Metastable Density Research Articles

Argon metastable and resonant densities in a low-pressure Ar–N2 inductively coupled plasma

Argon metastable (3P2 and 3P0) and resonant (3P1 and 1P1) atom densities have been measured in a low-pressure (1–20 mTorr) inductively coupled plasma generated in argon–nitrogen gas mixtures with nitrogen contents ranging from 0 to 90% using optical absorption spectroscopy techniques. A significant decrease of the metastable density is found for increasing nitrogen contents, more pronounced as the pressure is increased. As for the resonant atoms, their behaviour differs from that of metastable atoms, which results from the growing influence of radiation trapping with pressure. A simplified model accounting for several Ar metastable population/depopulation mechanisms (electron excitation/ionization, diffusion and excitation transfer from Ar metastable to nitrogen molecules) has been used to interpret these results. It shows that excitation transfer reactions from Ar metastable atoms to nitrogen become the main depopulation process, especially for large nitrogen contents and pressures above 10 mTorr.

Argon Ion and Neutral Metastable Levels Destruction in a Magnetically Enhanced Inductively Coupled Plasma Reactor

This paper deals with pure argon plasma studied in a magnetically enhanced inductively coupled reactor. Laser induced fluorescence technique was performed with an optical parametric oscillator laser on the 3d 4 F7/2 and 3d 2 G9/2 Ar + metastable levels and on the (2P3/2 0 )4s 2 neutral metastable state to determine their relative densities. Langmuir probe was used as a complementary diagnostic to measure the electron energy distribution functions. When increasing the confinement magnetic field, the metastable state density collapses for all cited levels, by contrast with n e which is continuously growing. Calculations were carried out for neutral argon to explain this behaviour. The results show that the metastable population is depleted by electron-impact excitation and ionization, these loss processes becoming dominant compared to the metastable state creation term.

A Two-Stage Contact Process on Scale-Free Networks

We study a two-stage contact process on scale-free networks as a model for the spread of epidemics. We show that any virus starting from a single vertex with arbitrarily small infection rates can last for a super-polynomial time with positive probability if the power law exponent α>2. This is in sharp contrast with the mean-field analysis. The estimation of the metastable density is also provided.

Determination of N and O – Atom and N2(A) Metastable Molecule Densities in the Afterglows of N2 and N2‐O2 Microwave Discharges

AbstractEarly afterglows of N2 and N2‐O2 flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atom and N2(A) metastable molecule densities are determined by optical emission spectroscopy after calibration by NO titration for N‐atoms and measurements of NO and N2 band intensities for O‐atoms and N2(A) metastable molecules. By using N2 tanks with 50 and 10 ppm impurity, it is determined in the afterglow an O‐ atom impurity of 150‐200 ppm. Variations of the N and O‐atom and N2(A) metastable molecule densities are obtained in the early afterglow of N2–(9·10–5–3·10–3)O2 gas mixtures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A theoretical insight into low-temperature atmospheric-pressure He+H2 plasmas

H2-containing low-temperature plasmas are used in a wide range of industrial applications. In recent decades, efforts have been made to understand and improve the performance of these plasmas, mainly when operated at low and medium pressures. Studies of hydrogen-containing plasmas at atmospheric pressure, however, are scarce despite the potential advantage of operation in a vacuum-free environment. Here the chemistry of low-temperature atmospheric-pressure He + H2 plasmas is studied by means of a global model that incorporates 20 species and 168 reactions. It is found that for a fixed average input power the plasma density decreases sharply when the H2 concentration is higher than ∼0.2%, whereas the atomic H density peaks at a H2 concentration of ∼2%. Operation at larger H2 concentrations leads to lower plasma densities and lower H concentrations because at high H2 concentrations significant power is dissipated via vibrational excitation of H2 and there is an increasing presence of negative ions (H−). Key plasma species and chemical processes are identified and reduced sets of reactions that capture the main physicochemical processes of the discharge are proposed for use in computationally demanding models. The actual waveform of the input power is found to affect the average density of electrons, ions and metastables but it has little influence on the density of species requiring low energy for their formation, such as atomic hydrogen and vibrational states of hydrogen.

Time-resolved tunable diode laser absorption spectroscopy of pulsed plasma

A method for time-resolved tunable diode laser absorption spectroscopy (LAS) has been developed. In this Letter, we describe in detail a developed electronic module that controls the time resolution of the LAS system. The transistor-transistor logic signal triggering the plasma pulse is used for generation of two signals: the first one triggers fine tuning of the laser wavelength and the second one controls time-defined signal sampling from the absorption detector. The described method and electronic system enable investigation of the temporal evolution of the density and temperature of selected particles in technological plasma systems. The high-power impulse magnetron sputtering system with a period of 10 ms and a duty cycle of 1% has been used to verify this method. The temporal evolution of argon metastable density was measured in the active part of the pulse and in the afterglow. The resulting density of Ar* displays a double-peak structure with a first peak in the plasma "ON" phase and a second peak in the afterglow approximately 1 ms after the end of the pulse.

Optical diagnostics of a surface-wave-sustained neon plasma by collisional–radiative modelling and a self-absorption method

A surface-wave-sustained microwave discharge in a coaxial tube configuration in neon at low pressure (300–700 Pa) was studied by optical emission spectroscopy. Spectra of the discharge were analysed by a self-absorption method and a collisional–radiative model. The self-absorption method was used to determine the densities of neon metastable states. The collisional–radiative model, using these densities, was applied to determine the axially resolved electron temperature and the reduced electric field strength in the discharge from the optical spectra. Two types of electron distribution functions were used in the calculations—a Maxwellian distribution function and a solution of electron Boltzmann kinetic equation.The spatially averaged neon metastable densities were in the range (0.12–0.53) × 1016 m−3 and (3.4–4.1) × 1016 m−3 for 1s3 and 1s5 states, respectively. The axial profile of the electron temperature was found to be approximately constant around 1.3–1.6 eV depending on the pressure. The reduced electric field strength decreased along the plasma column in the range 16.5–6.0 Td depending on the pressure. A large influence of gas cooling along the plasma column on the determined reduced electric field strengths was observed.

Line-ratio determination of atomic oxygen and metastable absolute densities in an RF nitrogen late afterglow

Optical emission spectroscopy line-ratio methods are developed in order to estimate the absolute densities of nitrogen and oxygen atoms and metastable N2(A) molecules in the nitrogen late afterglow of an RF discharge, operating at p = 8 Torr, Q = 1 slm and P = 100 W, in what constitutes an extension of the typical domain of application of these methods. [N] is obtained from the first positive (1+) emission with calibration by NO titration, [O] from the ratio of the NOβ to 1+ bands, and [N2(A)] from the ratios of (i) the NOγ and NOβ bands, (ii) the second positive (2+) and NOβ bands and (iii) the 1+ and 2+ bands. In addition to the determination of the N, O and N2(A) absolute densities, the present investigation gives an indication on the order of magnitude of the rate coefficient of the very important reaction N2(X, v ⩾ 13) + O → NO + N at room temperature.

Influence of metastable states on collective particle diffusion in the interacting lattice gas model

We analyze a system of particles diffusing over an anisotropic crystal surface represented as a lattice gas model. Every second row of adsorption sites has higher energy and the diffusion across rows occurs via metastable states. Double occupancy is forbidden and nearest neighbors along rows interact via repulsive or attractive couplings. A collective diffusion coefficient is derived within the framework of a variational approach. Analytic expressions are investigated as functions of metastable state energies, activation energies, interaction parameters and particle density. It is shown that the interactions of particles that reside at stable or metastable sites change the diffusion over the surface. For some parameters the diffusion coefficients along or across rows are totally changed due to the interactions between particles. Diffusion along rows can be amplified or suppressed by the particle density change for different values of the jump activation energies.

Effect of pretreatment on electrochemical etching behavior of Al foil in HCl–H2SO4

The aluminum foil for high voltage aluminum electrolytic capacitor was immersed in 0.5 mol/L H3PO4 or 0.125 mol/L NaOH solution at 40 °C for different time and then DC electro-etched in 1 mol/L HCl+2.5 mol/L H2SO4 electrolyte at 80 °C. The pitting potential and self corrosion potential of Al foil were measured with polarization curves (PC). The potentiostatic current—time curve was recorded and the surface and cross section images of etched Al foil were observed with SEM. The electrochemical impedance spectroscopy (EIS) of etched Al foil and potential transient curves (PTC) during initial etching stage were measured. The results show the chemical pretreatments can activate Al foil surface, facilitate the absorption, diffusion and migration of Cl− onto the Al foil during etching, and improve the initiation rate of meta-stable pits and density of stable pits and tunnels, leading to much increase in the real surface area and special capacitance of etched Al foil.

Noninvasive, real-time measurements of plasma parameters via optical emission spectroscopy

Plasma process control applications require acquisition of diagnostic data at a rate faster than the characteristic timescale of perturbations to the plasma. Diagnostics based on optical emission spectroscopy of intense emission lines permit rapid noninvasive measurements with low-resolution (∼1 nm), fiber-coupled spectrographs, which are included on many plasma process tools for semiconductor processing. Here the authors report on rapid analysis of Ar emissions with such a system to obtain electron temperatures, electron densities, and metastable densities in argon and argon/mixed-gas (Ar/N2, Ar/O2, Ar/H2) inductively coupled plasmas. Accuracy of the results (compared to measurements made by Langmuir probe and white-light absorption spectroscopy) are typically better than ±15% with a time resolution of 0.1 s, which is more than sufficient to capture the transient behavior of many processes, limited only by the time response of the spectrograph used.

Influence of the electrophysical processes in the discharge circuit on the energy characteristics of a copper vapor laser

The influence of the electrode arrangement in the gas-discharge tube (GDT) on the electrophysical processes in the discharge circuit is considered. The equivalent GDT circuit and the mechanism of its transformation depending on the electrode arrangement in the cold buffer or hot discharge channel zones are designed. The breakdown mechanism at the GDT channel ends is considered. It is demonstrated that the breakdown can be considered as the time moment since which the voltage is redistributed between the GDT ends and the active medium in favor of the last. A high relaxation rate of metastable copper atom states in the near afterglow is caused by ionization electron cooling in the active medium. In this case, the energy stored in the reactive components of the discharge circuit impedance during this time period is dissipated in the cold buffer zones. The energy is dissipated in the cold buffer zones until the time when the plasma resistance at the discharge channel ends reaches the value starting from which the duct capacitive components of the GDT shunt the cold buffer zones. As a result, a high-frequency circuit is formed, and the energy is further dissipated in the active medium. This reduces the relaxation rate of the metastable states during the time period between pulses and determines the characteristic bend in the time dependence of the metastable state population density determining the time moment of forming the high-frequency circuit.

Metastable densities for the contact process on power law random graphs

We consider the contact process on a random graph with fixed degree distribution given by a power law. We follow the work of Chatterjee and Durrett (2009), who showed that for arbitrarily small infection parameter $\lambda$, the survival time of the process is larger than a stretched exponential function of the number of vertices, $n$. We obtain sharp bounds for the typical density of infected sites in the graph, as $\lambda$ is kept fixed and $n$ tends to infinity. We exhibit three different regimes for this density, depending on the tail of the degree law.

Plasma-assisted cleaning by metastable-atom neutralization

Plasma-assisted cleaning by metastable atomic neutralization (PACMAN) is a process that can clean hydrocarbon from extreme ultraviolet photo masks and dissolve hydrocarbon particles. It was developed with semiconductor manufacturing and cleaning in mind. The PACMAN process works by utilizing helium metastable atoms to break apart the contamination to be cleaned. As helium metastables interact with the contaminant surface, bonding electrons from the surface are “stolen” by the metastable helium resulting in “holes” where a bonding electron used to be. In this way, the structure of the contamination is compromised and allows for the removal either through desorption of CxHy molecules or by chain scission of the hydrocarbon backbone. A model of the helium metastable density within the processing chamber has been developed in addition to experimental measurements of the metastable density at the sample surface. Cleaning efficiency has been linked to both helium metastable density as well as electric field in the plasma sheath. Electric field calculations in the plasma sheath reveal that an electric field pointing into the plasma is needed for achieving high cleaning rates of hydrocarbons since it pins the holes that are created to the surface and stops the hydrocarbon bonds from re-forming. Operating the PACMAN process in this fashion allows for cleaning rates of approximately 1.2 × 107 ± 5.1 × 105 nm3/min from a particle without causing damage to the surrounding structure of the sample being cleaned. Carbon contamination in the form of carbon films on lithographic material has been shown to clean at rates of approximately 11.4 ± 0.3 nm/min.

Metastable density in nitrogen barrier discharges: I. LIF diagnostics and absolute calibration by Rayleigh scattering

The density of the metastable state of the nitrogen molecule is measured by laser-induced fluorescence (LIF) spectroscopy in an asymmetric barrier discharge at 500 mbar in the filamentary mode. The absolute density calibration is performed by comparison of the time-resolved signals from LIF and Rayleigh scattering. Due to the different physics of both processes, the LIF signal is the temporal convolution of the Rayleigh signal with an exponential decay defined by the effective lifetime of the laser excited state. The result is a delay of the LIF signal with respect to the Rayleigh signal and a slower decay of the LIF signal. This is proven experimentally by a pressure variation from 100 to 1000 mbar. Furthermore, the calibration method allows an implicit measurement of the effective lifetimes and the fluorescence yield, which is important at atmospheric pressure due to the dominance of collisional de-excitation (quenching). The calculated densities in the range of 1013 cm−3 are in good agreement with results from the literature.

Relaxation time distribution of distant‐pair recombination: Application to QFRS of PL in a‐Si:H

AbstractThe distribution function of relaxation time of distant‐pair (DP), or nongeminate recombination is derived in an analytic form from the existing theories of DP recombination to analyse the data of quadrature frequency resolved spectroscopy (QFRS) of photoluminescence (PL) in a‐Si:H with the competing recombination model. The distribution function has a single parameter of metastable carrier density n and yields too narrow QFRS spectrum to fit the experimental data. This is improved with introducing the fluctuation Δ of the electron localisation radius ae. The devised formula is demonstrated to predict the dependence of the DP recombination on the photogeneration rate and the defect density in the QFRS data of the high‐quality and defective a‐Si:H. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solubilities, Densities, and Refractive Indices of the Ternary Systems KCl + RbCl + H2O and KCl + MgCl2 + H2O at 348.15 K

The solubilities of electrolyte mixtures ternary systems KCl + RbCl + H2O and KCl + MgCl2 + H2O at 348.15 K were obtained by isothermal evaporation method. The corresponding physicochemical properties of the solution such as density and refractive index were also determined. The Scherinemakers’ wet residue method was used to determine the composition of the solid phase. The metastable phase diagram, density versus composition diagram, and refractive index versus composition diagram were constructed on the basis of the experimental data. Results showed that these two ternary systems were both complex type. The incommensurare double salt carnallite (KCl·MgCl2·6H2O) was found in the KCl + MgCl2+ H2O system at 348.15 K. Comparisons between the metastable phase diagrams of KCl + MgCl2+ H2O system at (298.15, 323.15, and 348.15) K show that the crystallization zone of carnallite decreases with the increase in temperature. The solid solution [(K,Rb)Cl] was formed in the KCl + RbCl + H2O system at 348.15 K. Compa...

Optical emission diagnostics with electric probe measurements of inductively coupled Ar/O2/Ar-O2 plasmas

Physical properties of low-pressure inductively coupled argon, oxygen, and Ar-O2 mixture plasmas are investigated using optical emission spectroscopy (OES) combined with an rf-compensated Langmuir probe measurement. In each gas discharge, the electron density and the electron temperature were obtained by using the probe. The electron temperature was also obtained by OES models and compared with that measured by the probe. The electron temperature was observed to decrease with increasing power and pressure and also observed to decrease with increasing Ar content. Argon metastable densities were calculated based on an optical transition model. In Ar-O2 discharges, the dissociation fraction of O2 molecules was estimated using optical emission actinometry. The dissociation fraction was observed to increase with increasing power and Ar content.

A model for DC magnetron sputtering of bi-component target in Ar ambient: case study of Ti–Cu target

A simple model for magnetron sputtering of bi-component target was developed in Ar ambient at steady state with plasma chemistry consideration. Dependence of quantities such as Ar ion current, electron density, ground and metastable excited Ar atom densities, total pressure and mean deposition rate on total discharge current have been investigated. Area ratio is independent of total discharge current and only relates to alloy composition. Ar ion current linearly increases trend with total discharge current. The values of electron (ion) density, ground and metastable excited state Ar atom densities are of the orders of 109, 1014 and 1010 cm−3, respectively. Mean deposition rate is calculated with scattering (collisions) consideration.

Development of Barrier Discharges: Operation Modes and Structure Formation

AbstractA discharge cell configuration combining three high‐end diagnostic techniques (cross‐correlation spectroscopy, electro‐optic Pockels effect and laser induced fluorescence spectroscopy) has been developed. The joint application of these diagnostics enables the investigation of surface and bulk processes in barrier discharges in different arrangements. Surface charge density, N2(A) metastable state density, and optical radiation of plasma as three key parameters can be measured and correlated under identical and well‐defined experimental conditions. Systematic measurements of absolute surface charge densities and nitrogen metastable concentrations are presented for different operation modes. The operation mode is mainly controlled by the gas composition, the discharge cell geometry, and the properties of the applied voltage. In particular, in pure helium the unusual Townsend‐like discharge was investigated in detail. The described arrangements and the knowledge about reproducible control of discharge modes are an excellent base for the systematic investigation of the interaction between plasma and dielectric surface and its role on the formation, development, and structure of barrier discharges (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)