Showing papers in "Plasma Sources Science and Technology in 2007"

Efficient models for photoionization produced by non-thermal gas discharges in air based on radiative transfer and the Helmholtz equations

Abstract: This paper presents formulation of computationally efficient models of photoionization produced by non-thermal gas discharges in air based on three-group Eddington and improved Eddington (SP3) approximations to the radiative transfer equation, and on effective representation of the classic integral model for photoionization in air developed by Zheleznyak et al (1982) by a set of three Helmholtz differential equations. The reported formulations represent extensions of ideas advanced recently by S´ egur et al (2006) and Luque et al (2007), and allow fast and accurate solution of photoionization problems at different air pressures for the range 0.1

Oxygen discharges diluted with argon: dissociation processes

Abstract: We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O2/Ar discharge in the pressure range 1?100?mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Arm by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O? is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O2(a?1?g) are the dominating channels for creation of the negative oxygen ion O?.

A review of recent laboratory double layer experiments

Abstract: Recent developments in laboratory double layers from the late 1980s to the spring of 2007 are reviewed. The paper begins by a lead up to electric double layers in the laboratory. Then an overview of the main double layer devices and properties is presented with an emphasis on current-free double layers. Some of the double layer models and simulations are analysed before giving a more complete description of current-free double layers in radiofrequency plasmas expanding in a diverging magnetic field. Astrophysics double layers are briefly reported. Finally, applications of double layers to the field of plasma processing and electric propulsion are discussed.

Red sprite discharges in the atmosphere at high altitude: the molecular physics and the similarity with laboratory discharges

Abstract: An overview of the general phenomenology and physical mechanism of large-scale electrical discharges termed 'sprites' observed at high altitude in the Earth's atmosphere above thunderstorms is presented. The primary emphasis is placed on summarizing available experimental data on various emissions documented to date from sprites and interpretation of these emissions in the context of similar data obtained from laboratory discharges, in particular the pulsed corona discharges, which are believed to be the closest pressure-scaled laboratory analogue of sprite discharges at high altitude. We also review some of the recent results on modelling of laboratory and sprite streamers emphasizing the importance of the photoionization effects for the understanding of the observed morphological features of streamers at different pressures in air and provide a comparison of emissions obtained from streamer models with results of recent satellite-based observations of sprites.

Pulsed RF discharges, glow and filamentary mode at atmospheric pressure in argon

Abstract: The properties of a pulsed radio frequency capacitive discharge are investigated at atmospheric pressure in argon. The discharge can operate in two different modes: a homogeneous glow discharge or turn into filaments. By pulsing the 13.56 MHz generator both the filamentary and the glow modes can be selected depending on the pulse width and period. For a 5 µs pulse width (∼70 RF cycles in the pulse), short pulse periods (less than 100 µs) result in a filamentary discharge while long pulse periods (greater than 1 ms) result in a glow discharge. Optical emission spectroscopy and power measurements were performed to estimate the plasma temperature and density. Water vapour was introduced to the discharge as a source of hydrogen and the Stark broadening of the Balmer Hβ line was measured to allow the plasma density to be estimated as 10 15 cm −3 in the filamentary mode. The estimation of the glow mode density was based on power balance and yielded a density of 5 × 10 11 cm −3 . Emission line ratio measurements coupled with the Saha equation resulted in an estimate of electron temperature of approximatively 1.3 eV for the glow mode and 1.7 eV for the filaments. Using the glow mode at a duty cycle of 10% is effective in decreasing the hydrophobicity of polymer films while keeping the temperature low. (Some figures in this article are in colour only in the electronic version)

Plasma-induced ignition and plasma-assisted combustion in high-speed flow

Abstract: The suitability of the electrical discharge technique for application in plasma-induced ignition and plasma-assisted combustion in high-speed flow is reviewed Nonequilibrium, unsteady and nonuniform modes are under analysis to demonstrate the advantage of such a technique over heating A reduction in the required power deposition is possible due to unsteady operation and non-homogeneous spatial distribution Mixing intensification in non-premixed flow could be achieved by nonuniform electrical discharges The scheme of fuel ignition behind the wallstep and in the cavity is under consideration Experimental results on multi-electrode discharge maintenance in the separation zone of supersonic flow are presented The model test on hydrogen and ethylene ignition is demonstrated at direct fuel injection An energetic threshold of fuel ignition under separation and in the shear layer is measured under the experimental conditions

Plasma silane concentration as a determining factor for the transition from amorphous to microcrystalline silicon in SiH4/H2 discharges

Abstract: In this work, the microstructure transition from amorphous to microcrystalline silicon is defined in terms of the silane concentration in the plasma as opposed to the silane concentration in the input gas flow. In situ Fourier transform infrared absorption spectroscopy combined with ex situ Raman spectroscopy has been used to calibrate and validate this approach. Results show that a relevant parameter to obtain mu c-Si : H from SiH4/H-2 mixtures is the plasma composition, which is determined not only by the gas dilution ratio but also by the silane depletion fraction. It is also shown that mu c-Si : H can only be deposited efficiently, in terms of gas utilization, at a high rate by using high input concentration and depletion of silane.

Kinetic phenomena in charged particle transport in gases, swarm parameters and cross section data

Abstract: In this review we discuss the current status of the physics of charged particle swarms, mainly electrons. The whole field is analysed mainly through its relationship to plasma modelling and illustrated by some recent examples developed mainly by our group. The measurements of the swarm coefficients and the availability of the data are briefly discussed. More time is devoted to the development of complete electron?molecule cross section sets along with recent examples such as NO, CF4 and HBr. We extend the discussion to the availability of ion and fast neutral data and how swarm experiments may serve to provide new data. As a point where new insight into the kinetics of charge particle transport is provided, the role of kinetic phenomena is discussed and recent examples are listed. We focus here on giving two examples on how non-conservative processes make dramatic effects in transport, the negative absolute mobility and the negative differential conductivity for positrons in argon. Finally we discuss the applicability of swarm data in plasma modelling and the relationship to other fields where swarm experiments and analysis make significant contributions.

Decomposition of toluene in a gliding arc discharge plasma reactor

Abstract: The decomposition of toluene in a gliding arc discharge (glidarc) was performed and studied. Experimental results indicate that the glidarc technology can effectively decompose toluene molecules and has bright prospects of being applied as an alternative tool to decompose volatile organic compounds. It is found that a change in the electrode material had an insignificant effect on the toluene removal efficiency. The toluene removal efficiency increases with increasing inlet gas temperature. The water vapor present in the gas mixture has a favorable effect on the toluene decomposition in the plasma. The energy efficiency is 29.46 g (kWh−1) at a relative humidity of 50% and a specific energy input of 0.26 kWh m−3, which is higher than other types of non-thermal plasmas. Too much or too little oxygen content does not favor toluene decomposition. The major gas phase products detected by FT-IR from the decomposition of toluene with air participation were CO, CO2, H2O and NO2. Some brown depositions were found on the surface of the electrodes, which were polar oxygenous and nitrogenous compounds determined by the GC-MS analysis, such as benzaldehyde, benzoic acid, quinine and nitrophenol from the reaction of toluene with radicals. A possible mechanism for toluene destruction via glidarc technology is proposed and summarized.

Investigations of GMAW plasma by optical emission spectroscopy

Abstract: We report on investigations of gas metal arc welding plasma operated in pure argon and in a mixture of argon and CO2 at a dc current of 326?A. The spatially resolved electron densities and temperatures were directly obtained by measuring the Stark widths of the Ar?I 695.5?nm and Fe?I 538.3?nm spectral lines.Our experimental results show a reduction of the plasma conductivity and transfer from spray arc to globular arc operation with increasing CO2 concentration. Although the electron density ne increases while approaching the core of the plasma in the spray-arc mode, a drop in the electron temperature Te is observed. Moreover, the maximum Te that we measure is about 13?000?K. Our experimental results differ from the Haidar model where Te is always maximum on the arc axis and its values exceed 20?000?K. These discrepancies can be explained as a result of underestimation of the amount of metal vapours in the plasma core and of the assumption of local thermal equilibrium plasma in the model.

Mechanism of electrical breakdown of gases for pressures from 10−9 to 1 bar and inter-electrode gaps from 0.1 to 0.5 mm

Abstract: This paper discusses the mechanisms of gas breakdown at low values of pressure and inter-electrode gap, i.e. in the vicinity of the Paschen minimum. In this area of pressure and inter-electrode gap values, breakdown occurs either through gas or vacuum mechanisms, and also the so called anomalous Paschen effect appears. Electrical breakdown of electropositive, electronegative and noble gases has been investigated theoretically, experimentally and numerically. Based on the results obtained, regions in which particular breakdown mechanisms appear have been demarcated. Special attention has been devoted to the anomalous Paschen effect as well as to the avalanche vacuum breakdown mechanism.

{\rm N}_{2}(A\,^{3}\Sigma _{\rm u}^{+}) density measurement in a dielectric barrier discharge in N2 and N2 with small O2 admixtures

Abstract: This paper deals with the measurement of metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical?optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-? emissions, and of NO density by LIF. A metastable density of the order of 1013?cm?3 was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O2 and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.

Breakdown of methylene blue and methyl orange by pulsed corona discharge

Abstract: The recently developed corona above water technique is applied to water containing 10 mg l−1 methylene blue (MB) or methyl orange (MO). The corona discharge pulses are created with a spark gap switched capacitor followed by a transmission line transformer. The pulse amplitude is 40 kV; its duration is 50 ns. At a pulse repetition rate of 10 Hz this leads to an average power of 0.6 W into the discharge. MB and MO are completely decolourized in ~20 min. This corresponds to a yield of ~4.5 gr kW−1h−1, which is much higher than obtained with other discharge techniques or sonoluminescence. The high yield is reflected in the observed temperature increase of only ~1 K. Tests with additional chemicals show that the initial speed of the conversion can be influenced but the total time required for total decolourization is constant. Further, it follows that the main oxidation path of the dyes is by direct ozone attack and the conversion products are strong acids.

Non-equilibrium plasma kinetics: a state-to-state approach

Abstract: State-to-state approaches are used to shed light on (a) thermodynamic and transport properties of LTE plasmas, (b) atomic and molecular plasmas for aerospace applications and (c) RF sustained parallel plate reactors. The efforts made by the group of Bari in the kinetics and dynamics of electrons and molecular species are discussed from the point of view of either the master equation approach or the molecular dynamics of elementary processes. Recent experimental results are finally rationalized with a state-to-state kinetics based on the coupling of vibrational kinetics with the Boltzmann equation for the electron energy distribution function.

Analysis of the self-pulsing operating mode of a microdischarge

Abstract: The self?pulsing regime of a microhollow cathode discharge in argon is reported. The plasma is generated inside the hole drilled in an anode?dielectric?cathode device. The hole dimension ranges from 200 to 400??m and the gas pressure ranges from 40 to 200?Torr. It is shown by optical spectroscopy and fast CCD imaging that the current pulse is related to a fast expansion of the plasma outside the microhole on the cathode backside. The pulse current duration ranges from 0.4 to 2??s depending on the gas pressure. The self-pulsing regime occurs at medium current range (0.1?1?mA). At lower current the discharge is steady and the plasma is confined inside the hole (abnormal regime); at higher current, the plasma is steady and the plasma expands outside the hole on the cathode backside. The self-pulsing frequency is a linear function of the averaged discharge current and decreases with the device capacitance. The dependence of the self-pulsing characteristics (frequency, light emission, power deposition, etc) on the gas pressure follows a Paschen-like law; this is interpreted in considering that the fast expansion of the plasma outside the hole is similar to a gas breakdown. A simple electrical model, using a bistable voltage-controlled variable resistor to simulate the evolution of the plasma impedance, provides qualitative results in good agreement with the experiments.

A calibrated infrared imaging study on the steady state thermal behaviour of Hall effect thrusters

Abstract: The thermal behaviour of Hall effect thrusters was investigated by means of calibrated infrared thermal imaging performed in the 8?9??m spectral domain. Study on the variation of the steady state temperature of Hall thruster elements like discharge chamber (channel) walls and anodes along with discharge voltage and propellant (xenon) mass flow rate confirms that energy loss mechanisms, which are responsible for the heating of the thrusters, are a direct consequence of interactions between charged particles and surfaces. In order to obtain new insights into plasma surface interactions inside a thruster, the channel wall temperature was monitored over a broad range of electrical power stretching from 400?W to 5.5?kW for three types of thrusters with different designs, dimensions and operation domains, namely SPT100-ML, PPS ? 1350-G and PPSX000-ML. Note that over the range of thruster operating conditions the facility backpressure varies from 10?5 to 6 ? 10?5?mbar. In addition, the effect of discharge chamber wall material on temperature field was also investigated using dielectric BN?SiO2 and AlN walls as well as conducting graphite walls. For a given thruster geometry and material, a simple relationship between the mean wall temperature and the input electrical power can be established, in contradiction to the complex dynamics of such a magnetized plasma medium. Besides, thruster thermal history and degree of wear do not have a strong impact on power losses inside the channel.

Heating of a dual frequency capacitively coupled plasma via the plasma series resonance

Abstract: The behavior of dual frequency capacitively coupled plasma discharges (2f-CCP) is experimentally studied by Langmuir probe and rf current measurements and is compared with simulations from the literature. The driving frequency ratio, system pressure, high frequency (HF) power and low frequency (LF) power are varied in the experiments. An increase in LF power causes a moderate increase in electron density but a significant decrease in electron temperature. An increase in HF power causes a strong increase in electron density and populates the high energy part of the electron energy distribution function. These dependences can be explained on the basis of a global model. It is shown that the ratios of HF/LF power and driving frequency are the most important parameters. At integer frequency ratios a significant increase in electron density was found, which is explained by the indirect heating at the plasma series resonance. Several design guidelines are derived which address industrial applications and process stability.

Negative ions in single and dual frequency capacitively coupled fluorocarbon plasmas

Abstract: We have studied charged particle densities and fluxes in a customized industrial etch reactor, running in Ar/O 2 /c-C 4 F 8 gas mixtures at pressures in the region of 50 mTorr and driven by 2 and 27 MHz RF power, either separately or simultaneously. Independent control of ion flux and ion energy is the aim of using dual frequency plasmas. However, little experimental data exists regarding the charged particle dynamics in complex industrial gas mixtures. Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and they may even reach the surface and participate in etching. We have measured the electron density using a microwave hairpin resonator and the positive ion flux with a RF biased ion flux probe. The ratio of these two quantities, which depends on the negative ion fractions and other factors, is seen to vary strongly with gas chemistry, giving evidence for the presence of negative ions. Our results indicate high electronegativity for high c-C 4 F 8 flow rates. We have also examined the effect of varying the 2 and 27.12 MHz RF powers on both the electron density and the positive ion flux. This allows us to estimate the effect of varying power on the negative ion density. In addition, ultra-violet cavity ring-down spectroscopy was used to measure the F density directly (Booth et al 2006 Appl. Phys. Lett. 88 151502). This optical measurement was compared with the probe technique.

Modelling electron transport in magnetized low-temperature discharge plasmas

Abstract: Magnetic fields are sometimes used to confine the plasma in low-pressure low-temperature gas discharges, for example in magnetron discharges, Hall-effect-thruster discharges, electron-cyclotron-resonance discharges and helicon discharges. We discuss how these magnetized discharges can be modelled by two-dimensional self-consistent models based on electron fluid equations. The magnetized electron flux is described by an anisotropic drift–diffusion equation, where the electron mobility is much smaller perpendicular to the magnetic field than parallel to it. The electric potential is calculated either from Poisson's equation or from the electron equations, assuming quasineutrality. Although these models involve many assumptions, they are appropriate to study the main effects of the magnetic field on the charged particle transport and space charge electric fields in realistic two-dimensional discharge configurations. We demonstrate by new results that these models reproduce known phenomena such as the establishment of the Boltzmann relation along magnetic field lines, the penetration of perpendicular applied electric fields into the plasma bulk and the decrease in magnetic confinement by short-circuit wall currents. We also present an original method to prevent numerical errors arising from the extreme anisotropy of the electron mobility, which tend to invalidate model results from standard numerical methods.

Anode attachment modes and their formation in a high intensity argon arc

Abstract: The anode region of a high intensity argon arc has been studied experimentally. As the cold ambient gas enters, the arc-anode attachment can appear in four different modes. Besides the well-known diffuse and constricted anode attachment modes, two transition modes have been observed, namely a lift-up mode and a multiple-attachment mode. The electron temperature and the electron density distributions for these modes have been obtained with a laser Thomson scattering system, which can probe the plasma 50 µm in front of the anode surface. The measurement results show good agreement with previous Langmuir probe results. The electric field distributions for these modes have been obtained for the first time by solving the charge continuity equation. The results indicate that the entrainment of the ambient cold gas first generates a ring in the fringes of the arc, which has low electron temperatures, low electron densities and an increased electric field strength accelerating electrons towards the anode. These characteristics encourage the electron overheating instability to develop, and lead to the formation of the multiple-attachment mode. One of the constricted multiple attachments grows and takes over most of the current as more ambient gas enters to form an anode jet and a constricted mode. Moreover, directly from the electric field distributions, the anode potential fall has been calculated. A negative anode fall exists in the centres of diffuse attachments, while a positive anode fall exists in the centres of constricted attachments.

Plasma assisted combustion: effect of a coaxial DBD on a methane diffusion flame

Abstract: An experimental study of plasma assisted combustion has been initiated at ONERA in order to evaluate the potential application to supersonic combustion with various fuels. For this first step, we choose an easier and more versatile configuration where a coaxial dielectric barrier discharge is applied to a methane diffusion flame. The effects of this discharge on the flame structure and especially the reduction in its detachment height have been investigated. The role of the large scale motion of the flame on its anchoring by the plasma has then been revealed, along with the presence of a plasma channel linking the flame to the burner. Electrical and optical investigations demonstrated the pulsed nature of the light emission of the plasma and the spectroscopic study of OH, CH and exhibits a 10 mm overlap between the plasma and the flame.

Neutral density profiles in argon helicon plasmas

Abstract: We present an analysis of laser-induced fluorescence and emission spectroscopy profile measurements of the excited states of neutral argon for two different helicon source pressures. Through use of a collisional–radiative model that calculates excited state profiles based on measured electron densities, electron temperatures, and the measured edge neutral pressure, the ground state neutral density profile is determined for each pressure case. The results demonstrate that it is possible to extract the ground state neutral density profile from such measurements and that the degree of ionization at the center (r = 0) of a helicon source plasma can vary significantly for a small change in source pressure.

Nonlinear plasma dynamics in capacitive radio frequency discharges

Abstract: The excitation of harmonics in the current of capacitive radio frequency (RF) discharges is a frequently observed phenomenon. The effect is of interest for several reasons. It forms, for instance, the basis of a successful diagnostic concept for technical plasmas, and it is intimately connected to the process of electron heating in capacitive discharges. Recently, mathematical models were proposed which interpret the phenomenon as the self-excitation of the plasma series resonance by the nonlinearity of the boundary sheath. These models are surprisingly successful but suffer from the limitation that they analyse the plasma dynamics in terms of global equations with concentrated parameters. They are unable to account for the complex multi-mode current waveforms seen in experiments and also cannot resolve the electromagnetic effects which dominate contemporary/large area, high frequency processing discharges. This paper aims to correct the deficiency by presenting a spatially resolved, fully electromagnetic model of the nonlinear RF dynamics of a bounded plasma. The model holds for arbitrary plasma reactor geometries and external RF excitations and makes no assumptions on the homogeneity of the plasma or the characteristics of the boundary sheath. A functional analytic (Hilbert space) formulation of the model is given which allows for an exact solution in terms of an infinite power/Fourier series. For the case of an idealized cylindrical reactor, the model is also explicitly evaluated. The calculated RF current wave forms exhibit the complex multi-mode structure of the currents observed in experiments and follow the same scaling laws. It is concluded that the presented model is capable of describing the nonlinear dynamics of capacitive RF discharges of all sizes and that it provides a significant improvement over both the established nonlinear global models and linear models with spatial resolution.

Mass spectrometric detection of short-living radicals produced by a plasma needle

Abstract: A plasma needle is a radio-frequency (rf) micro-discharge operated in a mixture of helium and air at atmospheric pressure. This source is designed for medical treatment of living tissues. Therapeutic effects of plasma treatment depend on generation of short-living active radicals: reactive oxygen and nitrogen species (ROS/RNS). In this work we determine the concentration of several ROS and RNS (atomic oxygen, nitrogen and hydroxyl radical) by means of threshold ionization mass spectrometry. It is shown that molecular oxygen and nitrogen are substantially dissociated in the plasma. Atomic nitrogen and oxygen are the most abundant radicals: the densities are on average few times 10−4. Hydroxyl radicals are less abundant (10−5 fraction of the total gas density). As expected, the densities of active species increase with increasing plasma power. Spatial (axial) distributions have been determined; the radical density reaches a maximum at 2.5 mm away from the rf powered electrode, and it decreases at distances larger than 3.5 mm. The amount of active radical species is reasonably high, which explains the effectiveness of plasma in bacterial inactivation and tissue treatment.

Arbitrary substrate voltage wave forms for manipulating energy distribution of bombarding ions during plasma processing

Abstract: The energy distribution of ions (IED) bombarding a substrate during plasma etching has demonstrated effects on etch selectivity for integrated circuit fabrication. Accurate control of the IED is desired to better understand the nature of plasma–surface interaction and to control process outcomes. IED control can be achieved by tailoring the wave form shape of an rf bias applied to the substrate, using a programmable wave form generator in combination with a power amplifier. Due to the frequency dependence of the amplifier gain and the impedance of the plasma in contact with the substrate, it is not practical to predict the shape of the input wave form needed to produce a desired result at the substrate. Introduced here is an iterative approach using feedback control in the frequency domain to produce arbitrary wave form shapes at the substrate. A fast Fourier transform (FFT) of the substrate wave form is compared, one frequency at a time, with the FFT of a desired target wave form, to determine adjustments needed at the generator. This iterative procedure, which is fully automated and tested for several target wave form shapes, is repeated until the substrate wave form converges to the targeted shape.

Cleaning aluminum fluoride coatings from plasma reactor walls in SiCl4/Cl2 plasmas

Abstract: The formation of aluminum fluoride layers on the inner part of plasma reactor walls is known to be a serious issue for plasma etching processes: it causes process drifts and particle generation. AlFx coatings are formed on the reactor walls as soon as the Al2O3 reactor walls are exposed to fluorine (F)-based plasmas. Since plasma reactors are always cleaned in fluorine chemistries, SF6 for example, AlFx formation is inevitable in the typical plasma clean conditions used in manufacturing. AlFx material being extremely etch resistant, it cannot be removed from reactor walls: it accumulates until particles flake off, then imposing a wet clean of the plasma reactor. In this paper, we review the different methods to reduce or eliminate the formation of AlFx and we report that SiClx radicals/ions etch AlFx material by forming AlClx and SiFx products. By using a dedicated technique based on x-ray photoelectron spectroscopy to analyze the reactor walls, we show that AlFx (and also YFx) coatings formed on the reactor walls are cleared in SiCl4/Cl2 plasmas, restoring clean Al2O3 (Y2O3) walls before processing the next wafer. As a result the wafer-to-wafer reproducibility and the mean time between reactor wet cleans are probably significantly improved. Furthermore, SiCl4-based plasma chemistries efficiently remove other metal fluorides (such as HfFx) and more generally metallic oxides (high-k) from the reactor walls.

Experimental study of a pre-ionized high power pulsed magnetron discharge

Abstract: This paper is focused on experimental studies of a high power pulsed magnetron discharge stabilized by low current pre-ionization. Time resolved studies were performed for a Cu target by optical emission spectroscopy and electrical measurements for different pressures of Ar buffer gas. Due to the elimination of the statistical delay time and a fast discharge current rise the quasi-stationary state was reached in 6 µs. The quasi-stationary state is characterized by an extremely high and pressure independent discharge current density of ~10 A cm−2 and stable Cu+ and Cu++ emissions. Such fast discharge dynamics permits the magnetron cathode current to be driven with a pulse of duration of the order of a few µs, significantly shorter than in other devices. During this short time, the plasma does not have time to undergo the transition from the glow to the arc discharge even at the extremely high cathode loads met in our case. Different stages of the fast discharge development are identified and the composition of the magnetized plasma as a function of the pressure is discussed in detail.

The influence of ion-enhanced field emission on the high-frequency breakdown in microgaps

Abstract: This paper contains the results of the detailed simulation study of the role of ion-enhanced field emission on the breakdown voltage in argon, xenon and krypton at high frequencies. Calculations were performed by using a one-dimensional particle-in-cell/Monte Carlo collisions (PIC/MCC) code with the secondary emission model adjusted to include field emission effects in microgaps. The obtained simulation results clearly show that electrical breakdown across micron-size gaps may occur at voltages far below the minimum predicted by the conventional Paschen curve. The observed breakdown voltage reduction may be attributed to the onset of ion-enhanced field emission.

Neutral gas density depletion due to neutral gas heating and pressure balance in an inductively coupled plasma

Abstract: The spatial distribution of neutral gas temperature and total pressure have been measured for pure N2, He/5%N2 and Ar/5%N2 in an inductively coupled plasma (ICP) reactor, and a significant rise in the neutral gas temperature has been observed. When thermal transpiration is used to correct total pressure measurements, the total pressure remains constant regardless of the plasma condition. Neutral pressure is depleted due to the pressure balance when the plasma pressure (mainly electron pressure) becomes comparable to the neutral pressure in high density plasma. Since the neutral gas follows the ideal gas law, the neutral gas density profile was obtained from the neutral gas temperature and the corrected neutral pressure measurements. The results show that the neutral gas density at the centre of the plasma chamber (factor of 2–4 ×) decreases significantly in the presence of a plasma discharge. Significant spatial variation in neutral gas uniformity occurs in such plasmas due to neutral gas heating and pressure balance.

Deposition of protective coatings in rf organosilicon discharges

Abstract: The paper discusses the deposition of protective coatings ranging from organosilicon plasma polymers to SiO2-like films and hard diamond-like carbon/silicon oxide (DLC?:?SiOx) coatings in radio frequency capacitively coupled discharges using hexamethyldisiloxane (HMDSO). As a result of the optimization of the deposition conditions it was possible to obtain high performance protective coatings. In the HMDSO/O2 mixture, it was shown that rather than the SiO2-like film a hard cross-linked SiOxCyHz polymer film can be used as a protective coating for polycarbonate. The optimum conditions for the deposition of an almost stress-free film were 17% of HMDSO and dc bias voltage of ?240?V. The film hardness and elastic modulus were 10?GPa and 75?GPa, respectively. The refractive index at 600?nm was 1.5 and the extinction coefficient decreased from 0.02 at 240?nm down to zero at 600?nm.The films deposited from HMDSO/CH4 and HMDSO/CH4/H2 mixtures exhibited the attractive properties of DLC films with the partial elimination of some of their drawbacks, such as absorption in the visible and a high intrinsic stress. The optimum concentration of the HMDSO was approximately 21%. Under these conditions the concentration of SiOx in the films was approximately 9?at.%. The film hardness and elastic modulus were above 22?GPa and 120?GPa, respectively.