pressures triggers spontaneous formation of a current-free electric double layer. Ion acceleration through the double layer is detected through LIF. LIF-based velocity space tomography of the accelerated beam provides a two-dimensional mapping of the bulk and beam ion distribution functions. The driving frequency for the m = 1 helical antenna is continuously variable from 8.5 to 16 MHz and frequency dependent variations of the RF coupling to the plasma allow the spontaneously appearing double layers to be turned on and off without modifying the plasma collisionality or magnetic field geometry. Single and multi-species plasmas are created with argon, helium, nitrogen, krypton, and xenon. The noble gas plasmas have steep neutral density gradients, with ionization levels reaching 100% in the core of the plasma source. The large plasma density in the source enables the study of Aflvén waves in the HELIX device.

Interpretation of the gas flow field modification induced by guided streamer (‘plasma bullet’) propagation

Atmospheric-pressure non-equilibrium plasmas of noble gases in the form of ‘bullets’ have attracted considerable attention, against cold low-pressure or thermal atmospheric-pressure plasmas, for multidisciplinary scientific fields such as material science and biomedicine, due to their unique compatible features. A key factor for the efficiency of most of these systems is the interaction between the noble-gas channel, where the ‘bullets’ (streamers) propagate, and the plasma itself. It is the object of this paper to demonstrate this interaction and to provide the explanation on the gas flow field modification induced by the plasma ignition. A three-dimensional numerical model incorporating most of the governing equations, schlieren imaging and UV–visible high-resolution optical emission spectroscopy are applied. In accordance with the present results, the mechanism leading to the flow field alteration is clearly related to the electrohydrodynamic force, while it is demonstrated that the gas temperature plays a minor role.

Effects of He and Ar ion kinetic energies in protection of organosilicate glass from O2 plasma damage

In-situ x-ray photoelectron spectroscopy (XPS) and ex-situ Fourier transform infrared studies of He plasma and Ar+ ion bombardment pretreatments of organosilicate glass demonstrate that such pretreatments inhibit subsequent O2 plasma-induced carbon loss by forming a SiO2-like damaged overlayer, and that the degree of protection correlates directly with increased ion kinetic energies, but not with the thickness of the SiO2 overlayer. This thickness is observed by XPS to be roughly constant and <1 nm regardless of ion energies involved. The data indicate that ion kinetic energies are an important parameter in protective noble gas plasma pretreatments to inhibit O2 plasma-induced carbon loss.

Comparison of Damages on Tungsten Surface Exposed to Noble Gas Plasmas

Tungsten was exposed to pure Ar or Ne plasmas over 1550 K at several incident ion energies. Even under the irradiation condition that the tungsten nanostructure is formed by He plasma irradiation, holes/bubbles and fiberform nanostructures were not formed on the surface by exposure to Ar or Ne plasmas. In addition, the results from energy dispersive X-ray spectroscopy supported the facts that Ar and Ne did not remain in the sample. We will discuss the reason for the differences in the damage to the tungsten surface exposed to noble gas plasmas.

Electrical Conductivity of Noble Gas Plasmas in the Warm Dense Matter Regime

AbstractThe electrical conductivity of noble gas plasmas is investigated by using the relaxation‐time approximation in the density range 10–5–10 g cm–3 and temperature range 104–105 K. The electrical conductivity calculated shows reasonable consistency with shock wave experiments and theoretical simulations at above 2 × 104 K where the Coulomb interaction dominates. A nonmetal to metal transition in helium plasma is predicted at 2.4 g cm–3 and shown reasonable agreement with the most recent shock wave experiment (above 1.9 g cm–3). Furthermore, the insulator‐metal transition densities of all the noble gas plasmas are predicted and compared with available results (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)