Abnormal transition of the electron energy distribution with excitation of the second harmonic in low-pressure radio-frequency capacitively coupled plasmas

The self-excited second harmonic in radio-frequency capacitively coupled plasma was significantly enhanced by adjusting the external variable capacitor.At a lower pressure of 3 Pa,the excitation of the second harmonic caused an abnormal transition of the electron energy probability function,resulting in abrupt changes in the electron density and temperature.Such changes in the electron energy probability function as well as the electron density and temperature were not observed at the higher pressure of 16 Pa under similar harmonic changes.The phenomena are related to the influence of the second harmonic on stochastic heating,which is determined by both amplitude and the relative phase of the harmonics.The results suggest that the self-excited high-order harmonics must be considered in practical applications of lowpressure radio-frequency capacitively coupled plasmas.

Plasma density enhancement in radio-frequency hollow electrode discharge

The plasma density enhancement outside hollow electrodes in capacitively coupled radio-frequency(RF) discharges is investigated by a two-dimensional(2D) particle-in-cell/Monte-Carlo collision(PIC/MCC) model. Results show that plasma exists inside the cavity when the sheath inside the hollow electrode hole is fully collapsed, which is an essential condition for the plasma density enhancement outside hollow electrodes. In addition, the existence of the electron density peak at the orifice is generated via the hollow cathode effect(HCE), which plays an important role in the density enhancement. It is also found that the radial width of bulk plasma at the orifice affects the magnitude of the density enhancement, and narrow radial plasma bulk width at the orifice is not beneficial to obtain high-density plasma outside hollow electrodes.Higher electron density at the orifice, combined with larger radial plasma bulk width at the orifice,causes higher electron density outside hollow electrodes. The results also imply that the HCE strength inside the cavity cannot be determined by the magnitude of the electron density outside hollow electrodes.

Applications of cold atmospheric plasmas(CAPs)in agriculture:a brief review and the novel development of a radio-frequency CAP jet generator for plant mutation

Cold atmospheric plasmas(CAPs)have shown great applicability in agriculture.Many kinds of CAP sources have been studied in agricultural applications to promote plant growth and cure plant diseases.We briefly review the state-of-the-art stimulating effects of atmospheric-pressure dielectricbarrier-discharge(AP-DBD)plasmas,after the direct or indirect treatment of plants for growth promotion and disease control.We then discuss the special demands on the characteristics of the CAP sources for their applications in plant mutation breeding.An atmospheric and room temperature plasma(ARTP)jet generator with a large plasma irradiation area,a high enough concentration of chemically reactive species and a low gas temperature is designed for direct plant mutagenesis.Experimental measurements of the electrical,thermal and optical features of the ARTP generator are conducted.Then,an ARTP-P(ARTP for plant mutagenesis)mutation breeding machine is developed,and a typical case of plant mutation breeding by the ARTP-P mutation machine is presented using Coreopsis tinctoria Nutt.seeds.Physical and agricultural experiments show that the newly-developed ARTP-P mutation breeding machine with a large irradiation area can generate uniform CAP jets with high concentrations of chemically reactive species and mild gas temperatures,and have signiflcant mutagenesis effects on the Coreopsis tinctoria Nutt.seeds.The ARTP-P mutation breeding machine may provide a platform for systematic studies on mutation mechanisms and results for various plant seeds under different operating conditions in future research.

Characteristics of a radio-frequency cold atmospheric plasma jet produced with a hybrid cross-linear-field electrode configuration

Cold atmospheric plasma(CAP)jet has wide applications in various fields including advanced materials synthesis and modifications,biomedicine,environmental protection and energy saving,etc.Appropriate control on the volume,temperature and chemically reactive species concentratio ns of the CAP jet is of great importance in actual applications.In this paper,an radio-frequency atmospheric-pressure glow discharge(RF-APGD)plasma generator with a hybrid cross-linear-field electrode configuration is proposed.The experimental results show that,with the aid of the copper mesh located at the downstream of the traditional co-axial-type plasma generator with a cross-field electrode configuration,a linear field between the inner powered electrode of the traditional plasma generator and the copper mesh can be established.This linerfield can,to some extent,enhance the discharges at the upstream of the copper mesh,resulting in small increments(all less than 12.5%)of the species emission intensities,electron excitation temperatures and gas temperatures by keeping other parameters being unchanged.And due to the intrinsic transparent and conducting features of the grounded copper mesh to the gas flowing,electric current and heat flux of the plasma plumes,a plasma region with higher concentrations of chemically reactive species and larger plasma plume diameters is obtained at the downstream of the grounded copper mesh on the same level of the gas temperature and electron excitation temperature compared to those of the plasma free jet.In addition,the charged particle number densities at the same downstream axial location of the grounded copper mesh decrease significantly compared to those of the plasma free jet.This means that the copper mesh is also,to some extent,helpful for separating the chemically reactive neutral species from the charged particles in side a plasma environment.The preceding results indicate that the cross-linear-field electrode configuration of the plasma generator is an effective approach for tuning the characteristics of the RF-APGD plasma jet in order to obtain an appropriate combination of the plasma jet properties with higher chemically reactive species concentrations,especially relative higher number densities of neutral species,larger plasma volumes and lower gas temperatures.

Study of electron-extraction characteristics of an inductively coupled radio-frequency plasma neutralizer

Inductively coupled radio-frequency（RF） plasma neutralizer（RPN） is an insert-free device that can be employed as an electron source in electric propulsion applications.Electron-extraction characteristics of the RPN are related to the bulk plasma parameters and the device＇s geometry.Therefore,the effects of different electron-extraction apertures and operational parameters upon the electron-extraction characteristics are investigated according to the global nonambipolar flow and sheath model.Moreover,these models can also be used to explain why the electron-extraction characteristics of the RPN strongly depend upon the formation of the anode spot.During the experimental study,two types of anode spots are observed.Each of them has unique characteristics of electron extraction.Moreover,the hysteresis of an anode spot is observed by changing the xenon volume-flow rates or the bias voltages.In addition,the rapid ignited method,gas-utilization factor,electron-extraction cost and other factors that need to be considered in the design of the RPN are also discussed.

Simulations of Ion Behaviors in a Photoresist Trench During Plasma Etching Driven by a Radio-Frequency Source

Ion＇s behavior plays an important role in plasma etching processes and is determined by the local electric potential in the etched trenches. In this study, with the trench powered by a radio frequency （rf） source, the Laplace equation is solved to obtain the electric potential. The ion trajectories and the ion energy distribution （IED） at the bottom of the trench are obtained self-consistently by tracking the ions in the trench. The results show that the aspect ratio of depth- to-width of the photoresist trench and the voltage amplitude of the rf source applied to the electrode are important parameters. The larger the aspect ratio and the smaller the amplitude are, the more ions hit the sidewalls, which results in a notching phenomenon. Meanwhile, there are a higher high-energy peak and a lower low-energy peak in the IED with the increase in aspect ratio.

Experimental Investigation on Electromagnetic Attenuation by Low Pressure Radio-Frequency Plasma for Cavity Structure

This paper reports on an experiment designed to test electromagnetic（EM）attenuation by radio-frequency（RF）plasma for cavity structures.A plasma reactor,in the shape of a hollow cylinder,filled with argon gas at low pressure,driven by a RF power source,was produced by wave-transmitting material.The detailed attenuations of EM waves were investigated under different conditions：the incident frequency is 1-4 GHz,the RF power supply is 13.56 MHz and1.6（-3） k W,and the argon pressure is 75-200 Pa.The experimental results indicate that 5-15 d B return loss can be obtained.From a first estimation,the electron density in the experiment is approximately（1.5-2.2）×1016m（-3）and the collision frequency is about 11（-3）0 GHz.The return loss of EM waves was calculated using a finite-difference time-domain（FDTD）method and it was found that it has a similar development with measurement.It can be confirmed that RF plasma is useful in the stealth of cavity structures such as jet-engine inlet.

Gas pressure effect on plasma transport in a magnetic-filtered radio-frequency plasma source

Volume negative ion production relies on a magnetic filter(MF),where the plasma downstream of the MF is characterized by a strip-like pattern that consists of a bright and dense plasma region.In this work,we study,in a radio-frequency plasma source,the effects of operating pressure on this strip.This investigation,conducted using a Langmuir probe,shows that the plasma uniformity might be controlled through the gas pressure.Moreover,the operating pressure determines on which hemi-cylinder(side of magnetic field lines)the strip forms.This side inversion of the high-density plasma hemi-cylinder is due to an inversion of an ambipolar electric field that changes the E?×?B drift direction.

Effects of power on ion behaviors in radio-frequency magnetron sputtering of indium tin oxide(ITO)

This study delves into ion behavior at the substrate position within RF magnetron discharges utilizing an indium tin oxide(ITO)target.The positive ion energies exhibit an upward trajectory with increasing RF power,attributed to heightened plasma potential and initial emergent energy.Simultaneously,the positive ion flux escalates owing to amplified sputtering rates and electron density.Conversely,negative ions exhibit broad ion energy distribution functions(IEDFs)characterized by multiple peaks.These patterns are clarified by a combination of radiofrequency oscillation of cathode voltage and plasma potential,alongside ion transport time.This elucidation finds validation in a one-dimensional model encompassing the initial ion energy.At higher RF power,negative ions surpassing 100 e V escalate in both flux and energy,posing a potential risk of sputtering damages to ITO layers.

Influence of Discharge Parameters on Tuned Substrate Self-Bias in an Radio-Frequency Inductively Coupled Plasma

The tuned substrate self-bias in an rf inductively coupled plasma source is controlled by means of varying the impedance of an external LC network inserted between the substrate and the ground. The influencing parameters such as the substrate axial position, different coupling coils and inserted resistance are experimentally studied. To get a better understanding of the experimental results, the axial distributions of the plasma density, electron temperature and plasma potential are measured with an rf compensated Langmuir probe; the coil rf peak-to-peak voltage is measured with a high voltage probe. As in the case of changing discharge power, it is found that continuity, instability and bi-stability of the tuned substrate bias can be obtained by means of changing the substrate axial position in the plasma source or the inserted resistance. Additionally, continuity can not transit directly into bi-stability, but evolves via instability. The inductance of the coupling coil has a substantial effect on the magnitude and the property of the tuned substrate bias.

Fabrication and characterization of He-charged ODS-FeCrNi films deposited by a radio-frequency plasma magnetron sputtering technique

He-charged oxide dispersion strengthened （ODS） FeCrNi fills were prepared by a radiofrequency （RF） plasma magnetron sputtering method in a He and Ar mixed atmosphere at 150 °C.As a comparison,He-charged FeCrNi films were also fabricated at the same conditions through direct current （DC） plasma magnetron sputtering.The doping of He atoms and Y2O3 in the FeCrNi fills was realized by the high backscattered rate of He ions and Y2O3/FeCrNi composite target sputtering method,respectively.Inductive coupled plasma （ICP） and x-ray photoelectron spectroscopy （XPS） analysis confirmed the existence of Y2O3 in FeCrNi fills,and Y2O3 content hardly changed with sputtering He/Ar ratio.Cross-sectional scanning electron microscopy （SEM） shows that the FeCrNi films were composed of dense colunnarnanocrystallines and the thickness of the films was obviously dependent on He/Ar ratio.Nanoindentation measurements revealed that the FeCrNi films fabricated through DC/RF plasma magnetron sputtering methods exhibited similar hardness values at each He/Ar ratio,while the dispersion of Y2O3 apparently increased the hardness of the fills.Elastic recoil detection （ERD） showed that DC/RF magnetron sputtered FeCrNi films contained similar He amounts （～17 at.%）.Compared with the minimal change of He level with depth in DC-sputtered films,the He amount decreases gradually in depth in the RF-sputtered fills.The Y2O3-doped FeCrNi films were shown to exhibit much smaller amounts of He owing to the lower backscattering possibility of Y2O3 and the inhibition effect of nano-sized Y2O3 particles on the He element.

Enhanced removal of X-ray-induced carbon contamination using radio-frequency Ar/H_2 plasma

Removal of X-ray-induced carbon contamination on beamline optics was studied using radio-frequency plasma with an argon/hydrogen(Ar/H_2) mixture. Experiments demonstrated that the carbon removal rate with Ar/H_2 plasma was higher than that with pure hydrogen or argon. The possible mechanism for this enhanced removal was discussed. The key working parameters for Ar/H_2 plasma removal were determined, including the optimal vacuum pressure, gas mixing ratio, and source power. The optimal process was performed on a carbon-coated multilayer, and the reflectivity was recovered.

Aerodynamic actuation characteristics of radio-frequency discharge plasma and control of supersonic flow

In this paper, aerodynamic actuation characteristics of radio-frequency(RF) discharge plasma are studied and a method is proposed for shock wave control based on RF discharge. Under the static condition, a RF diffuse glow discharge can be observed; under the supersonic inflow, the plasma is blown downstream but remains continuous and stable.Time-resolved schlieren is used for flow field visualization. It is found that RF discharge not only leads to continuous energy deposition on the electrode surface but also induces a compression wave. Under the supersonic inflow condition, a weak oblique shock wave is induced by discharge. Experimental results of the shock wave control indicate that the applied actuation can disperse the bottom structure of the ramp-induced oblique shock wave, which is also observed in the extracted shock wave structure after image processing. More importantly, this control effect can be maintained steadily due to the continuous high-frequency(MHz) discharge. Finally, correlations for schlieren images and numerical simulations are employed to further explore the flow control mechanism. It is observed that the vortex in the boundary layer increases after the application of actuation, meaning that the boundary layer in the downstream of the actuation position is thickened. This is equivalent to covering a layer of low-density smooth wall around the compression corner and on the ramp surface, thereby weakening the compressibility at the compression corner. Our results demonstrate the ability of RF plasma aerodynamic actuation to control the supersonic airflow.

Influences of the inner diameter of the solid shielding tube on the characteristics of the radio-frequency cold atmospheric plasma jet

Active control of the local environment of the cold atmospheric plasma(CAP) jet is of great importance in actual applications since the CAP operates in an open atmosphere with the inevitable entrainment of the surrounding cold air. In this paper, the solid shielding effects of the cylindrical quartz tubes with different inner diameters on the characteristics of the CAP jets driven by a radio-frequency(RF) power supply are studied experimentally. The experimental results show that the total length of the shielded plasma jet can be increased significantly by an appropriate combination of the quartz tube inner diameter and that of the plasma generator nozzle exit with other parameters being unchanged. This phenomenon may be qualitatively attributed to the loss of diffusion of the charged particles in the radial direction under different inner diameters of the quartz tubes. Compared with the plasma free jet, the plasma shielding jet is produced with optimized parameters including longer plasma jet length, higher concentrations of chemically reactive species, higher rotational, vibrational, and electron excitation temperatures when the inner diameters of the solid shielding tube and the generator nozzle exit are the same. A maximum plasma jet length of 52.0 cm is obtained in contrast to that of 5.0 cm of the plasma free jet in this study. The experimental results indicate that the solid shielding effect provides a new method for the active control of the local environment of the RF-CAP jet operating in an open atmosphere.

Electronic dynamic behavior in inductively coupled plasmas with radio-frequency bias

The inflexion point of electron density and effective electron temperature curves versus radio-frequency （RF） bias voltage is observed in the H mode of inductively coupled plasmas （ICPs）. The electron energy probability function （EEPF） evolves first from a Maxwellian to a Druyvesteyn-like distribution, and then to a Maxwellian distribution again as the RF bias voltage increases. This can be explained by the interaction of two distinct bias-induced mechanisms, that is： bias- induced electron heating and bias-induced ion acceleration loss and the decrease of the effective discharge volume due to the sheath expansion. Furthermore, the trend of electron density is verified by a fluid model combined with a sheath module.

Radio-frequency compressed electron pulse-width characterization by cross-correlation between electron bunches and laser-induced plasma

A method is proposed to determine the temporal width of high-brightness radio-frequency compressed electron pulses based on cross-correlation technique involving electron bunches and laser-induced plasma. The temporal evolution of 2-dimensional transverse profile of ultrafast electron bunches repelled by the formed transient electric field of laser-induced plasma on a silver needle is investigated, and the pulse-width can be obtained by analyzing these time-dependent images.This approach can characterize radio-frequency compressed ultrafast electron bunches with picosecond or sub-picosecond timescale and up to 105 electron numbers.

Observation of the Emission Spectra of an Atmospheric Pressure Radio-frequency Plasma Jet

An atmospheric pressure plasma jet （APPJ） using radio-frequency （13.56 MHz） power has been developed to produce homogeneous glow discharge at low temperature. With optical emission spectroscopy, we observed the excited species （atomic helium, atomic oxygen and metastable oxygen） generated in this APPJ and their dependence on gas composition ratio and RF power. O and O2（b1∑g^＋） are found in the effluent outside the jet by measuring the emission spectra of effluent perpendicular to the jet. An interesting phenomenon is found that there is an abnormal increase of O emission intensity （777.4 nm） between 10 mm and 40 mm away from the nozzle. This observation result is very helpful in practical operation.

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.

Numerical Simulation of Hydrogen Dilution Effects on Deposition of Silicon Film at Atmospheric Pressure Radio-Frequency Argon Silane Plasma

Based on the one-dimensional fluid model, the characteristics of homogeneous discharges with hydrogen diluted silane and argon at atmospheric pressure are numerically investigated. The primary processes of excitation and ionization and sixteen reactions of radicals with radicals in silane/hydrogen/argon discharges are considered. The effects of hydrogen dilution on the densities of species （e, H, SiH3^＋, SiH3^-, SiH3,） are analyzed. The simulation results show that the highest densities of e, Si113^＋, H, SiH3^-, SiH3 correspond to the optimal dilution concentration of H2. The deposition rate of μc-Si：H film depends on the SiH3 concentration, and atomic hydrogen in the plasma is found to play an important role in the crystallization fraction of the deposited films. This model explains the effects of H2 dilution on the deposition rate and crystallized fraction of μc-Si：H film growth.

A Numerical Study on Tuned Substrate Self-Bias in a Radio-Frequency Inductively Coupled Plasma

The tuned substrate self-bias in a radio-frequency inductively coupled plasma is controlled by varying the impedance of an external tuning LCR （inductor, capacitor and resistor） network inserted between the substrate and the ground. In experiments, it was found that the variation of the tuned substrate self-bias with the tuning capacitance demonstrated three features, namely, continuity, instability and bistability. In this paper, a numerical study is focused on the elucidation of the physical mechanisms underlying continuity and bistability. For the sake of simplicity and feasibility to include the key factors influencing the tuned substrate bias, the tedious calculation of inductive-coupling to obtain the plasma density axtd electron temperature is omitted, and discussion of the tuned substrate self-bias is made under the prescribed plasma density and electron temperature. On the other hand, the parameters influencing capacitive- coupling are retained in modeling the system with an equivalent circuit. It is found that multi-stable state appears when one of the parameters, such as the resistance in LCR, substrate area and plasma density, decreased to its critical value, or the rf voltage or electron temperature increased to the critical value individually. In the reverse cases, the tuned substrate self-bias varies continuously with the tuning capacitance.