Noble Gas Plasma Research Articles

Are all atmospheric pressure cold plasma jets electrically driven?

Up to now, all studies on the dynamics behavior of non-equilibrium plasma plumes were focussed on noble gas plasma plumes. It was found out that they are electrically driven rather than gas flow dependent. Our study on the dynamics of a non-equilibrium N2 plasma plume reveals that the propagation velocity of the N2 plasma plume is several orders magnitude lower than those previously reported and further studies show that it is close to the gas flow velocity. The gas flow has a significant effect on the length of the plasma plume, and the results provide some fundamental knowledge about atmospheric pressure plasma jets.

Inverse bremsstrahlung heating rate in atomic clusters irradiated by femtosecond laser pulses

In the interaction of atomic clusters with femtosecond laser pulses, nanoplasmas with high density and high temperature are created. The heating is mainly determined by inverse bremsstrahlung (IB) due to electron-ion collisions. In many approaches for the calculation of the IB heating rate such as the Born approximation, large-angle scattering events are underestimated. However, rescattering events of an electron on the same atomic ion play an important role because they increase the amount of energy exchanged between the electrons and the laser field. In noble gas plasmas, the electron-ion interaction is often considered to take place between point-like particles. For typical noble gas clusters studied in experiments, one is advised to take into account not only the screening by the surrounding plasma medium but also the inner structure of the ions what can be accomplished by the use of appropriate model potentials. In the present paper, the IB heating rate is calculated from the classical simulation of individual electron trajectories. Results are presented for xenon clusters and argon clusters with different degree of ionization. Especially for higher energies, the consideration of the ionic structure increases the heating rate compared with the scattering on point-like particles. The Born approximation, however, overestimates this effect.

Enhancement of Laser-Induced Fluorescence by Intense Terahertz Pulses in Gases

The enhancement of laser-induced fluorescence by intense terahertz pulses was studied both theoretically and experimentally using selected gases. Semiclassical physical picture incorporating photoionization, electron heating, impact excitation, and dissociative recombination was used to explain the plasma dynamics under terahertz radiation in picosecond scale. The dependences of enhanced fluorescence on the terahertz field, laser intensity, and atomic properties were systematically investigated with neon, argon, krypton, xenon, methane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{CH}_{4}$</tex></formula> ), ethane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $\hbox{C}_{2}\hbox{H}_{6}$</tex></formula> ), propane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{C}_{3}\hbox{H}_{8}$ </tex></formula> ), and n-butane ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $\hbox{C}_{4}\hbox{H}_{10}$</tex></formula> ) gases. In noble gas plasma, both narrow line emission and broad continuum emission were found to be enhanced by the terahertz field. Their enhancement ratios and time-dependent enhanced fluorescence are greatly influenced by the scattering cross section and ionization potential of atoms. In alkane gas plasma, the enhancement of the fluorescence emission from various molecular fragments was also observed.

On the application of the theory of Lorentzian plasma to calculation of transport properties of multicomponent arc plasmas

The application of the Lorentzian plasma theory for the calculation of the properties of multicomponent electric arc plasma is considered. It is shown that this model gives satisfactory results in the temperature range, which is corresponding to the weakly ionized arc plasma and arc plasma with dominant first ionization. The calculations are provided for the arc plasmas of noble gases and their mixtures between them and also that with metals. It is also pointed that the discrepancy between the electron temperature and gas temperature can be significant even at relatively weak electric fields, that fact must be taken into account under the simulation of arc discharges.

Low-temperature synthesis of multiwalled carbon nanotubes by graphite antenna CVD in a hydrogen-free atmosphere

Multiwalled carbon nanotubes (MWCNTs) were synthesized at 390 °C in a hydrogen-free atmosphere by graphite antenna chemical vapor deposition, which provides carbon radicals by the etching of the antenna itself in a He- or Ar-based noble gas plasma. An increase in the number of defects in the graphite layers of the CNTs was observed with increasing hydrogen partial pressure. The results of X-ray photoelectron spectroscopy analysis suggested that these defects were caused by the transformation from an sp 2 to an sp 3 structure in the graphite layers of CNTs due to hydrogen radicals.

Carbon atom and cluster sputtering under low-energy noble gas plasma bombardment

Exit-angle resolved carbon atom and cluster (C2 and C3) sputtering yields are measured during different noble gas (Xe, Kr, Ar, Ne, and He) ion bombardments from a plasma, for low incident energies (75–225 eV). A quadrupole mass spectrometer (QMS) is used to detect the fraction of sputtered neutrals that is ionized in the plasma and to obtain the angular distribution by changing the angle between the target normal and the QMS aperture. A one-dimensional Monte Carlo code is used to simulate the interaction of the plasma and the sputtered particles in the region between the sample and the QMS. The effective elastic scattering cross sections of C, C2, and C3 with the different bombarding gas neutrals are obtained by varying the distance between the sample and the QMS and by performing a best fit of the simulation results to the experimental results. The total sputtering yield (C+C2+C3) for each bombarding gas is obtained from weight-loss measurements and the sputtering yield for C, C2, and C3 is then calculated from the integration of the measured angular distribution, taking into account the scattering and ionization of the sputtered particles between the sample and the QMS. We observe undercosine angular distributions of the sputtered atoms and clusters for all the studied bombarding gases and a clear decrease of the atom to cluster (C2 and C3) sputtering ratio as the incident ion mass increases, changing from a carbon atom preferential erosion for the lower incident ion masses (He, Ne, and Ar) to a cluster preferential erosion for the higher incident ion masses (Kr and Xe).

The dominant role of impurities in the composition of high pressure noble gas plasmas

We present in this letter how a molecular gas such as nitrogen at different levels of impurity dominates the ionic composition of an atmospheric pressure noble gas plasma such as in helium. The positive charge in the discharge is only determined by helium ions if the discharge gas contains less than 1ppm of impurity. Above this impurity level, the positive charge is completely determined by the impurity nitrogen. The higher the relative nitrogen concentration, the more N4+ dominates over N2+. If the impurity level is between 1 and about 20ppm, N2+ is clearly the most abundant positive ion but for higher levels of impurity, N4+ almost completely determines the positive charge.

Electrical conductivity of noble gases at high pressures

Theoretical results for the electrical conductivity of noble gas plasmas are presented in comparison with experiment. The composition is determined within a partially ionized plasma model. The conductivity is then calculated using linear response theory, in which the relevant scattering mechanisms of electrons from ions, electrons, and neutral species are taken into account. In particular, the Ramsauer-Townsend effect in electron-neutral scattering is discussed and the importance of a correct description of the Coulomb logarithm in electron scattering by charged particles is shown. A detailed comparison with recent experiments on argon and xenon plasmas is given and results for helium and neon are also revisited. Excellent agreement between theory and experiment is observed, showing considerable improvement upon previous calculations.

The Hall Effect in Dense Noble Gas Plasmas

AbstractIn recent years, noble gas plasmas in magnetic fields have been investigated. Experimental results for the Hall coefficient have been made available. We compare these with results calculated using linear response theory in the Zubarev formalism. In this work, we investigate the possibility of using Hall voltage measurements to determine the electron concentration of partially ionized plasmas. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Some transport characteristics in plasmas in the mixtures of xenon, argon and caesium

In the present paper, the xenon plasmas containing argon and caesium as additives and pertaining to the above-mentioned type (high-pressure, low-temperature) are studied, and a theoretical evaluation of their basic transport parameters are presented. The numerical calculations were based on the assumption that the system is kept under constant pressure and temperature and has attained local thermodynamical equilibrium (LTE). Particular attention is given to the evaluation of electron collision frequencies within the temperature interval from 2000 to 20000K, and for initial pressures ranging from 0.1 to 1.5 MPa in plasmas in the mixtures of xenon, argon and caesium. The equilibrium plasma composition was evaluated with the use of the modified Debye radius.It is found that the addition of as little as 1% of caesium profoundly alters the properties of the plasma. The presence of the added caesium in the thermal xenon and argon plasma is indeed quite significant for its transport properties, especially at temperatures below and around 10000K. Thus, electron number density, monotonically increasing with temperature (with pressure kept constant) much as in the pure noble-gas plasma, becomes considerably higher. The characteristics of the electrical and thermal conductivities are basically caused by the specificities of the processes of thermal ionization taking place in the system considered, as these processes determine the multiplier electron number density.

Role of trace impurities in large-volume noble gas atmospheric-pressure glow discharges

A computational study of capacitively coupled atmospheric-pressure glow discharges in high-purity helium is presented, and the role of trace impurities in such noble gas plasmas is established. Trace impurities result in generation of significant amounts of charged species through Penning ionization and charge exchange reactions. The altered charged species balance in the plasma causes a large change in the discharge impedance when compared to a completely pure noble gas discharge. Peak electron temperatures of the order of 50 000 K and electron densities of order 1011 cm−3 are predicted. Significant electron generation is observed inside the sheath region due to the highly collisional nature of the plasma. This study underscores the importance of modeling the effect of even trace amounts of impurities in noble gas atmospheric-pressure glow discharges.

Gas-phase and plasma-surface reactions in radiofrequency discharges of C 2H 2–N 2–noble gas mixtures

Actinometric optical emission spectrometry was used to determine the relative concentrations of the species H, CH, N 2 and CN as a function of the ratio of the nitrogen flow rate to the acetylene flow rate in acetylene–nitrogen–helium–argon mixtures. The forms of these concentration functions are discussed in terms of gas-phase molecular fragmentation and plasma/film-surface interactions. In plasmas fed only noble gases, additional experiments revealed the occurrence of plasma/film-surface reactions that produce significant plasma concentrations of the species CN provided that a nitrogenated carbon film was previously deposited in the chamber. In contrast, there is no appreciable concentration of the species CH in plasmas of noble gases in the presence of a-C:H:N films on the chamber walls.

Measurements of the negative ion density in SF6/Ar plasma using a plane electrostatic probe

A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.

Polymers and Cold Plasmas

In the last 15 years the use of plasma for materials processing has received a great amount of interest. The scope of possible applications is expanding rapidly and covers a large range of different fields. Applications may be found in microelectronics, food packaging, decorative and functional coatings; many different materials as metals, semiconductors, ceramics or polymers are involved by the plasma technology as bulk material, surface or interface. Plasma treatment is probably the most versatile surface treatment technique. Different types of gases such as argon, oxygen, nitrogen, fluorine, water etc. can produce the unique surface properties required by various applications. For example, O2 plasma treatment can increase the surface energy of polymers, whereas fluorine-containing plasma treatment can decrease the surface energy and improve the chemical inertness. Cross-linking at a polymer surface can be introduced by noble gas plasmas. Thin polymer films with unique chemical and physical properties are produced by plasma polymerisation. This technology is still in its infancy, and the plasma chemical process is not fully understood. The present article gives a short and non-exhaustive introduction on plasma polymer interaction and our activities in this field.

NANOCRYSTALLINE DIAMOND FILMS

▪ Abstract The synthesis of nanocrystalline diamond films from carbon-containing noble gas plasmas is described. The nanocrystallinity is the result of new growth and nucleation mechanisms, which involve the insertion of C2, carbon dimer, into carbon-carbon and carbon-hydrogen bonds, resulting in hetereogeneous nucleation rates on the order 1010cm−2s−1. Extensive characterization studies led to the conclusion that phase-pure diamond is produced with a microstructure consisting of randomly oriented 3–15-nm crystallites.By adjusting the noble gas/hydrogen ratio in the gas mixture, a continuous transition from micro- to nanocrystallinity is achieved. Up to 10% of the total carbon in the nanocrystalline films is located at 2 to 4 atom-wide grain boundaries. Because the grain boundary carbon is π-bonded, the mechanical, electrical, and optical properties of nanocrystalline diamond are profoundly altered.Nanocrystalline diamond films are unique new materials with applications in fields as diverse as tribology, cold cathodes, corrosion resistance, electrochemical electrodes, and conformal coatings on MEMS devices.

Plasma etch-induced conduction changes in gallium nitride

The effect of plasma-etching damage on carrier transport properties in GaN has been studied under various plasma conditions by monitoring the changes in sheet resistivity (ρ s) and mobility (μ s) or the resistivity (R). All the etching experiments were performed in an electron cyclotron resonance microwave plasma reactive ion etching (ECR-RIE) system. Consistent changes in the transport properties have been observed with increasing dc bias (ion energy) in all plasmas except in those containing chlorine. With noble gas plasmas, the largest change in conductance was created when Ar, the heaviest ion, was accelerated to its highest voltage. In these Ar sputtering cases, substantial surface micro-roughening has been observed. These surfaces also display considerable nitrogen deficiency as measured by Auger electron spectroscopy. These observations suggest that preferential sputtering of nitrogen from the surface of GaN is one form of ion damage. The other is displacement damage. Both of these forms of ion damage are considered to be the direct cause of the observed changes in the electrical properties.

Surface cleaning by plasma-enhanced desorption of contaminants (PEDC)

Low-pressure plasma processes were applied for cleaning the surface of glass, metals, carbon and polymers from contaminants such as air pollutants, fingerprints, coupling agents, oxide layers, weakly bonded surface layers, slip agents, light stabilizer or additive enrichments at the surface. It is possible to remove contaminations by simple plasma sputtering (particle bombardment) in plasmas of noble gases, by oxidation of organic contaminants with oxygen plasma or reduction of oxides or sulphides by hydrogen plasma. Examples were mentioned for purification of glass or glass fibre surface and the consequences for optical properties or adhesion in polymer–fibre composites. Degreasing of oil-contaminated sheet metals and removal of weakly bonded oxides were performed by plasma cleaning. The consequences of surface contamination of polymers in gluing and metallization are demonstrated.

On dense hydrogen and noble gases plasma spectra

In the optical properties of a plasma, nonideality manifests itself in the deformation of the energy spectrum of radiating particles. In particular, it can influence the character of light absorption (emission) and energy transfer both in dense plasma of astrophysical objects and terrestrial applications [1, 2]. The effects of nonideality on the absorption coefficients in a strongly coupled plasma (SCP) may be considered as a consequence of two main physical causes. The first is the strong action of quasistatic (F0 ≈ ni2/3) and dynamic (E0 ≈ ne1/2) electric fields on atoms and ions immersed in a dense plasma. We mean by this both individual and collective fields, including cases of suprathermal plasmas and ion-acoustic waves. The second source of nonideal effects is the strong Coulomb correlations between the free charges. In this paper some aspects of dense plasma light absorbtion are discussed, and an attempt is made to analyze the available and new experimental data for dense hydrogen and noble gas plasmas to clarify the density effects in opacity.

AFM measurements of the topography and the roughness of ECR plasma treated polypropylene

Polypropylene (PP) samples have been treated in an ECR-rf plasma with several gases and at different treatment times and rf-potentials. Modifications of the surface topography have been analyzed by AFM and the results were correlated with previous XPS measurements of the surface chemistry. Plasma treatments with reactive gases (N 2, O 2) lead to the incorporation of new chemical species in the PP surface, whereas plasma treatments with noble gases (He, Ar, Xe) induce a desorption of hydrogen and a graphitization. The untreated PP sample has a rough surface with a granular structure. Plasma treatments with reactive gases induce weak morphology changes, but no new defined structures. Moreover, the modifications of the surface roughness are very sensitive to the treatment conditions. Noble gas plasma treatments, on the contrary, create a completely new surface morphology, which consists of a network of chains of 40–100 nm in diameter oriented in a random way. The size and the shape of these structures are very sensitive to the nature of the gas and to the treatment conditions (ion energy and dose, total energy deposition).

Application of an acousto-optic spectrometer for plasma etching process quality control

The solution is proposed to the problem of the relative concentration dynamics control of chemically active particles in the low temperature gas plasma as well as the investigation of their generation and annihilation by means of time-resolved actinometry. A special small-size acousto-optic spectrometer has been constructed capable of spectral resolution of 0.1–0.3 nm in the visible range (430–850 nm) and the maximum time resolution of 2.0 μs. The software package developed allows: 1. (1) measuring of the chronograms of the relative concentrations of various active particles in parallel mode at eight spectral channels; 2. (2) generating impulse plasma modulation and measuring the microdynamics of the generation and annihilation of the active particles in the frame-by-frame mode (1024 points) in four parallel spectral channels, including the two corresponding background values. To shorten the measurement time and to improve the measurement accuracy of the particle annihilation kinetic characteristics in the interval between two stationary (long) pulses, it is proposed to form a series of probing (short) pulses. Some preliminary results are presented of the monitoring of the fluorine radicals (703.7 nm) relative concentration in the SF 6 gas r.f. plasma, containing as an actinometer a small amount (up to 5%) or argon (750.4 nm) at different discharge powers. Using the measurement results of the fluorine radicals, relative concentration in the pulse-modulated noble gas plasma obtained at monosilicon etching, the constants of the radical accumulation, as well as their life time values, are determined for the loaded and unloaded reaction chamber.