Metastable States Of Argon Research Articles

Study Of The Optimum Conditions And Parameters To Perform Simulation Of Low Temperature Capacitive Radio Frequency Argon Discharge

Capacitively coupled plasma (CCP) is used extensively in plasma processing, such as, plasma etching, deposition, and sputtering. Particle-in-Cell (PIC) is used to explore the discharge dynamics microscopically. Considering all species in PIC simulation is challenging. Also, considering various atomic transitions, e.g., ionization, recombination, excitation, and deexcitation is not doable. The state of the art is to carry out fluid simulation. We compare between PIC and fluid simulation for radio frequency CCP to reveal the effect of various approximations as assuming constant temperature, simplifying Navier-Stokes equations in terms of particles mobility and diffusivity, and considering artificial boundary conditions at the electrodes. The fluid model predicts qualitatively PIC results in few minutes. For Argon discharge in geometrically symmetric CCP, the plasma bulk is quasineutral. Over the electrodes, two sheaths are built up due to the difference between ion and electron fluxes. The dynamics of two sheaths are out of phase. In the fluid model, chemical reactions and atomic processes can be considered. Argon metastable states density is maximum in the plasma bulk. In semi dark sheaths, metastable states concentration is small. The proposed fluid model could be used as a simulation platform to find the optimum conditions and to interpret experimental results.

Spatio-temporal distribution of absolute densities of argon metastable 1s5 state in the diffuse area of an atmospheric pressure nanosecond pulsed argon microplasma jet propagating into ambient air

Atmospheric microplasma jets (MPJs) sustained in rare gases have gained increased attention due to their potential to generate highly reactive species. In this paper, we present space- and time-resolved argon metastable densities, Ar(1s5), measured in an atmospheric pressure MPJ operated in Ar and propagating into ambient air using tunable diode laser absorption spectroscopy (1s5 → 2p9 optical transition). The MPJ was produced using a dielectric barrier discharge energized by short duration (230 ns) high-voltage positive pulses (4.2–6.2 kV) at a repetition frequency of 20 kHz. The spectral absorption line profile was recorded and allowed measurements of the absolute metastable Ar(1s5) density integrated in the line-of-sight of the laser beam under various operating conditions of the MPJ. The results reveal a sensitive dependence of the Ar(1s5) density on spatial coordinates, i.e., distance from the exit of the capillary tube of the discharge and from the axis of the argon jet. The highest Ar(1s5) densities of about 3 × 1013 cm−3 were measured at the axis of the argon jet at longitudinal distances between 4 and 5.5 mm downstream from the nozzle of the tube. The temporal distribution of the Ar(1s5) density, which presents three maxima, is thoroughly discussed in this paper. The spatial distribution of the effective Ar(1s5) lifetime (<250 ns) is also reported, giving some insight into the surrounding environment of the argon metastable atoms. The determined spatiotemporal distributions of the Ar(1s5) density can be useful for the optimization of argon MPJs for different applications like surface or biomedical processes.

Emission spectral diagnosis of argon-helium plasma produced by radio frequency capacitive discharge

Optically pumped metastable rare-gas laser (OPRGL) have been proposed to overcome the shortcomings of diode-pumped alkali-vapor laser in the recent years. The OPRGL promises to realize high-scale output. But how to achieve enough particle density of metastable atoms is still an open problem. Usually, plasma produced by discharge serves as a gain medium of the OPRGL. Here in this paper, we are to reveal the effects of different discharge parameters on the plasma properties, such as particle density of metastable argon atoms. Gas discharge at a radio frequency of 13.56 MHz is adopted to excite argon atoms. Emission spectrum is employed to study argon and helium radio frequency discharge of optically pumped argon laser at high pressure, different powers of discharge and various content of argon. Gas temperature is obtained by analyzing rotational spectrum (A2∑+ → X2Π) of OH radical generated by residual water vapor and comparing simulated spectrum with the measured spectrum. The electronic excitation temperature relating to electron temperature is obtained by the method of Boltzmann's plot. Stark broadening of the spectrum is used to determine the electron density. The results show that gas temperature rises slightly with the increase of pressure and varies little with content and discharge power changing. The electronic excitation temperature increases with the decrease of pressure evidently and decreases slightly with the increase of content. The electron density is on the order of 1015 cm-3 under various conditions controlled by us. Long time discharge test reveals that residual water vapor can lead to the decrease of electron temperature, and thus reducing the yield of argon metastable state. In conclusion, considering that the higher gas temperature can improve the collision relaxation rate of helium and argon, and the higher electron temperature can improve the rate of production of argon metastable state. Thus a proposal is put forward that appropriately heating gas and reducing gas pressure can obtain higher particle density of metastable argon. Furthermore, It can be found from these results that heating and cleaning the gas during discharge may be candidate methods to obtain and sustain the higher particle density in the plasma.

Argon metastables in HiPIMS: validation of the ionization region model by direct comparison to time resolved tunable diode-laser diagnostics

The volume plasma interactions of high power impulse magnetron sputtering (HiPIMS) discharges operated with a Ti target is analyzed in detail by combining time-resolved diagnostics with modeling of plasma kinetics. The model employed is the ionization region model (IRM) with an improved and detailed treatment of the kinetics of the argon metastable (Arm) state, called m-IRM. The diagnostics used is tunable diode-laser absorption spectroscopy (TD-LAS) of the Arm state, which gives the line-of-sight density integrated along the laser path parallel to the target surface. The TD-LAS recordings exhibit quite complex temporal evolutions Arm(t), with distinct features that are shown to reflect the time evolution of the plasma (the electron density and temperature), and of the argon gas (gas rarefaction and refill). The Arm(t) function is thus a tracer for the most important aspects of internal discharge physics, and therefore suitable for model testing and validation. The IRM model is constructed to be locked to obey specific experimental macroscopic discharge parameters, specifically the discharge current ID(t) and the voltage UD(t). It has to this purpose been run with the appropriate process gas pressures (from 0.67 to 2.67 Pa), with the experimentally applied voltage pulse profiles UD(t), and with the resulting current pulse profiles ID(t) (with maxima from 0.5 to 70 A). It is shown that the model reproduces the features in the TD-LAS measurements: both the Arm(t) evolution in single pulses, and how the pulse shapes change with gas pressure and with pulse amplitude. The good agreement between the measurements and model output is in this work taken to validate the basic assumptions of the m-IRM. In addition, the m-IRM results have been used to unravel the connections between volume plasma kinetics and various features recorded in the TD-LAS measurement, and to generalize the foremost characteristics of the studied discharges.

An unstructured shock-fitting solver for hypersonic plasma flows in chemical non-equilibrium

A CFD solver, using Residual Distribution Schemes on unstructured grids, has been extended to deal with inviscid chemical non-equilibrium flows. The conservative equations have been coupled with a kinetic model for argon plasma which includes the argon metastable state as independent species, taking into account electron–atom and atom–atom processes. Results in the case of an hypersonic flow around an infinite cylinder, obtained by using both shock-capturing and shock-fitting approaches, show higher accuracy of the shock-fitting approach.

Normal and abnormal evolution of argon metastable density in high-density plasmas

A controversial problem on the evolution of Ar metastable density as a function of electron density (increasing trend versus decreasing trend) was resolved by discovering the anomalous evolution of the argon metastable density with increasing electron density (discharge power), including both trends of the metastable density [Daltrini et al., Appl. Phys. Lett. 92, 061504 (2008)]. Later, by virtue of an adequate physical explanation based on a simple global model, both evolutions of the metastable density were comprehensively understood as part of the abnormal evolution occurring at low- and high-density regimes, respectively, and thus the physics behind the metastable evolution has seemed to be clearly disclosed. In this study, however, a remarkable result for the metastable density behavior with increasing electron density was observed: even in the same electron density regime, there are both normal and abnormal evolutions of metastable-state density with electron density depending on the measurement position: The metastable density increases with increasing electron density at a position far from the inductively coupled plasma antenna but decreases at a position close to the antenna. The effect of electron temperature, which is spatially nonuniform in the plasma, on the electron population and depopulation processes of Argon metastable atoms with increasing electron density is a clue to understanding the results. The calculated results of the global model, including multistep ionization for the argon metastable state and measured electron temperature, are in a good agreement with the experimental results.

On the difference between breakdown and quench voltages of argon plasma and its relation to 4p–4s atomic state transitions

Using a relaxation oscillator circuit, breakdown (VBD) and quench (VQ) voltages of a DC discharge microplasma between two needle probes are measured. High resolution modified Paschen curves are obtained for argon microplasmas including a quench voltage curve representing the voltage at which the plasma turns off. It is shown that for a point to point microgap (e.g., the microgap between two needle probes) which describes many realistic microdevices, neither Paschen's law applies nor field emission is noticeable. Although normally VBD > VQ, it is observed that depending on environmental parameters of argon, such as pressure and the driving circuitry, plasma can exist in a different state with equal VBD and VQ. Using emission line spectroscopy, it is shown that VBD and VQ are equal if the atomic excitation by the electric field dipole moment dominantly leads to one of the argon's metastable states (4P5 in our study).

Shock-Fitting Versus Shock-Capturing Modeling of Strong Shocks in Nonequilibrium Plasmas

In this paper, a supersonic flow of an argon plasma around a cylinder has been investigated comparing shock fitting and shock capturing techniques. Shock-capturing codes are algorithmically simple, but are plagued by a number of numerical troubles, particularly evident when the shocks are strong and the grids unstructured. On the other hand, shock-fitting algorithms allow to accurately compute solutions on coarse meshes, but tend to be algorithmically complex. The kinetic scheme adopted includes the argon metastable state as an independent species and takes into account for electron-atom and atom-atom processes. Electron density distributions have been reported.

Characteristics of a non-Maxwellian electron energy distribution in a low-pressure argon plasma

The generality of the non-Maxwellian electron energy distribution function (EEDF) is demonstrated by using optical emission spectroscopy (OES) and Langmuir probe measurements in inductively- and capacitively-coupled low-pressure argon plasmas to analyze the shape factor of the EEDF. To measure the shape factor of the EEDF, we propose a corona — equilibrium (CE) — based analysis model operating at low density, which uses the line intensity ratio of the Ar I to the Ar II emission lines. The Ar I line is chosen to represent the relatively low-energy state, and the Ar II line is chosen to represent the high-energy state. Thus, an analysis of the shape factor is equivalent to monitoring the variation in the high-energy electron fraction represented in the tail of the EEDF. Results show a depleted tail for the Maxwellian distribution in most of the low-density argon plasmas. The analysis reveals that the generation and the stepwise ionization of metastable argon atoms by inelastic collisions with high-energy (∼ 10 eV) electrons are dominant processes of argon plasma generation and cause serious high-energy electron loss in a low-density system compared to the loss in an ideal Maxwellian plasma. The existence of argon metastable states is inevitable; thus, the general shape of the electron energy distribution in low-pressure argon plasmas is non-Maxwellian.

The Role of Argon Metastables in an Inductively Coupled Plasma for Treatment of PET

Metastable states play an important role in many plasmas because they are often one of the main ionization channels and therefore essential for the discharge dynamics. Because of their high population, density fluxes of metastable states on surfaces are not negligible compared to fluxes of reactive species or ions. If the energy of argon metastable states (Arm) is high enough to cause bond breakage or other surface reactions, they can be an alternative component to treat substrates. In view of the role of Arm in an inductively coupled PECVD reactor for the deposition of barrier films on PET, the densities, lifetimes, and fluxes of Arm are quantified by means of tuneable diode laser absorption (TDLAS) and compared to the ion flux.

Investigation of the Dependence of Argon Metastable State Population on the Conditions of High-Frequency Glow Discharge Excitation

The population of metastable states of argon atoms in high-frequency glow discharge plasma is calculated. Dependences of the population of metastable states of argon atoms in the high-frequency glow discharge plasma on the standard gas flow rate (14–30 sccm) and vibration generator power (10–50 W) in the atmospheres of pure argon and argon-silane (95% Ar + 5% SiH4) mixture are investigated together with the dependence of the population of metastable states of argon atoms in the high-frequency glow discharge plasma on the silane content (0.5–5%) in the argon-silane mixture.

Theoretical and experimental study of emission spectrum of the second positive system of N2

The optical emission spectrum of the second positive system of N2(C3uB3g)isanalyzed and calculated based on the energy structure of nitrogen radical triplet system. Some key parameters of the equation for the radiative transition intensity are evaluated theoretically, including the potentials of the upper and lower states obtained from diagonalizing their Hamiltonian matrices, the electronic transition moments calculated by using r-centroid approximation, and the Einstein coefficients of different vib-rotational levels. For comparing with the theoretical spectrum, we achieve the measured results from corona discharge experiments of N2 and Ar. By fitting the measured spectral intensities and the calculated ones, the vibrational and the rotational temperatures are determined approximately to be 4300 K and 800 K. The results also demonstrate that with the reduction of nitrogen concentration, the intensity of N2 radiative state first increases and then decreases due to Penning excitation from argon metastable states. The experimental results verify the correctness of the theoretical calculations on the second positive system.

Estimations of Electron Temperature and Electron Density in Argon-Containing Reactive Plasma Based on Diode Laser Absorption Spectroscopy

We propose an optical method for estimating both the electron temperature and the electron density in an argon-containing reactive plasma. This method is based on density measurements of the two metastable states of argon in the plasma. Diode laser absorption spectroscopy is utilized for measuring the metastable densities. A population-kinetics model is used for finding the electron temperature and the electron density for which the measured densities of the metastable states are realized. This method has a potential application for the in-situ monitoring of plasma processing tools in factories because of the low cost, small footprint, and maintenance-free operation of a diode laser. We demonstrated this method in an inductively coupled Ar/N2 mixture plasma.

Argon metastable state densities in inductively coupled plasma in mixtures of Ar and O2

We have measured the densities of 1s5 and 1s3 argon metastables as a function of the abundance of molecular oxygen in an inductively coupled plasma (ICP) in mixtures of Ar and O2. Laser absorption spectroscopy was used to determine the densities of the metastables. It was found that even small abundances of oxygen lead to large increases in metastable density, mostly due to the reduction in the electron number density, since electron-induced quenching determines the metastable density. At abundances higher than 7% to 15% for powers between 50 and 150 W, quenching by oxygen molecules begins to dominate and the metastable density drops again.

The study of VUV emissions of Ar*2 excimers using three-photon absorption laser-induced fluorescence

The VUV emissions of the second continuum of argon at ∼130 nm were investigated for pressures ranging from 100 to 600 Torr after selectively populating the resonant atomic state of the 3p54s configuration of argon by using three-photon absorption laser-induced fluorescence. For brief and selective excitation conditions, a kinetic study was performed by analysing the VUV postluminescence decay of argon. This study clearly shows that only three-body collisions lead to the formation of the excimer Ar2(A1u). The three-body decay rate constant of the metastable state Ar4s[3/2]02 was measured for the first time under selective excitation (k′3 = (19.5 ± 2.1) Torr−2 s−1). The role of the first resonant and metastable states of argon in the formation of excimers responsible for the VUV emissions is discussed. A kinetic scheme showing the main reaction processes is proposed and compared with those already obtained with other rare gases.

Populations of argon metastable states in an argon-silane plasma of a high-frequency electric discharge

We have determined the absolute populations of argon metastable states in the plasma of a high-frequency discharge in a pure argon atmosphere and in a mixture of argon and silane.

Photoionization spectroscopy of even-parity autoionizing Rydberg states of argon: Experimental and theoretical investigation of Fano profiles and resonance widths

Using the $4{s}^{\ensuremath{'}}{[1/2]}_{0}$ and $4s{[3/2]}_{2}$ metastable states of argon created in a plasma discharge we perform photoionization spectroscopy of even-parity autoionizing $\text{Ar}(n{p}^{\ensuremath{'}})$ Rydberg states between the first and second ionization limits. Fitting the data to a sum of Fano-type or Shore-type profiles allows us to extract the widths and energies of the three overlapping resonance states ${[1/2]}_{1}$, ${[3/2]}_{1}$, and ${[3/2]}_{2}$ in the $11{p}^{\ensuremath{'}}$ manifold. We compare the experimental analysis of the $11{p}^{\ensuremath{'}}$ multiplet to theoretical calculations which are based on the configuration interaction Pauli-Fock approach with core polarization. Agreement between the experimental and theoretical resonance parameters is good.

Optical emission spectroscopy of Ar–N 2 mixture plasma

Optical emission spectroscopy measurements are presented to characterize the different excitation and ionization processes of both atomic and molecular species in Ar–N 2 mixture plasma under different discharge conditions. Particularly, the emission intensities of nitrogen (0–0) band of second positive system at 337.1 nm and (0–0) band of first negative systems at 391.4 nm are used to determine the dependence of their radiative states [ N 2 ( C 3 Π u ) , N 2 + ( B 2 Σ u + ) ] on argon fraction in the mixture. It is observed that the addition of argon gas influences the radiative states differently due to their different populating mechanisms. The results demonstrate that the addition of argon to nitrogen plasma remarkably enhance the population of N 2 ( C 3 Π u ) radiative state through Penning excitation involving argon metastable states. The electron temperature is determined from Ar-I spectral line intensities, using Boltzmann's plot method and is found to depend on argon fraction in the mixture.

Model study of DC ignition of fluorescent tubes

Breakdown in a discharge tube is investigated by means of a fluid model. The discharge tube is similar to a compact fluorescent lamp tube, containing argon at 3 Torr and mercury at a few millitorr. It was found that the minimum breakdown voltage is decreased substantially compared with a tube containing pure argon. Penning ionization of mercury via an argon metastable state plays an important role in this effect. This is illustrated for a lamp operated on a DC voltage, where significant Penning ionization takes place in the wake of the ionization front. Furthermore, contrary to what is suggested in earlier literature, the development of the surface potential of the lamp is shown to be not only determined by surface charges, but also by volume charges.

Functional separation in two frequency operation of an inductively coupled plasma

Measurements of densities of excited atoms and metastables were performed in pure Ar and in mixtures of Ar and CF4 in inductively coupled plasma sustained by a high frequency (13.56 MHz) source and biased by a low frequency (500 kHz) voltage applied to the wafer supporting electrode. The measurements are made in front of the biased electrode with a goal to understand the effects of different parameters on the plasma profile and to test whether functional separation between plasma sustaining and biasing voltage is achieved. We find a very efficient separation with small or no observable effects of biasing voltage both in pure argon and in mixtures. These results have been achieved at all pressures (5–50 mTorr) and were confirmed by additional microwave measurements of electron density. The effect of flow rate, pressure, power, and distance from the biased electrode was studied from the spatial profiles of short lives excited states and metastable states of argon. We have also compared the profiles close to the biasing electrode, close to the coil and in extended processing chamber, and found a slight increase of metastable density close to the biasing electrode due to reduced electron quenching far from plasma source.