Showing papers on "Argon published in 1995"

Gas flow in micro-channels

Abstract: An experimental and theoretical investigation of low Reynolds number, high subsonic Mach number, compressible gas flow in channels is presented. Nitrogen, helium, and argon gases were used. The channels were microfabricated on silicon wafers and were typically 100 μm wide, 104 μm long, and ranged in depth from 0.5 to 20 μm. The Knudsen number ranged from 10-3 to 0.4. The measured friction factor was in good agreement with theoretical predictions assuming isothermal, locally fully developed, first-order, slip flow.

Transport coefficients of air, argon-air, nitrogen-air, and oxygen-air plasmas

Abstract: Calculated values of the viscosity, thermal conductivity and electrical conductivity of air and mixtures of air and argon, air and nitrogen, and air and oxygen at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. The calculations, which assione local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30,000 K. The results for air plasmas are compared with those of published theoretical and experimental studies. Significant discrepancies are found with the other theoretical studies; these are attributed to differences in the collision integrals used in calculating the transport coefficients. A number of the collision integrals used here are significantly more accurate than values used previously, resulting in more reliable values of the transport coefficients.

Spatially averaged (global) model of time modulated high density argon plasmas

Abstract: The behavior of argon plasmas driven by time modulated power in ‘‘high density’’ plasma reactors is investigated using a global model. The time evolution of the electron temperature and the plasma density is calculated by solving the particle and energy balance equations. In the first stage of power application during the ‘‘on’’ time, the electron temperature rapidly increases above the steady state value. In this region, charged particles accumulate in the plasma due to the relatively higher power applied than for the continuous wave (cw) case. In the first stage of the ‘‘off’’ time, the electron temperature drops quickly, yielding a smaller particle loss (Bohm) velocity. These effects give rise to higher time‐average plasma densities than for the cw plasma driven by the same average power. The highest average plasma density obtained was more than twice the density of the cw plasma for a duty ratio of 25%. Even higher plasma densities were obtained for shorter duty ratios. The possibility of controlling chemical reactions in the plasma by changing the modulation period is also shown.

A model of the cathode region of atmospheric pressure arcs

Abstract: The paper deals with calculation of parameters in the near-cathode plasma layer, on the cathode surface and in the body of a cathode in high-pressure arc discharges. These parameters can be calculated independently of the arc column if the heat flux coming from the column to the edge of the near-cathode layer does not play a decisive role in the energy balance of the layer, which, according to the estimates presented, is a likely case. The physics of the near-cathode layer is reconsidered in view of major contradictions that have appeared in the literature recently, in particular with regard to the role of the near-cathode space charge sheath. A model of a near-cathode layer is developed that is based on a multifluid description of the plasma and takes into account multiply charged ions. The model is employed to calculate parameters of the layer as functions of the voltage drop in the layer and of the local value of the surface temperature. By means of these data, an approximate asymptotic theory of arc spots is extended to cathode spots in high-pressure plasmas. Calculated spot parameters are presented for the following combinations cathode/plasma: tungsten/argon, thoriated-tungsten/argon, thoriated-tungsten/nitrogen, and zirconium/nitrogen. The obtained results agree with the recent measurements of the spot temperature.

An Inductively Coupled Plasma Source for the Gaseous Electronics Conference RF Reference Cell.

Abstract: In order to extend the operating range of the GEC RF Reference Cell, we developed an inductively coupled plasma source that replaced the standard parallel-plate upper-electrode assembly. Voltage and current probes, Langmuir probes, and an 80 GHz interferometer provided information on plasmas formed in argon, chlorine, and nitrogen at pressures from 0.1 Pa to 3 Pa. For powers deposited in the plasma from 20 W to 300 W, the source produced peak electron densities between 1010/cm3 and 1012/cm3 and electron temperatures near 4 eV. The electron density peaked on axis with typical full-width at half maximum of 7 cm to 9 cm. Discharges in chlorine and nitrogen had bimodal operation that was clearly evident from optical emission intensity. A dim mode occurred at low power and a bright mode at high power. The transition between modes had hysteresis. After many hours of high-power operation, films formed on electrodes and walls of one Cell. These deposits affected the dim-to-bright mode transition, and also apparently caused generation of hot electrons and increased the plasma potential.

Dynamics of laser ablation plume penetration through low pressure background gases

Abstract: The dynamics of laser‐ablated yttrium plume propagation through background argon have been investigated with fast time‐ and spatially‐resolved plasma diagnostics in order to characterize a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). During expansion into low‐pressure background gases, the ion flux in the laser ablation plasma plume is observed to split into fast and slow components over a limited range of distances including those typically utilized for PLD. Optical absorption and emission spectroscopy are employed to simultaneously identify populations of both excited and ground states of Y and Y+. These are correlated with intensified‐CCD (ICCD) photographs of visible plume luminescence and ion fluxes recorded with fast ion probes. These measurements indicate that plume‐splitting in background gases is consistent with scattering of target constituents by ambient gas atoms. The momentum transfer from these collisions produces a transition from the initial,...

Nitrogen content of the mantle inferred from N 2 -Ar correlation in oceanic basalts

Abstract: RARE gases have proved to be particularly useful in modelling the early evolution of the Earth's atmosphere1–3. But it is not straightforward to extend this approach to the main volatile species (such as hydrogen, carbon and nitrogen) that comprise the atmosphere, hydrosphere and sediments, as these elements are chemically reactive and may have experienced different geodynamic histories. A way around this problem is to calibrate major volatile species relative to rare gases4–8. Here I use a recently developed static mass spectrometry method that allows simultaneous analysis of nitrogen, carbon, helium and argon9 to analyse gases trapped in vesicles of mid-ocean-ridge basalt glasses. The results show that the abundances of N2 and 40Ar (a radiogenic isotope that has been produced through geological time by the decay of 40K in the solid Earth) correlate well over several orders of magnitude, suggesting that the N2/40Ar ratio in the mantle source is near-constant and comparable to the present-day atmospheric value. In contrast, the inferred mantle N2/36Ar ratio (where 36Ar is a primordial isotope of argon) is two orders of magnitude higher than the atmospheric ratio. This observation, when combined with argon isotope systematics, allows a better estimate to be made of the nitrogen content of the mantle.

Modeling of metastable argon atoms in a direct current glow discharge

Abstract: To calculate the behavior of metastable argon atoms in a direct-current glow discharge, a balance equation is constructed, taking into account all known production and loss processes of the metastable atoms. Density profiles and fluxes of the metastable atoms are computed. The relative importance of different production and loss processes determining the metastable density is calculated for the case of a molybdenum cathode in pure argon. Besides electron-impact excitation, fast-ion and atom-impact excitation are found to be the dominant production processes at the high voltages used here, while loss of the metastable atoms is caused predominantly by diffusion and also by electron quenching to the nearby resonant states. The role of metastable atoms in the total discharge is investigated. They are found to play a minor role in the ionization of argon atoms, but their part in the ionization of sputtered molybdenum cathode atoms appears to be rather important. Moreover, they seem to have a significant effect on the secondary electron emission at the cathode. The investigation also includes the influence of pressure, voltage, and current on the metastable densities and fluxes, on the relative importance of the different production and loss processes, and on the role of metastable atoms in the entire discharge.

Investigation of the Air Separation Properties of Zeolites Types A, X and Y by Monte Carlo Simulations

Abstract: The air separation properties of zeolite types A, X, and Y have been studied using grand canonical Monte Carlo simulations of nitrogen, oxygen, and argon adsorbed in these zeolite lattices. Nitrogen is adsorbed preferentially due to the quadrupole-ion electrostatic interactions with the extra framework cations. The localization of adsorption sites for nitrogen near cations and the more diffuse distributions of oxygen and argon within zeolite cavities are clearly illustrated. Predicted nitrogen/oxygen selectivity for 5A from simulations is in good agreement with that determined experimentally. The effect of the calcium-sodium ion exchange on the predicted nitrogen/oxygen selectivity is examined, and is shown to be sensitive to the magnitude of the charges assigned to the extra framework cations.

Molecular Simulation of Fluid Adsorption in Buckytubes

Abstract: We report grand canonical Monte Carlo (GCMC) molecular simulation studies of simple fluid adsorption in buckytubes. Buckytubes are graphitic tubes with internal diameters of 1-5 nm and a regular pore structure. Adsorption isotherms and isosteric heats of adsorption are shown for argon and nitrogen in a microporous buckytube at 77 K, for argon and nitrogen in a mesoporous buckytube at 77 K, and for argon in a mesoporous buckytube at 55 K. The smaller buckytube shows type I adsorption behavior. Layering and hysteresis (type VI and IV adsorptions, respectively) are observed for the larger buckytube. A temperature of 77 K is shown to be below the critical temperature for capillary condensation and above the critical temperature for layering transitions for both nitrogen and argon adsorption. A temperature of 65 K is shown to be below the critical temperature for layering transitions for argon adsorption. Argon adsorption at 77 K in an open-ended mesoporous model buckytube is also presented. A comparison is made with argon adsorption at 77 K in a `semiinfinite` model buckytube of the same diameter. The open-ended model gives a narrower, more rounded hysteresis loop. 26 refs., 13 figs.

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

Abstract: Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

Carbon dimer, C2, as a growth species for diamond films from methane/hydrogen/argon microwave plasmas

Abstract: It was shown recently that nanocrystalline diamond films can be grown using fullerenes as precursors in an argon microwave plasma without the addition of hydrogen or oxygen. Extensive fragmentation of C60 in the microwave discharge leads to a copious production of the carbon dimer molecule, C2, as evidenced by intense Swan‐band emission. Here we have investigated hydrogen–methane–argon plasmas and found that high argon fractions (≳50%) lead to intense C2 emission, indicating significant production of C2 in the plasma. In situ measurements of the substrate reflectivity were used to determine the growth rate. A correlation between the C2 emission intensity and growth rate was observed. These results prompted us to propose a scheme for diamond film growth on the (100)–(2×1): H reconstructed diamond surface with C2 as the growth species. Each surface carbon atom (bonded twice to carbons in the bulk, once to a surface carbon, creating a ‘‘dimer’’ and then forming a five‐membered ring) is terminated with hydrog...

Residual stresses in nitride hard coatings prepared by magnetron sputtering and arc evaporation

Abstract: Nitride hard coatings such as TiN or (Ti,Al)N produced by magnetron sputtering and arc evaporation processes are characterized by large compressive stresses up to −15 GPa. In the case of magnetron sputtering the highest stress values can be observed at high bias voltages and low substrate temperatures. Coatings produced by arc evaporation, exhibit high compressive stresses even in the case of no bias voltage. The stresses are also influenced by substrate temperatures. In both cases the growing layer is bombarded with accelerated particles. However, there are differences in the kinds of particles. We assume that in the case of sputtering Ar ions and in the case of arc evaporation metal ions with high kinetic energy push nitrogen atoms from regular lattice sites into tetrahedral holes of the rock salt type lattice and occupy the previous nitrogen sites. During arc evaporation the formation of vacancies (Frenkel defects) is also possible. Both effects, the incorporation of argon or metal atoms on nitrogen sites and the nitrogen interstitials, produce high compressive residual stresses. During annealing interstitial nitrogen diffuses to inner boundaries or to the surface. High residual stresses are decreased.

Langmuir Probe Measurements in the Gaseous Electronics Conference RF Reference Cell.

Abstract: The use of a Langmuir probe system in two GEC cells is reviewed. The major problems associated with probe diagnostics in a GEC cell are outlined and discussed. While the data base is still insufficient to give definitive values for many parameters, a number of standard measurements are put forward. The plasma density in argon is 9×109 cm-3 (±20 %) at an applied rf voltage of 250 V (500 V peak to peak) and a gas pressure of 13.3 Pa (100 mTorr). The electron density scales linearly with applied voltage. The plasma to ground sheath resistance is shown to be very important with a value of 810 Ω in argon at a pressure of 13.3 Pa (100 mTorr) and discharge current of 0.1 A. The value of plasma to ground resistance scales inversely with discharge current and sublinear with pressure. Two standard features in the electron energy distribution function (EEDF) have been proposed as a test of the ability of a probe system to resolve features, first, the transition from a low temperature (<1 eV) bi-Maxwellian distribution to a Druyveysten distribution (3 eV) at 13.3 Pa (100 mTorr) in argon, and the "hole" in the EEDF at 2 eV to 4 eV in nitrogen plasmas.

Surface temperature measurements for tungsten-based cathodes of high-current free-burning arcs

Abstract: Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.

Simulations of trench‐filling profiles under ionized magnetron sputter metal deposition

Abstract: Numerical simulation results are presented for microscopic profile evolutions of deposited metal films in trench structures under ionized magnetron sputter deposition. The model used for the simulations takes account of the deposition of both ionized and neutral metal species and sputtering (i.e., etching) of the deposited film by the bombardment of metal and inert‐gas (such as argon) ions. The evolution of the surface topography is calculated numerically using the shock‐ tracking algorithm. Numerical results are also compared with experimental observations. A primary application of this metal deposition technique is interconnect metallization on semiconductors.

On the use of the line-to-continuum intensity ratio for determining the electron temperature in a high-pressure argon surface-microwave discharge

Abstract: This work presents an experimental study of argon microwave discharges maintained by a surface wave at 2.45 GHz, at atmospheric pressure and under low flow, in a capillary tube. Emission spectroscopy techniques are used to characterize these discharges. Special attention is paid to the determination of the electron temperature by using the line-to-continuum intensity ratio method. We show that, even in situations very close to local thermal equilibrium, simple substitution of the excitation temperature Texc by the electron temperature Te in the equation governing the line radiation emitted by a plasma should not be done. Causes for such an assertion are analysed and a parametric study is conducted to determine the sensitivity of the proposed method. This method is then applied to provide Te from experimental data corresponding to an atmospheric pressure argon plasma produced by a surface microwave.

Plasma Polymerization of Sputtered Poly(tetrafluoroethylene)

Abstract: Plasma polymerization of sputtered poly(tetrafluoroethy1ene) (F'TFE) using nonequilibrium helium, neon, argon, nitrogen, and hydrogen glow discharges has been followed by X-ray photoelectron spectroscopy ( X P S ) , infrared spectroscopy, and UV emission spectroscopy. The chemical character of the resultant fluoropolymer deposits is found to be strongly influenced by the type of carrier gas employed.

The argon-hydrogen expanding plasma : model and experiments

Abstract: An argon expanding cascaded arc plasma, with small amounts (0-10 vol.%) of hydrogen added to the flow, is investigated by means of Thomson-Rayleigh scattering and optical emission spectroscopy. The results, especially the electron density behaviour as a function of the distance from the onset of the expansion, are interpreted by comparison with results of a quasi one-dimensional model. The associative charge exchange reaction between Ar+ ions and H2 molecules plays a dominant role in the model. Assuming that H2 molecules from the wall enter the plasma in the shock region, the large ionization loss can be explained. Good agreement between model and experiment is found for the electron and neutral density and the electron temperature behaviour. This makes plausible the existence of a recirculation flow inside the vacuum vessel, which transports wall-associated hydrogen molecules towards the plasma.

Two‐dimensional argon metastable density measurements in a radio frequency plasma reactor by planar laser‐induced fluorescence imaging

Abstract: Two‐dimensional, relative measurements of the argon 1s5 metastable density distribution were obtained in a low‐pressure, 13.56 MHz, parallel‐plate, Gaseous Electronics Conference reference cell discharge using planar laser‐induced fluorescence imaging. For the conditions examined (pure argon, 75–300 V, 13.3–133.3 Pa), the measured density fields show significant radial and axial variations that depend more strongly on pressure than applied voltage. Generally speaking, the metastable density increases radially from the center to the edge of the discharge by ∼10%–30%. As the pressure is increased, the peak metastable density increases by ∼4 times and the axial distribution changes from a center‐peaked parabolic‐like profile to an asymmetric profile peaked near the powered electrode. Comparisons of centerline metastable and excited‐state emission profiles indicate that, while the metastable distribution is largely determined by the spatially dependent electron‐impact excitation function, variations in quench...

The Effects of Gas‐Phase Convection on Carbon Contamination of Czochralski‐Grown Silicon

Abstract: Measurements of carbon incorporated into a 2 in. diam Czochralski silicon crystal are compared with predictions computed using a three-step simulation : (i) the temperature field is computed throughout the melt and all soli regions of the furnace without heat transfer by gas convection ; (ii) the temperature and flow fields for the argon purge gas are computed with the temperatures at all solid surfaces set from the first step in the simulation ; (iii) the concentrations of dilute SiO and CO in the purge gas, as well as the concentration of carbon in the melt, are computed using the temperature and flow fields computed in step (ii). The concentration of carbon in the crystal is readily computed from the concentration of carbon in the melt. The predicted carbon concentrations are approximately a factor of 2 lower than the measured concentrations, with the discrepancy likely due to inaccurate thermodynamic and kinetic data. Additional simulations show that the concentration of carbon in the crystal increases with increasing gas pressure and decreases with increasing mass flux of gas through the furnace at constant pressure. Finally, convective cooling by the flowing gas is shown to be unimportant relative to radiation thereby justifying the decoupling used in the first step of the simulation.

The role of fast argon ions and atoms in the ionization of argon in a direct-current glow discharge: A mathematical simulation

Abstract: A model is developed for a direct‐current glow discharge in argon by a combination of a hybrid Monte Carlo fluid model of electrons and ions in the entire discharge and a Monte Carlo model of ions and fast atoms in the cathode dark space, in which fast ion and atom impact ionization are incorporated. The relative importance of these processes, compared to electron impact ionization is investigated, as a function of distance from the cathode and at different discharge conditions. It is found that they are dominant close to the cathode, and that they gain importance with increasing voltages. With the incorporation of these processes it was possible to predict current–voltage relations which are in excellent agreement with experiment. Also, the length of the cathode dark space, as a function of pressure and voltage, is calculated; the results agree with Aston’s empirical formula.

Local thermodynamic equilibrium in the cathode region of a free burning arc in argon

Abstract: Measurements of normalized plasma emission coefficients for the 696.55 nm Ar I line for free burning argon arcs at 1 atm are presented for various arc parameters. These coefficients show variations in the cathode region of the arc, depending upon the arc current, the cathode composition and the mode of operation of the arc, and suggest departures from local thermodynamic equilibrium in the plasma near the cathode tip. In this paper, these results are discussed and an explanation is proposed based on the overpopulation of the ground state level of the neutral atoms. This overpopulation is due to the injection of a large mass flow of cold gas into the arc core under the influence of pressure gradients resulting from the magnetic pinch force in the cathode region.

Hydrogen incorporation in boron-doped 6H-SiC CVD epilayers produced using site-competition epitaxy

Abstract: We report on the initial investigations of using site-competition epitaxy to control boron incorporation in chemical vapor deposition (CVD) 6H-SiC epilayers. Also reported herein is the detection of hydrogen in boron-doped CVD SiC epilayers and hydrogen-passivation of the boron-acceptors. Results from low temperature photoluminescence (LTPL) spectroscopy indicate that the hydrogen content increased as the capacitance-voltage (C-V) measured net hole concentration increased. Secondary ion mass spectrometry (SIMS) analysis revealed that the boron and the hydrogen incorporation both increased as the Si/ C ratio was sequentially decreased within the CVD reactor during epilayer growth. Epilayers that were annealed at 1700°C in argon no longer exhibited hydrogen-related LTPL lines, and subsequent SIMS analysis confirmed the outdiffusion of hydrogen from the boron-doped SiC epilayers. The C-V measured net hole concentration increased more than threefold as a result of thel700°C anneal, which is consistent with hydrogen passivation of the boron-acceptors. However, boron related LTPL lines were not observed before or after the 1700°C anneal.

Method for depositing a hard protective coating

Abstract: A method for depositing a hard protective coating on at least one part, wherein 10-70 % of nitrogen combined with 10-70 % of a mixture of helium and argon, or 10-70 % of neon for strongly exciting and dissociating the nitrogen and hydrogen, is added to a precursor gas, and the energy delivered to each atom of the precursor gas is selected so as to achieve so-called 'Lifschitz' conditions which define the energy range in which a maximum number of diamond-like tetrahedral structures are produced instead of graphite structures.