Metastable Argon Atoms Research Articles

Study of Mid-Pressure Ar Radiofrequency Plasma Used in Plasma-Enhanced Atomic Layer Deposition of α-Al2O3

This study investigated the characteristics of radiofrequency, middle-pressure argon plasma used in the atomic layer deposition (ALD) of Al2O3 films. Based on the electrical characteristics—the current, voltage, and phase shift between them—and the stability of the plasma plume, the optimum plasma power, allowing reliable switching on of the plasma for any step of an ALD cycle, was determined. Spectral measurements were performed to determine the gas temperature and reactive species that could be important in the ALD process. The density of metastable argon atoms was estimated using tunable laser absorption spectroscopy. It was concluded that plasma heating of substrates did not affect film growth. The crystallization-enhancing effect of plasma observed in these experiments was due to the action of OH radicals produced in the plasma.

Modeling of the influence of field electron emission from a cathode with a thin insulating film on its sputtering in a gas discharge in a mixture of argon and mercury vapor

A model of the low-current gas discharge in a mixture of argon and mercury vapor in the presence of a thin insulating film on the cathode surface is proposed. The model takes into account that in such a mixture a substantial contribution to the ionization of the working gas can come from the ionization of mercury atoms during their collisions with metastable excited argon atoms. In the discharge, positive charges accumulate on the film surface, creating an electric field in the film sufficient to cause field emission of electrons from the cathode metal substrate into the insulator. Such electrons are accelerated in the film by the field and can escape from it into the discharge volume. As a result, the effective yield of ion-electron emission from the cathode increases. The temperature dependences of discharge characteristics are calculated and it is shown that, due to a rapid decrease in the concentration of mercury vapor in the mixture with decreasing temperature, the electric field strength in the discharge gap and the discharge voltage increase. The presence of a thin insulating film on the cathode can result in an improvement in its emission characteristics and a significant reduction in the discharge voltage. This causes a decrease in the energies of the ions and atoms bombarding the cathode surface, and, consequently, in the intensity of cathode sputtering in the discharge.

Plasma properties and discharging of dust particles in an Ar/C2H2 plasma afterglow

A global (volume averaged) model is developed for an argon-acetylene plasma afterglow. The model is used to study the electron and ion densities, electron temperature and densities of argon metastable atoms in the afterglow plasma. The calculated time dependence for the electron density is found to be in agreement with the existing experimental data. These calculated plasma characteristics are used to investigate the dust charge distribution function (DCDF) for particles with radii of 10–200 nm. The DCDF is found by solving numerically the master equation describing dust discharging as a one-step stochastic process and is also calculated as a Gaussian distribution with mean dust charge and variance, which are functions of time. The time dependences for mean dust charge, variance and dust charging time are obtained and analysed. If the electronegativity of the plasma in the steady-state is low, negative ions do not affect much discharging of dust particles in the afterglow, while at large electronegativity their role is essential. In the case of low electronegativity, discharging of dust particles is mainly due to deposition of positive ions with small and moderate masses (less than the mass of C10H6 + ions). Increasing electronegativity, the effect of heavy positive ions on dust discharging in the late afterglow is important. Secondary electron emission from dust surface at collisions of metastable atoms appears to be negligible.

Production of high-density metastable argon atoms with periodic fast pulsed DC discharge.

Optically pumped metastable rare gas laser (OPRGL), as a potential high-energy laser has attracted much attention. During extensive research on OPRGL, the realization of volume discharge with high-metastable density has always been a priority and is essential to the efficient lasing of OPRGL. A large-volume plasma of He/Ar with high metastable density at atmospheric pressure can be generated by applying peaking capacitors near the electrodes. In this article, 0.8 cm3 of He/Ar plasma at a pressure of 900 mBar, with a peak value of metastable argon density higher than 1014 cm-3 was realized. The metastable density was measured by absorption spectroscopy based on the absorption bandwidths acquired at a pressure range of 400-900 mbar where a probe beam at a weak line with wavelength 772.38 nm was applied. Furthermore, the high metastable density was verified by the laterally-observed laser-induced fluorescence, as well as the laser oscillation of 100 mW under a longitudinal pumping with a short gain length of 8 mm.

Experimental and numerical investigation on the uniformity of nanosecond pulsed dielectric barrier discharge influenced by pulse parameters

Nanosecond (ns) pulsed dielectric barrier discharge (DBD) is considered as a promising method to produce controllable large-volume and high activity low-temperature plasma at atmospheric pressure, which makes it suitable for wide applications. In this work, the ns pulse power supply is used to excite Ar DBD and the influences of the pulse parameters (voltage amplitude, pulse width, pulse rise and fall times) on the DBD uniformity are investigated. The gas gap voltage (U g) and conduct current (I g) are separated from the measured voltage and current waveforms to analyze the influence of electrical parameters. The spectral line intensity ratio of two Ar excited species is used as an indicator of the electron temperature (T e). The time resolved discharge processes are recorded by an intensified charge-coupled device camera and a one-dimensional fluid model is employed to simulate the spatial and temporal distributions of electrons, ions, metastable argon atoms and T e. Combining the experimental and numerical results, the mechanism of the pulse parameters influencing on the discharge uniformity is discussed. It is shown that the space electric field intensity and the space particles’ densities are mainly responsible for the variation of discharge uniformity. With the increase of voltage and pulse width, the electric field intensity and the density of space particles increased, which results in the discharge mode transition from non-uniform to uniform, and then non-uniform. Furthermore, the extension of pulse rise and fall times leads to the discharge transition from uniform to non-uniform. The results are helpful to reveal the mechanism of ns pulsed DBD mode transition and to realize controllable and uniform plasma sources at atmospheric pressure.

Metastable argon atom kinetics in a low-pressure capacitively coupled radio frequency discharge

The kinetics of excited atoms in a low-pressure argon capacitively coupled plasma source are investigated by an extended particle-in-cell/Monte Carlo Collisions simulation code coupled with a diffusion-reaction-radiation code which considers a large number of excited states of Ar atoms. The spatial density distribution of Ar atoms in the 1s5 state within the electrode gap and the gas temperature are also determined experimentally using tunable diode laser absorption spectroscopy. Processes involving the excited states, especially the four lower-lying 1s states are found to have significant effects on the ionization balance of the discharge. The level of agreement achieved between the computational and experimental results indicates that the discharge model is reasonably accurate and the computations based on this model allow the identification of the populating and de-populating processes of the excited states.

Formation mechanisms of striations in a filamentary dielectric barrier discharge in atmospheric-pressure argon

Formation mechanisms of striations along the discharge channel of a single-filament dielectric barrier discharge (DBD) in argon at atmospheric pressure are investigated by means of a time-dependent, spatially two-dimensional fluid-Poisson model. The model is applied to a one-sided DBD arrangement with a 1.5 mm gap using a sinusoidal high voltage at the powered metal electrode. The discharge conditions are chosen to mimic experimental conditions for which striations have been observed. It is found that the striations form in both half-periods during the transient glow phase, which follows the streamer breakdown phase. The modelling results show that the distinct striated structures feature local spatial maxima and minima in charged and excited particle densities, which are more pronounced during the positive polarity. Their formation is explained by a repetitive stepwise ionisation of metastable argon atoms and ionisation of excimers, causing a disturbance of the spatial distribution of charge carriers along the discharge channel. The results emphasise the importance of excited states and stepwise ionisation processes on the formation of repetitive ionisation waves, eventually leading to striations along the discharge channel.

Role of excimer formation and induced photoemission on the Ar metastable kinetics in atmospheric pressure Ar–NH3 dielectric barrier discharges

Tunable diode laser absorption spectroscopy was used to record the space-and time-resolved number density of argon metastable atoms, Ar(1s3) (Paschen notation), in plane-to-plane dielectric barrier discharges (DBDs) operated in a Penning Ar–NH3 mixture at atmospheric pressure. In both low-frequency (LF 650 V, 50 kHz) discharges and dual LF–radiofrequency (RF 190 V, 5 MHz) discharges operated in α–γ mode, the density of Ar(1s3) revealed a single peak per half-period of the LF voltage, with rise and decay times in the sub-microsecond time scale. These results were compared to the predictions of a 1D fluid model based on continuity and momentum equations for electrons, argon ions (Ar+ and Ar2 +) and excited argon 1s atoms as well electron energy balance equation. Using the scheme commonly reported for Ar-based DBDs in the homogeneous regime, the Ar metastable kinetics exhibited much slower rise and decay times than the ones seen in the experiments. The model was improved by considering the fast creation of Ar2 * excimers through three-body reactions involving Ar(1s) atoms and the rapid loss of Ar2 * by vacuum ultraviolet light emission. In optically thin media for such photons, they can readily reach the dielectric barriers of the DBD electrodes and induce secondary electron emission. It is shown that Ar2 * and photoemission play a significant role not only on the Ar metastable kinetics, but also on the dominant ionization pathways and possible α–γ transition in dual frequency RF–LF discharges.

Pressure broadening of 772.376 and 772.421 nm argon lines and kinetics of argon metastable atoms

Tunable diode laser absorption spectroscopy is used for recording the spectrally mixed profiles of argon 772.376 nm (2p7–1s5) and 772.421 nm (2p2–1s3) lines at different argon background pressures between 0.3 and 1 atmosphere. The absorbing atoms in the Ar*(1s5) and Ar*(1s3) metastable states are produced in a plane-to-plane dielectric barrier glow discharge operating at 20 kHz. The pressure broadening coefficient w and wavelength shift, s of the lines, deduced from the analysis of their variation versus pressure of recorded profiles, are w1=28.3 ± 2.0 pm (14.2 ± 1.0 GHz) and s1=11.5 ± 0.8 pm (-5.8 ± 0.4 GHz) for 772.376 nm line and w2=36.3 ± 0.5 pm (18.2 ± 0.3 GHz) and s2=12.8 ± 0.7 pm (-6.4 ± 0.4 GHz) for 772.4207 nm line, at 1 bar and 300 K. The progressive enhancement with argon pressure of the peak densities ratio [Ar*(1s3)]/[Ar*(1s5)], reaching 0.3 at 1 bar, reveals the importance of Ar-induced collisions for the population transfer from resonant 1s2 to the metasTable 1s3 state. Also, the time variation of Ar*(1 s) densities indicates the presence of a weak excitation channel outside the discharge current pulse initiated by the breakdown. This channel results from the heating of electrons by the electric field seen by the gas in the DBD gap.

Influence of Metastable Argon Atoms and Dust Particles on Gas Discharge Plasma

The model of a DC glow discharge with metastable argon atoms and dust particles based on the Boltzmann equation for the electron energy distribution function (EEDF), dust particle charging, and balance equation for metastable argon atoms is presented. The processes of direct and stepwise electron impact ionization, metastable-metastable collisions, and recombination of electrons and ions on the dust particle surface and discharge tube wall are taken into account. The results show that the densities of metastable argon atoms and dust particles are in strong correlation, and both sufficiently influence gas discharge parameters.

Experiment and modeling of the pulsed lasing in a diode-pumped argon metastable laser

Diode-pumped metastable rare gas lasers are showing their potential for high-energy laser systems with intensive investigations carried out to understand their lasing characteristics. Here, we demonstrated pulsed lasing of a diode-pumped argon metastable laser based on a pulsed DC glow discharge in argon and helium mixtures for different discharge voltages and pressures. A transient rate equation model was established and solved to explain the cause of the pulsed lasing in our experiment. The insufficient production rate of argon metastable atoms (1s5) and the slow transfer rate out of the 1s4 state in the plasma during the lasing process were found to be the key reasons. This work may shed some light on the future design of plasma sources that have higher metastable atom production rates to achieve highly efficient continuous wave lasing.

Formation and evolution of striation plasma in high-pressure argon glow discharge

The self-organized striation phenomenon in the positive column region of glow discharge plasma is a basic problem in gas discharge physics, which involves rich physics such as electron dynamics, transport process, discharge instability and nonlinear phenomenon. It is an important topic in basic physics and practical application. In this work an argon glow discharge striation plasma at high pressure is reported. The electrical, optical and ionization wave propagation characteristics of the striation plasma, and the evolution of the striation plasma with pressure and impurity gas are investigated experimentally. The generation and quenching mechanism of argon striation plasma are analyzed. The results show that the striation length is about 1.5 mm, and decreases with pressure increasing, and the velocity and frequency of the ionization wave are estimated at 1.87 m/s and 1.25 kHz, respectively. The measurement of optical emission spectrum shows that the generation of striation plasma is probably related to the argon metastable atoms. The stepwise ionization process caused by metastable atoms triggers off an ionization instability. The instability propagates in the form of ionization wave, which leads the plasma parameters to be modulated longitudinally, thus, forming an alternating bright and dark striation plasma. The adding of nitrogen can effectively quench metastable argon atoms and change the electron energy distribution function, which destroys the instability conditions of the plasma, therefore, the striation plasma disappears. This work provides a new insight into the understanding of the formation and annihilation mechanism of glow discharge striation plasma at high pressure.

Cross-comparison of diagnostic and 0D modeling of a micro-hollow cathode discharge in the stationary regime in an Ar/N2 gas mixture

A micro-hollow cathode discharge (MHCD) operated in Ar/N2 gas mixture, working in the normal regime, was studied both experimentally and with a 0D (volume-averaged) model in this work. This source provides high electron densities (up to 1015 cm−3) at low injected power (1 W). To understand the mechanisms leading to the production of N atoms, the densities of electrons, N atoms and argon metastable atoms (Ar*) were monitored over a wide range of experimental conditions. Electrons, N atoms and Ar* densities were probed by means of optical emission spectroscopy, vacuum ultra violet Fourier transform spectroscopy and tunable diode laser absorption spectroscopy, respectively. Measurements showed that using a smaller hole diameter enables to work with less injected power, while increasing the power density inside the hole and, subsequently, increasing the densities of excited species. Varying the percentage of N2 in the gas mixture highlighted that, up to 80%, the density of N atoms increases although the dissociation rate drops. Looking at the processes involved in the production of N atoms with the help of the 0D model, we found that at very low N2 fraction, N atoms are mostly produced through dissociative electron-ion recombination. However, adding more N2 decreases drastically the electron density. The density of N atoms does not drop thanks to the contribution of Ar* atoms, which are the main species dissociating N2 between 5% and 55% of N2 in the gas mixture. A reasonable agreement is found between the experiments and the model results. This study shows that, with this MHCD, it is possible to significantly modify the production of N atoms when modifying the physical parameters, making it particularly relevant for applications requiring a N atoms source, such as nitride deposition.

A linewidth narrowed diode laser for metastable Argon atom pumping

A volume Bragg grating-based external cavity diode laser array produced more than 100 W output power for metastable argon atoms pumping centered at 811.53 nm with a 0.15 nm spectral linewidth (FWHM). The device has only 4.6 % loss compared to the free-running case operating at full power and a spectral contrast of 40 dB. The emission spectrum can be precisely tuned to 811.53 nm and has an over 200 pm tuning range. Pump absorption was demonstrated by using a repetitively pulsed glow discharge operating with 620 Torr Ar/He mixture, and an average absorption of 24 % of the pump radiation was realized.

Characterising a tunable, pulsed atomic beam using matter-wave interferometry

We describe the creation and characterisation of a velocity tunable, spin-polarized beam of slow metastable argon atoms. We show that the beam velocity can be determined with a precision below 1% using matter-wave interferometry. The profile of the interference pattern was also used to determine the velocity spread of the beam, as well as the Van der Waals (VdW) co-efficient for the interaction between the metastable atoms and the multi-slit silicon nitride grating. The VdW co-efficient was determined to be C 3 = 1.84 ± 0.17 a.u., in good agreement with values derived from spectroscopic data. Finally, the spin polarization of the beam produced during acceleration of the beam was also measured, demonstrating a spatially uniform spin polarization of 96% in the m = +2 state.

Current-voltage characteristics of a magnetron with a hot titanium target in chemically active environments

This work analyzes the results of original experimental studies focused on the discharge current-voltage (I-V) characteristics of a magnetron with a hot titanium target in environments containing reactive gases. Our work presents a fundamental study of a direct current gas discharge with a hot cathode in chemically active environments with a magnetic field applied. The results obtained are useful for practical applications in the development of technologies for the deposition of oxide, nitride and other films. It is found that regardless of the gas environment composition, the I-V characteristics reflect the physical processes in the discharge with a hot target. At low currents, there is a competition of two processes on the target surface, namely, the formation of a titanium compound film and its removal due to sputtering with argon ions and evaporation. A distinctive feature of the hot target discharge is the influence of the thermionic emission of the target, the quenching of metastable argon atoms by atoms and molecules of reactive gases and possibly an additional increase in the rarefaction of the gas near the target due to its heating not only by sputtered atoms but also by evaporated ones on the I-V characteristics. Furthermore, calculations using the Richardson–Dushman equation show that the presence of reactive gases entails a change in the electron work function of titanium.

Spatio-temporal dynamics of a nanosecond pulsed microwave plasma ignited by time reversal

In the present paper, a detailed investigation of the spatio-temporal dynamics of the recently developed time reversal microwave plasma source is presented. This novel source allows to ignite a plasma at a desired location in a reverberant cavity by focusing the electromagnetic energy in time and space. An important feature is the possibility to control the plasma position only by changing the input microwave waveform. The source is operated in a repetitive pulsed mode with very low duty cycle (typically 5 × 10−2%). Nanosecond pulses have rise time lower than 1 ns. The generated plasmas have typical sizes in the millimeter range and are observed using imaging for dozens of nanoseconds. The plasma behavior is investigated for different pressures and repetition frequencies. A strong dependence is observed between each discharge pulse suggesting the existence of an important memory effect. The latter is probably due to argon metastable atoms and/or residual charges remaining in the post-discharge and allowing the next breakdown to occur at a moderate electric field.

Argon/dust and pure argon pulsed plasmas explored using a spatially-averaged model

The properties (densities of electrons and metastable argon atoms, effective electron temperature and dust charge) of argon/dust and pure argon pulsed plasmas are studied using a spatially-averaged model. The calculated time-dependencies for the densities of electrons and metastable atoms are compared with the experimental measurements and are found to be in a good qualitative agreement. It is analyzed how the plasma properties depend on the shape of the electron energy probability function (EEPF), the pulsing frequency and the duty cycle for both dust-free and dusty plasma. The analysis reveals that the agreement between theory and experiment is better with Druyvesteyn EEPF than the Maxwellian EEPF. Further, the variation in the pulsing frequency ν p differently affects the metastable density n m in a dust-free and in a dusty plasma. For large ν p, the metastable density in the dust-free pulsed plasma is larger than in the continuous-wave (CW) discharge, while the opposite is obtained in the presence of dust particles. This difference probably arises because of faster variation in the effective electron temperature in the dusty plasma due to collection of electrons by dust particles. Our calculations also show that dust particles may affect the behavior of electron density in the beginning of the on-period due to an enhancement in electron collection by dust particles.

Temperature measurements of the background gas and metastable argon atoms in N2/Ar nanosecond repetitively pulsed discharges

Characterization of plasma properties that underpin kinetic processes in nanosecond repetitively pulsed discharges (NRPDs) is necessary to understand and manipulate the behavior of these discharges for a wide variety of applications. Here, the neutral gas temperature in N2 and 50% N2/50% Ar NRPDs during the discharge is determined by characterizing the rotational temperature of rovibrational spectra from the N2 2nd positive system. At the conditions investigated, it is shown that the timescale for rotational–translational relaxation is shorter than the effective lifetime of the N2(C) state, thereby, rendering the rotational temperature measurements a reasonable representation of the background gas temperature. The measurements show that the translational temperature of ground state nitrogen molecules does not increase significantly above ambient temperature during the discharge generated at a constant pressure of 20 Torr, 10 kHz pulse repetition frequency, and pulse energy of 50 μJ. An absorption based detection technique with a 2 ns time-resolution used to measure the translational temperature history of the metastable argon atoms (Ar(1s3)) in the N2/Ar NRPDs shows that the Ar(1s3) atoms and the neutral background gas are in thermal non-equilibrium during the discharge. Furthermore, the addition of nitrogen gas is shown to significantly reduce the translational energy enhancement of the metastable argon atoms produced in the N2/Ar discharges compared to that in pure argon discharges.

Influence of metastable atoms in low pressure magnetized radio-frequency argon discharges

One-dimensional particle-in-cell simulations with Monte Carlo collisions are used to investigate the influence of metastable atoms in low pressure radio-frequency argon discharges with magnetic fields ranging from 0 G to 60 G. Two metastable levels of argon species are included and tracked as particles, enabling multistep ionization and metastable pooling. At low magnetic fields, the metastable argon atoms have little influence on the discharge. At higher magnetic fields, the electron density increases and the electron temperature decreases at the center of the discharge with the inclusion of metastable atoms. The reduction in electron temperature is attributed to the depletion of energetic electrons due to low-energy-threshold reactions related to the metastable atoms. The reduced electron temperature leads to a reduction in the total ionization rate, albeit the contribution of multistep ionization increases with the magnetic field. The suppression of plasma diffusion at low electron temperatures plays a greater role than the reduction in ionization rate, resulting in a higher electron density. A transformation in metastable density profiles from parabolic to saddle type is observed with the increase in the magnetic field. Metastable atoms may play an important role in modulating the electron temperature in low pressure magnetized discharges.