Radio frequency glow discharge source with integrated voltage and current probes used for sputtering rate and emission yield measurements at insulating samples

Radio frequency glow discharge optical emission spectroscopy (RF-GD-OES) is routinely used for the chemical analysis of solid samples. Two independent electrical signals from the discharge are required for quantification. When sputtering insulating samples, the voltage over the discharge is not directly measurable. The coupling capacity of the sample is required in order to calculate the discharge voltage. A procedure is outlined where the coupling capacity is determined using an electrical measurement without discharge. The calculated time-dependent discharge voltage and current are evaluated using a plasma equivalent circuit. An insulating sample is sputtered at constant cathode voltage and current. The emission yield for an aluminium line is comparable to that of conducting reference material.     

A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane

The authors recently developed a high-frequency pulsed plasma process for methane conversion to acetylene and hydrogen using a co-axial cylindrical (CAC) type of reactor. The energy efficiency represented by methane conversion rate per unit input energy has been improved so that such a pulsed plasma has potential for commercial acetylene production. A pulsed plasma consists of a pulsed corona discharge and a pulsed spark discharge. Most of energy is injected over the duration of the pulsed spark discharge. Methane conversion using this kind of pulsed plasma is a kind of pyrolysis enhanced by the pulsed spark discharge. In this study, a point-to-point (PTP) type of reactor that can produce a discharge channel over the duration of a pulse discharge was used for the pulsed plasma conversion of methane. The energy efficiency and carbon formation on electrodes have been improved. The influences of pulse frequency and pulse voltage on methane conversion rate and product selectivity were investigated. The features of methane conversion using PTP and CAC reactors were discussed.     

Pulsed discharge in a hollow cathode with the detection of ions in a time-of-flight mass spectrometer: Analytical capabilities in the analysis of solid samples

The results of a study of a new analytical system that includes an ion source with a hollow-cathode pulsed gas discharge and a time-of-flight mass spectrometer are reported. It was found that such a system can be successfully used for the direct analysis of metal and dielectric solid samples. The results of the optimization of various ionizer parameters, such as the delay of an ejecting pulse with respect to a discharge pulse and the bias voltage of the anode-cathode pair, are given. It was proposed to use an additive of hydrogen as a reaction gas for a considerable decrease in the intensities of gas components in the mass spectrum, and the effectiveness of this approach was demonstrated. The possibility of the rapid and efficient direct analysis of solid samples with the use of a hollow-cathode pulsed glow discharge was demonstrated using an example of the analysis of high-purity copper samples and the glassy slag of molten lead.     

Ion formation processes in the afterpeak time regime of pulsed glow discharge plasmas

The formation of ions following the termination of power in a pulsed glow discharge ion source is investigated. The populations of ionized species containing sputtered atoms M⁺, M ₂ ¹ :, and MAr⁺ are observed to maximize after the termination of discharge power. Collisions involving sputtered atoms and metastable argon atoms, Penning and associative ionization, are considered to be responsible for the formation of ions in the discharge afterpeak time regime. The domination of these ion formation processes during the afterpeak time regime is supported by the results from investigations of discharge operating parameters, metastable argon atom quenching, and ion kinetic energy distributions.     

Electronic perturbation investigations into excitation and ionization in the millisecond pulsed glow discharge plasma

This study employed a power perturbation method to examine the energy transfer processes at different locations within the afterpeak regime of a millisecond pulsed glow discharge plasma. Brief power perturbation pulses were applied during the afterpeak regime altering the environment of the collapsing plasma. Responses of several transitions to the power perturbations were measured via atomic emission and absorption spectroscopic methods at various distances from the surface of the cathode. The experimental data provide further insight into the energy transfer processes that occur at different spatial locations and in different temporal regimes of these pulsed glow discharge plasmas. Although the enhancement of the large population of metastable argon atoms is again confirmed, the mechanism responsible for this enhancement remains unclear. The most likely possibility involves some form of ion–electron recombination followed by radiative relaxation of the resulting species. The metastable argon atoms subsequently Penning ionize sputtered copper atoms which then appear to undergo a similar ion–electron recombination process yielding variable degrees of observable afterpeak emission for copper atom transitions. The kinetic information of these processes was approximated from the corresponding relaxation time. The electron thermalization time allowing for recombination with ions was found to be ∼25 μs after the discharge power termination.     

Monte Carlo analysis of the electron thermalization process in the afterglow of a microsecond dc pulsed glow discharge

A Monte Carlo model is utilized for studying the behavior of electrons in the afterglow of an analytical microsecond dc pulsed glow discharge. This model uses several quantities as input data, such as electric field and potential, ion flux at the cathode, the fast argon ion and atom impact ionization rates, slow electron density, the electrical characterization of the pulse (voltage and current profiles) and temperature profile. These quantities were obtained by earlier Monte Carlo — fluid calculations for a pulsed discharge. Our goal is to study the behavior of the so-called Monte Carlo electrons (i.e., those electrons created at the cathode or by ionization collisions in the plasma which are followed by using the Monte Carlo model) from their origin to the moment when they are absorbed at the cell walls or when they have lost their energy by collisions (being transferred to the group of slow electrons) in the afterglow of the pulsed discharge. The thermalization of the electrons is a phenomenon where the electron-electron Coulomb collisions acquire a special importance. Indeed, in the afterglow the cross sections of the other electron reactions taken into account in the model are very low, because of the very low electron energy. We study the electron energy distributions at several times during and after the pulse and at several positions in the plasma cell, focusing on the thermalization and on the behavior of the electrons in the afterglow. Also, the time evolution of the rates of the various collision processes, the average electron energy, the densities of Monte Carlo and slow electrons and the ionization degree are investigated.     

The laser as an analytical probe in glow discharge mass spectrometry

A pulsed dye laser was used alone and in combination with an auxiliary glow discharge for diagnostic studies. We have compared the mass ablation rate and ion signal in high vacuum and in the presence of an ambient buffer gas. Laser pulse energy and repetition rate were studied over the analytical range of the system. The effect of ambient gas pressure on redeposition was determined for three different gases: helium, neon, and argon. The laser/auxiliary discharge system was also shown to have analytical utility for the analysis of nonconducting samples. Spectra are included to illustrate the enhancement of the laser atomization/discharge ionization scheme over laser atomization/ionization.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Spectral,spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization

Two-dimensional maps of the spatial distributions of excited and ionized sputtered copper atoms are presented for a millisecond pulsed argon glow discharge. These maps demonstrate the temporal as well as spatial dependence of different excitation and ionization processes over the pulse cycle. Transitions from the low energy electronic states for the atom, characterized by emission such as that at 324.75 nm (3.82→0.00 eV), dominate the plateau time regime at a distance of 2.5 mm from the cathode surface. These processes originate from the electron excitation of ground state copper atoms. Transitions from high-energy electronic states, such as that characterized by emission at 368.74 nm (7.16→3.82 eV), predominate during the afterpeak time regime at a distance of 5.0–6.0 mm from the cathode surface. This observation is consistent with the relaxation of highly excited copper atoms produced by electron recombination with copper ions during the afterpeak time regime. Analyses of afterpeak and plateau intensities for a series of copper emission lines indicate an electron excitation temperature equivalent to 5.78 eV at 0.8 torr and 1.5 W. Temporal profiles exhibit copper ion emission only during the plateau time regime.     

Scaling of Shielded Sliding Discharges for Environmental Applications

Shielded sliding discharges are nanosecond streamer discharges which develop along a dielectric between metal foil electrodes, with one of the foils extended over the entire rear of the dielectric layer. The electrode configuration not only allowed rearranging discharges in parallel due to the decoupling effect of the metal layer, but also to modify the electric field distribution in such a way that components normal to the surface are enhanced, leading to an increased energy density in the discharge plasma. By varying the electrode gap, the applied voltage, and the repetition rate, it is shown that by keeping the average electric field constant, the discharge voltage can be reduced from tens of kV to values on the order of a few kV, but only at the expense of a reduced energy density of the plasma. Varying the repetition rate from 20 to 500 Hz resulted in a slightly reduced energy per pulse, likely caused by residual charges on the dielectric surface. Measurements of the NO conversion to NO₂ and ozone synthesis in dry air showed that the conversion is only dependent on the energy density of the discharge plasma. Although reducing the pulse voltage from the tens of kV range to that of few kV, and possibly even lower, causes a reduction in energy density, this loss can be compensated for by increasing the electrode gap area. This and the possibility to form discharge arrays allows generating large volume discharge reactors for environmental applications, at modest pulsed voltages.     

Low Cost Compact Nanosecond Pulsed Plasma System for Environmental and Biomedical Applications

Nanosecond pulsed non-thermal atmospheric-pressure plasmas are promising for numerous applications including air and water purification, ozone synthesis, surface sterilization, material processing, and biomedical care. However, the high cost of the nanosecond pulsed power sources has hindered adaptation of the plasma-based technologies for clinical and industrial use. This paper presents a low cost (<100US$) nanosecond pulsed plasma system that consists of a Cockcroft–Walton high voltage charging circuit, a compact nanosecond pulse generator using a spark gap as switch, and a plasma reactor. The nanosecond pulse power source requires only a 12 V DC input, hence is battery operable. Through the optimization of the experimental parameters, pulses with a peak voltage >10 kV, a 3 ns rise time (10 to 90 %), and a 10 ns pulse duration (full width at half maximum) at a pulse repetition rate of up to 500 Hz were achieved in the present study. It has been successfully tested to power three different plasma reactors to form pulsed corona discharges, dielectric barrier discharges, and sliding discharges. The energy efficiency of such a nanosecond pulsed sliding discharge system was assessed in the context of ozone synthesis using air or oxygen as the feed gas, and was found comparable to a previously reported non-thermal plasma system that used commercial high voltage pulsed power sources. This study demonstrated that this low-cost nanosecond pulsed power source can prove to be an energy efficient and simple supply to drive various non-thermal atmospheric-pressure plasma reactors for environmental, medical and other applications.     

Development of resonance‐enhanced multiphoton ionization SNMS for state‐selective detection of sputtered atoms under low‐energy ion irradiation

An instrument for a sputtered neutral mass spectrometry with a quadrupole mass spectrometer (QMS) by resonance‐enhanced multiphton ionization method is developed to study sputtered neutrals emission phenomena under ion irradiation in a low‐energy region. We have prepared a pulsed primary ion beam and an ion counting system, and have optimized the operation parameter including a sample bias, energy analyzer voltages, pulsed timing of laser and ion beam, etc. A yield ratio of the lowest‐lying excited state a⁵S₂ to the ground state a⁷S₃ for sputtered Cr atoms has been measured as a function of incident energy of Ar⁺ and O₂⁺ down to 600 eV using a polycrystalline Cr sample. The yield ratio has become a constant value for the Ar⁺ incidence, while it has exponentially increased below 1 keV for the O₂⁺ incidence. It is found that the internal energy distribution of sputtered Cr atoms has been significantly influenced by oxygen density at the surface. Copyright © 2011 John Wiley & Sons, Ltd.     

The effects of pulsed introduction of buffer gas on ion storage and detection efficiencies in a quadrupole ion trap

The quadrupole ion trap is commonly operated with a constant background pressure of an inert, low molecular weight buffer gas. This inclusion of a buffer gas has been shown to increase the sensitivity and mass resolution of the instrument. Research to gain an understanding of these effects, both experimental and through simulations, has typically assumed that it is optimal to maintain a constant buffer gas pressure throughout the entire experiment This article describes the effects of the pulsed introduction of buffer gas at strategic points within the analytical scan and evaluates those events during which the presence of buffer gas is critical. By incorporating a pulsed valve within the ion trap manifold, both the presence and pressure of the buffer gas can be controlled and varied during the individual steps of the scan. The presence of helium buffer gas just before the ion ejection and detection event showed a greater increase in intensity of the ion signal than at any other time in the analytical scan. In addition, this increase in intensity upon pulsed introduction of buffer gas prior to detection is constant over a wide range of pulsed valve open times (i.e., pressures), whereas the signal enhancement upon pulsed introduction of the buffer gas before ionization is observed only over a narrow range of pulsed valve open times.     

Pulsed rf-GD-TOFMS for depth profile analysis of ultrathin layers using the analyte prepeak region

Direct solid analysis of ultrathin layers is investigated using pulsed radiofrequency (rf) glow discharge (GD) time-of-flight mass spectrometry (TOFMS). In particular, previous studies have always integrated the detected ion signals in the afterglow region of the rf-GD pulse, which is known to be the most sensitive one. Nevertheless, the analytical capabilities of other pulse time regions have not been evaluated in detail. Therefore, in this work, we investigate the analyte prepeak region, which is the pulse region where the analyte ions peak after the initial sputtering process of each GD pulse, aiming at obtaining improved depth profile analysis with high depth resolution and with minimum polyatomic spectral interferences. To perform these studies, challenging ultrathin Si-Co bilayers deposited on a Si substrate were investigated. The thickness of the external Si layer was 30 nm for all the samples, whilst the internal Co layer thicknesses were 30, 10, 5, 2 and 1 nm, respectively. It should be remarked that the top layer and the substrate have the same matrix composition (Si > 99.99%). Therefore, the selected samples are suitable to evaluate the response of the Si ion signal in the presence of an ultrathin Co layer as well as the possible oxygen contaminations or its reactions. Additionally, these samples have been evaluated using time-of-flight secondary ion mass spectrometry, and the results compare well to those obtained by our pulsed rf-GD time-of-flight mass spectrometry results.     

Two-laser infrared and ultraviolet matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization (MALDI) was performed using two pulsed lasers with wavelengths in the IR and UV regions. A 10.6 micro m pulsed CO(2) laser was used to irradiate a MALDI target, followed after an adjustable delay by a 337 nm pulsed nitrogen laser. The sample consisted of a 2,5-dihydroxybenzoic acid matrix and bovine insulin guest molecule. The pulse energy for both of the lasers was adjusted so that the ion of interest, either the matrix or guest ion, was not produced by either of the lasers alone. The delay time for maximum ion yield occurs at 1 micro s for matrix and guest ions and the signal decayed to zero in approximately 400 micro s. A mechanism is presented for enhanced UV MALDI ion yield following the IR laser pulse based on transient heating.     

Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization

This paper describes TiO2 nanotube arrays prepared by anodic oxidation of Ti substrates using pulse voltage waveforms. Voltages were pulsed between 20 and -4 V or between 20 and 0 V with varying durations from 2 to 16 s at the lower limit of the pulse waveform. Ammonium fluoride or sodium fluoride (and mixtures of both) was used as the electrolyte with or without added medium modifier (glycerol, ethylene glycol, or poly (ethylene glycol) (PEG 400)) in these experiments. The pulse waveform was optimized to electrochemically grow TiO2 nanotubes and chemically etch their walls during its cathodic current flow regime. The resultant TiO2 nanotube arrays showed a higher quality of nanotube array morphology and photoresponse than samples grown via the conventional continuous anodization method. Films grown with a 20 V/-4 V pulse sequence and pulse duration of 2 s at its negative voltage limit afforded a superior photoresponse compared to other pulse durations. Specifically, the negative voltage limit of the pulse (-4 V) and its duration promote the adsorption of NH4+ species that in turn inhibits chemical attack of the growing oxide nanoarchitecture by the electrolyte F- species. The longer the period of the pulse at the negative voltage limit, the thicker the nanotube walls and the shorter the nanotube length. At variance, with 0 V as the low voltage limit, the longer the pulse duration, the thinner the oxide nanotube wall, suggesting that chemical attack by fluoride ions is not counterbalanced by NH3/NH4+ species adsorption, unlike the interfacial situation prevailing at -4 V. Finally, the results from this study provide useful evidence in support of existing mechanistic models for anodic growth and self-assembly of oxide nanotube arrays on the parent metal surface.     

Pulsed Discharge Regeneration of Diesel Particulate Filters

A novel method for the removal of soot from a diesel particulate filter using pulsed electric discharges is presented. High voltage pulses of between 18 and 25 kV of nano to microsecond duration and with pulse energies of typically 100–200 mJ were applied to the filter via a series spark gap. Initial slow erosion of the soot layer proceeds via the formation of microdischarges. Subsequent spark discharges removed the accumulated soot more effectively from a larger filter volume. Average soot removal rates of ～0.1–0.2 g/min were achieved at 50 Hz breakdown frequency by optimizing both electrode geometry and breakdown voltage. On-engine long term testing of the technology showed soot removal by pulsed discharge to be reliable, efficient and uniform; a total of 100 g of soot was deposited and removed over 18 filter regeneration cycles.     

Some aspects of plasma polymerization investigated by pulsed R.F. discharge

The polymerization of organic compounds in glow discharge (plasma polymerization) was investigated by using pulsed R.F. discharge (100 μsec on, 900 μsec off). The effects of pulsed discharge on polymer deposition rate, pressure change in plasma, ESR signals of free spins in both plasma polymer and substrate, and the contact angle of water on the plasma polymer surface were investigated for various organic compounds. The results are correlated to the mechanisms of polymer formation in plasma (plasma polymerization) which has been postulated as repeating processes of stepwise (propagation) reactions. The effect of the pulse is different from one group of organic compounds to another depending on whether or not they contain an olefinic double bond and/or a triple bond. The main difference seems to be the addition polymerization which can occur exclusively during the off-period of pulsed discharge. Ultraviolet emission from pulsed discharge is much less than from continuous discharge. Consequently, the fragmentation of the monomer and the free-radical formation in the substrate are less with the pulsed discharge. Properties of polymers from some organic compounds formed in continuous and in pulsed discharge were found to be significantly different, and the differences were postulated from the changes of polymerization mechanisms in the pulsed discharge.     

Electrical characteristics of a millisecond pulsed glow discharge

A μs and ms pulsed argon glow discharge was investigated with respect to the breakdown condition (Paschen curve). Moreover, current–voltage profiles were acquired for different discharge frequencies, pulse durations, cathode–anode spacing and discharge pressures. The breakdown voltage was dependent on the cathode material (Cu, steel, Ti and Al). No severe change in the breakdown voltage was observed for a 1 ms pulse at different frequencies. However, the theoretical breakdown curve, calculated based on the Paschen equation did not fit the experimental data. The current plots for different cathode–anode spacing showed a maximum at intermediate distance (8–10 mm). These data were consistent with mass spectrometric data acquired using the same instrument in a GC-GD-TOFMS chemical speciation study.     

Parametric evaluation of sputtering in a planar, diode glow discharge—I. Sputtering of oxygen-free hard copper (OFHC)

A parametric evaluation of the principal factors which affect cathodic sputtering rates in glow discharge sources is performed. Employing a planar, disk cathode in the simple diode geometry, the roles of discharge voltage, current, and pressure are evaluated for the sputtering of oxygen-free hard copper (OFHC). Samples were sputtered at discharge currents of 5–70 mA over an argon pressure range of 1.5–8 torr (200–1064 Pa). In addition, the relationship between applied power and sputter weight loss is investigated. Studies indicate that the current density at the cathode surface and the discharge voltage are directly related and are keys in the determination of sputtering rates. Through factor analysis, an empirical formula is developed which is useful in quantifying sputter rates for a given set of discharge conditions.     

Development and fundamental investigation of Laser Ablation Glow Discharge Time-Of-Flight Mass Spectrometry (LA-GD-TOFMS)

Glow Discharge (GD) spectroscopy is a well known and accepted technique for the bulk and surface composition analysis, while laser ablation (LA) provides analysis with high spatial-resolution analysis in LIBS (laser-induced breakdown spectroscopy) or when coupled to inductively coupled plasma spectrometry (ICP-OES or ICP-MS). This work concerns the construction of a Laser Ablation Glow Discharge Time-Of-Flight Mass Spectrometry (LA-GD-TOFMS) instrument to study the analytical capabilities resulting from the interaction of a laser-generated sample plume with a pulsed glow discharge. Two ablation configurations were studied in detail. In a first approach, the laser-generated plume was introduced directly into the GD, while the second approach generated the plume inside the GD. The ablated material was introduced at different times with respect to the discharge pulse in order to exploit the efficient ionization in the GD plasma. For both LA-GD configurations, direct ablation into the afterglow of the pulsed glow discharge leads to an ion signal enhancement of up to a factor of 7, as compared to the ablation process alone under the same experimental conditions. The LA-GD enhancement was found to occur exclusively in the GD afterglow, with a maximum ablation S/N occurring in a few hundred microseconds after the termination of the glow discharge. The duration of the enhanced signal is about two milliseconds. Both the laser pulse energy and the position of the ablation plume (with respect to the sampling orifice) were found to affect the amount of mass entering the afterglow region and consequently, the enhancement factor of ionization.