Study on an Atmospheric Pressure Plasma Jet and its Application in Etching Photo-Resistant Materials

An atmospheric pressure radio-frequency plasma jet that can eject cold plasma has been developed. In this paper, the configuration of this type of plasma jet is illustrated and its discharge characteristics curves are studied with a current and a voltage probe. A thermal couple is used to measure the temperature distribution along the axis of the jet stream. The temperature distribution curve is generated for the He/O2 jet stream at the discharge power of 150 W. This jet can etch the photo-resistant material at an average rate of 100 nm/min on the surface of silicon wafers at a right angle.     

Characteristics of Low Power CH4/Air Atmospheric Pressure Plasma Jet

A low power atmospheric pressure plasma jet driven by a 24 kHz AC power source and operated with a CH_4/air gas mixture has been investigated by optical emission spectrometer.The plasma parameters including the electron excitation temperature,vibrational temperature and rotational temperature of the plasma jet at different discharge powers are diagnosed based on the assumption that the kinetic energy of the species obeys the Boltzmann distribution.The electron density at different power is also investigated by H_β Stark broadening.The results show that the plasma source works under non-equilibrium conditions.It is also found that the vibrational temperature and rotational temperature increase with discharge power,whereas the electron excitation temperature seems to have a downward trend.The electron density increases from 0.8×10~(21) m~(-3) to 1.1×10~(21)m~(-3) when the discharge power increases from 53 W to 94 W.     

Diagnosis of Electron, Vibrational and Rotational Temperatures in an Ar/N2 Shock Plasma Jet Produced by a Low Pressure DC Cascade Arc Discharge

In this paper,a low pressure Ar/N2 shock plasma jet with clearly multicycle alternating zones produced by a DC cascade arc discharge has been investigated by an emission spectral method combined with Abel inversion analysis.Plasma emission intensity,electron,vibrational and rotational temperatures of the shock plasma have been measured in the expansion and compression zones.The results indicate that the ranges of the measured electron temperature,vibrational temperature and rotational temperature are 1.1 eV to 1.6 eV,0.2 eV to 0.7 eV and 0.19 eV to 0.22 eV,respectively,and it is found for the first time that the vibrational and rotational temperatures increase while the electron temperature decreases in the compression zones.The electron temperature departs from the vibrational and the rotational temperatures due to non-equilibrium plasma efects.Electrons and heavy particles could not completely exchange energy via collisions in the shock plasma jet under the low pressure of 620 Pa or so.     

Experimental Study of the Unsteady Actuation Effect on Induced Flow Characteristics in DBD Plasma Actuators

The main aim of this paper is to investigate unsteady actuation effects on the operation of dielectric barrier discharge(DBD) plasma actuators and to study induced flow characteristics of steady and unsteady actuators in quiescent air.The parameters affecting the operation of unsteady plasma actuators were experimentally measured and compared with the ones for steady actuators.The effects of excitation frequency and duty cycle on the induced flow pattern properties were studied by means of hot-wire anemometers,and the smoke visualization method was also used.It was observed that the current and the mean induced velocity linearly increase with increasing duty cycle while they are not sensitive to excitation frequency.Furthermore,with increasing excitation frequency,the magnitude of vortices shedding from the actuator decreases while their frequency increases.Nevertheless,when the excitation frequency grows beyond a certain level,the induced flow downstream of the actuator behaves as a steady flow.However,the results for steady actuators show that by increasing the applied voltage and carrier frequency,the velocity of the induced flow first increases and then decreases with actuator saturation and the onset of the emission of streaky glow discharge.     

Comparison of Sterilizing Effect of Nonequilibrium Atmospheric-Pressure He/O2 and Ar/O2 Plasma Jets

The sterilizing effect of the non-equilibrium atmospheric pressure plasma jet by applying it to the Bacillus subtilis spores is invesigated. A stable glow discharge in argon or helium gas fed with active gas （oxygen）, was generated in the coaxial cylindrical reactor powered by the radio-frequency power supply at atmospheric pressure. The experimental results indicated that the efficiency of killing spores by making use of an Ar/O2 plasma jet was much better than with a He/O2 plasma jet. The decimal reduction value of Ar/O2 and He/O2 plasma jets under the same experimental conditions was 4.5 seconds and 125 seconds, respectively. It was found that there exists an optimum oxygen concentration for a certain input power, at which the sterilization efficiency reaches a maximum value. It is believed that the oxygen radicals are generated most efficiently under this optimum condition.     

Discharge Characteristics of an Atmospheric Pressure Argon Plasma Jet Generated with Screw Ring-Ring Electrodes in Surface Dielectric Barrier Discharge

An atmospheric-pressure argon plasma jet with screw ring-ring electrodes in surface dielectric barrier discharge is generated by a sinusoidal excitation voltage at 8 kHz. The discharge characteristics, such as rotational and vibrational temperature of nitrogen, electronic excitation temperature, oxygen atomic density, nitrogen molecular density, and average electron density, are estimated. It is found that the rotational temperature of nitrogen is in the range of 352 ～ 392 K by comparing the simulated spectrum with the measured spectrum at the C3Πu→ B3Πg (△ν = 2) band transition, the electronic excitation temperature is found to be in the range of 3127 ～ 3230 K by using the Boltzmann plot method, the oxygen atomic and nitrogen molecular density are of the order of magnitude of 1016 cm-3 by the actinometry method, and the average electron density is of the order of magnitude of 1012 cm-3 by the energy balance equation. Besides, the effective power, conduction, and displacement current are measured during the discharge.     

Investigation of E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma

In this paper, E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%–100%), operating pressure(1 Pa, 5 Pa, 10 Pa and 50 Pa), and radio frequency(RF) power(5–100 W). An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study. Owing to the lower ionization potential and higher collision cross-section of argon, when its fraction in the discharge is increased, the mode transition occurs at lower RF power; i.e. for 0% argon and1 Pa pressure, the threshold power of the E–H mode transition is 65 W, which reduces to 20 W when the argon fraction is increased. The electron density increases with the argon fraction at afixed pressure, whereas the temperature decreases with the argon fraction. The relaxation length of the low-energy electrons increases, and decreases for high-energy electrons with argon fraction, due to the Ramseur effect. However, the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path. The electron energy probability function(EEPF) profiles are non-Maxwellian in E-mode, attributable to the nonlocal electron kinetics in this mode; however, they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode. The tail of the measured EEPFs is found to deplete in both E-and H-modes when the argon fraction in the discharge is increased, because argon has a much lower excitation potential(11.5 eV) than neon(16.6 eV).     

An Experimental Study on Atmospheric Pressure Glow Discharge in Different Gases

Usually,the electrical breakdown of dielectric barrier discharge(DBD) at atmospheric pressure leads to a filamentary non-homogeneous discharge,However,it is also possible to obtain a diffuse DBD in homogeneous form,called atmospheric pressure glow discharge(APGD).We obtained a uniform APGD in helium and in the mixture of argon with alcohol,and studied the electrical characteristics of helium APGD.It if found that the relationship between discharge current and source frequency is different depending on the difference in gas gap when the applied voltage is kept constant.The discharge current shows an increasing trend with the increased frequency when gas gap is 0.8cm ,but the discharge current tends to decrease with the increased frequency when the gas gap increases.The discharge current always increases with the increased applied voltage when the source frequency is kept constant.We also observed a glow-like discharge in nitrogen at atmospheric pressure.     

Numerical Study of Fluid Dynamics and Heat Transfer Induced by Plasma Discharges

A numerical investigation is conducted to explore the evolution of a plasma discharge and its interaction with the fluid flow based on a self-consistent fluid model which couples the discharge dynamics with the fluid dynamics.The effects of the applied voltage on the distribution of velocity and temperature in initially static air are parainetrically studied.Furthermore,the spatial structure of plasma discharge and the resulting force contours in streamwise and normal directions are discussed in detail.The result shows that the plasma actuator produces a net force that should always be directed away from the exposed electrode,which results in an ionic wind pushing particles into a jet downstream of the actuator.When the energy added by the plasma is taken into account,the ambient air temperature is increased slightly around the electrode,but the velocity is almost not affected.Therefore it is unlikely that the induced flow is buoyancy driven.For the operating voltages considered in this paper,the maximum induced velocity is found to follow a power law,i.e.,it is proportional to the applied voltage to the 3.5 power.This promises an efficient application in the flow control with plasma actuators.     

Influences of the cold atmospheric plasma jet treatment on the properties of the demineralized dentin surfaces

Improvement of the bonding strength and durability between the dentin surface and the composite resin is a challenging job in dentistry. In this paper, a radio-frequency atmosphericpressure glow discharge(RF-APGD) plasma jet is employed for the treatment of the acid-etched dentin surfaces used for the composite restoration. The properties of the plasma treated dentin surfaces and the resin–dentin interfaces are analyzed using the x-ray photoemission spectroscopy, contact angle goniometer, scanning electron microscope and microtensile tester.The experimental results show that, due to the abundant chemically reactive species existing in the RF-APGD plasma jet under a stable and low energy input operating mode, the contact angle of the plasma-treated dentin surfaces decreases to a stable level with the increase of the atomic percentage of oxygen in the specimens; the formation of the long resin tags in the scattered clusters and the hybrid layers at the resin–dentin interfaces significantly improve the bonding strength and durability. These results indicate that the RF-APGD plasma jet is an effective tool for modifying the chemical properties of the dentin surfaces, and for improving the immediate bonding strength and the durability of the resin-dentin bonding in dentistry.     

Nonequilibrium Atmospheric Pressure Ar/O2 Plasma Jet: Properties and Application to Surface Cleaning

In this study an atmospheric pressure Ar/O_2 plasma jet is generated to study the effects of applied voltage and gas flux rate to the behavior of discharge and the metal surface cleaning.The increase in applied voltage leads to increases of the root mean square(rms) current,the input power and the gas temperature.Furthermore,the optical emission spectra show that the emission intensities of metastable argon and atomic oxygen increase with increasing applied voltage.However,the increase in gas flux rate leads to a reduction of the rms current,the input power and the gas temperature.Furthermore,the emission intensities of metastable argon and atomic oxygen decrease when gas flux rate increases.Contact angles are measured to estimate the cleaning performance,and the results show that the increase of applied voltage can improve the cleaning performance.Nevertheless,the increase of gas flux rate cannot improve the cleaning performance.Contact angles are compared for different input powers and gas flux rates to search for a better understanding of the major mechanism for surface cleaning by plasma jets.     

OES diagnostic of radicals in 33MHz radio-frequency Ar/C_2H_5OH atmospheric pressure plasma jet

Ar/C_2H_5OH plasma jet is generated at atmospheric pressure by 33 MHz radio-frequency power source. This RF excitation frequencies which are higher than 13.56 MHz had rarely been used in atmospheric pressure plasma. The plasma characteristics of ethanol are investigated. The introduction of ethanol leads to the generation of four excited carbonaceous species C, CN, CH and C_2 in plasma, respectively. Optical emission intensities of four carbonaceous species were strengthened with ethanol content increasing in the range of 0-4600 ppm. The ethanol content increase results in all the Ar spectra lines decrease. The reason is that the electron temperature decreases when ethanol content is high. The emission intensity ratios of C/C_2, CN/C_2 and CH/C_2 decrease with the increase of ethanol content, showing that the relative amount of C_2 is increasing by increasing the ethanol flow. The emission intensity ratios of excited species did not change much with the increase of RF power in stable discharge mode.     

Analysis of an Ar plasma jet in a dielectric barrier discharge conjugated with a microsecond pulse

In this work, an Ar plasma jet generated by an AC-microsecond-pulse-driven dielectric barrier discharge reactor, which had two ring-shaped electrodes isolated from the ambient atmosphere by transformer oil, was investigated. By special design of the oil insulation, a chemically active Ar plasma jet along with a safe and stable plasma process as well as low emission of CO and NO_x were successfully achieved. The results indicated that applied voltage and frequency were basic factors influencing the jet temperature, discharge power, and jet length, which increased significantly with the two operating parameters. Meanwhile, gas velocity affected the jet temperature in a reverse direction. In comparison with a He plasma jet, the Ar plasma jet had relatively low jet temperature under the same level of the input parameters, being preferable for bio-applications. The Ar plasma jet has been tested to interact with human skin within 5 min without the perception of burnt skin and electrical shock.     

On the green aurora emission of Ar atmospheric pressure plasma

The Ar atmospheric pressure plasma was found to be an excellent laboratorial source for green aurora emission. However, the characteristic and production mechanism of the green aurora emission of the Ar atmospheric pressure plasma are still not clear. In this work, an Ar plasma in a long glass tube which emits intense green aurora light is investigated. With the long glass tube, it can be concluded that the green aurora emission in the Ar plasma is not owing to the mixture of Ar plasma plume with the surrounding air. It is also found that the green aurora emission often appeared beyond the active electrode when the active electrode is placed at the downstream of the gas flow. The green emission disappears when the traces amount of O₂ or N₂ (about 0.05%–0.07%) is added to Ar. This is because the O₂ molecules deactivate the upper state O(1S), which results in the decrease of the green emission. On the other hand, when N₂ is added, Ar metastable atoms are quenched by N₂, which results in the decrease of O atoms and eventually leads to the decrease of the green emission intensity. The intensity of the green aurora emission increases when the driving voltage frequency increases from 1 to 10 kHz. More importantly, it is found that the green aurora emission is not affected when a grounded stainless steel needle is in contact with the plasma plume. Thus, the green emission is not driven electrically. All these findings are helpful for the understanding of the physics and its applications of atmospheric pressure plasma jet in space physics, laser physics and other application areas.     

Study of spatial and temporal evolution of Ar and F atoms in SF6/Ar microsecond pulsed discharge by optical emission spectroscopy

The study of sulfur hexafluoride (SF₆) discharge is vital for its application in gas-insulated equipment. Direct current partial discharge (PD) may cause SF₆ decomposition, and the decomposed products of SF₆, such as F atoms, play a dominant role in the breakdown of insulation systems. In this study, the PD caused by metal protrusion defects is simulated by a needle-plate electrode using pulsed high voltage in SF₆/Ar mixtures. The spatial and temporal characteristics of SF6/Ar plasma are analyzed by measuring the emission spectra of F and Ar atoms, which are important for understanding the characteristics of PD. The spatial resolved results show that both F and Ar atom spectral intensities increase first from the plate anode to the needle and then decrease under the conditions of a background pressure of 400 Pa, peak voltage of −1000 V, frequency of 2 kHz, pulse width of 60 μs, and electrode gap of 5–9 mm. However, the distribution characteristics of F and Ar are significantly different. The temporal distribution results show that the spectral intensity of Ar decreases first and then increases slowly, while the spectral intensity of F increases slowly for the duration of the pulsed discharge at the electrode gap of 5 mm and the pulse width of 40–80 μs.     

Cleaning of carbon-contaminated optics using O2/Ar plasma

Cleaning of carbon-contaminated beamline optics was studied by RF plasma discharge process using O2/At.Carbon-coated samples were prepared,and through their cleaning processes key parameters were determined,such as the optimal RF output power,mixing rates of O2/Ar,and chamber vacuum.Considerations were made against possible adverse effects in cleaning the beamline optics,such as comparing the roughness of samples before and after cleaning,and possible detrimental kinetic effects on cable insulation.Under the cleaning parameters to clean the beamline optics,the thickness of removed carbon film and the change in beamline photon flux were analyzed.     

Effect of methane content and the oscillating electric field between electrodes on atmospheric Ar/methane plasma jet and DLC coating deposition

In this work, the effects of the methane gas flow and the internal oscillating electric field between electrodes on radio-frequency(RF) atmospheric pressure argon/methane plasma jet and process of diamond-like carbon(DLC) film deposition have been investigated. Properties of RF atmospheric Ar/methane plasma jet such as active species density, length, electron temperature,appearance and ionization process of argon/methane plasma jet are changed due to the changing of methane flow content and electric field vector and its gradient. With increasing methane flow,the formation of C2 hydrocarbon and CH band content is decreased because injected electrical energy to a mixture of Ar/methane gases is insufficient to stabilize the ionization process of methane gas and the electrical-chemical reaction rate is decreased. With shortening the gas gap between two electrodes, electric field strength and its gradient are increased leading to more energy injection to the electron. Electrical-chemical reactions are strengthened leading to increasing the CH band content. These phenomena introduce the Ar/methane plasma jet in different modes causing to deposit the DLC film with different structures and properties. With using quartz glass and alumina ceramic as dielectric barriers tubes, RF atmospheric pressure Ar/methane plasma jet has been used to deposit DLC coating in different modes. Increasing methane content and shortening the gas gap leads to decreasing sp3 bonded content and the quality of the deposited film.     

Study of Non-Thermal DC Arc Plasma of CH_4/Ar at Atmospheric Pressure Using Optical Emission Spectroscopy and Mass Spectrometry

Non-thermal C/H/Ar plasmas are widely applied to carbonaceous material production and processing.In this work,plasma parameters and gaseous species of the atmospheric non-thermal C/H/Ar plasmas produced by an atmospheric-pressure DC arc discharge generator in CH_4/Ar were investigated.The voltage-current characteristics were measured for different CH_4/Ar ratios.Optical emission spectroscopy was employed to analyze the electron excitation temperature,gas temperature and electron density under various discharge conditions.The hydrocarbon molecules produced in the CH_4/Ar plasmas were detected with photoionization mass spectrometry.The optical spectral results demonstrated that the electron excitation temperature was 0.4-1 eV,the gas temperature was 2800-4200 K and the electron density was in the range of(5-20)×10~(15) cm~(-3).The mass spectrum indicated that a variety of unsaturated hydrocarbons(C_2H_4,C_3H_6,C_6H_6,etc.) and several highly unsaturated hydrocarbons(C_4H_2,C_5H_6,etc.) were produced in the non-thermal arc plasmas.