RF Plasma Research Articles

Dependence of plasma dynamics on the strength of spatiotemporal-changing magnetic field in an RF plasma thruster using rotating magnetic field acceleration method

Abstract The rotating magnetic field (RMF) acceleration method is a newly proposed approach to enhance the performance of electrodelessradio frequency (RF)plasma thruster. In the previous study, electron current drivein the azimuthal direction was observed using the RMF method,which resultedin the electromagnetic force in the presence of a magnetic nozzle. To further optimizethe acceleration effect with the RMF method, we investigated the dependenceofspatial profiles of plasmaparameters and the driven current density on the RMF-field strength.We observed a higherazimuthal-currentdensityin plasma compared to the previous campaign.According to spatial electrostatic probe measurements,the ion Mach numberspatially increaseswith the increase in RMF strength. The ion acceleration on thez-axis can result from the presence of spatial convergence of electron pressure due to radial electron transport. Total thrust composed of a static pressure termand electromagnetic force increases with higher RMF strength under the full penetration condition of the RMF. We clarified the strengthening RMF-field contributes to enhancementof the azimuthal current and spatial ion acceleration effect, leading to the thrust increment. These findings, although the thrust performance is not yet at a practical level, hold significant potential for the future optimization of the RMF acceleration method applied to electrodeless RF plasma thrusters.

In Situ Pre-Metallization Cleaning of CoSi2 Contact-Hole Patterns with Optimized Etching Process

We examined how controlling variables in a pre-metallization Ar sputter-etching process for in situ contact-hole cleaning affects the contact-hole profile, etching rate, and substrate damage. By adjusting process parameters, we confirmed that increasing plasma power lowered the DC bias but enhanced the etching rate of SiO2, while increasing RF power raised both, with RF power having a more pronounced effect. Higher Ar flow rate reduced etching uniformity and slightly lowered the DC bias. There was no significant difference in the amount of etching between the oxide film types, but the nitride/oxide selectivity ratio was about 1:2. Physical damage during Ar sputter-etching was closely linked to DC bias. finally, Finally, etching of the Si and CoSi2 sublayers was performed on the device contact hole model. At this time, Si losses of up to about 31.7 Å/s occurred, and the etch speed was strongly affected by the DC bias. By optimizing the RF power and plasma power, we achieved a Si/CoSi2 etch selectivity ratio of about 1:2.

Impact of the surface on He(23S1) and He2(a3Σu+) metastables densities in atmospheric pressure RF plasma

Abstract The dynamic of helium metastable formation along an atmospheric pressure helium plasma channel with different bounding surfaces is analysed. The densities of He(23S1) and He2(a3Σu +) are measured using optical absorption spectroscopy. A simple model for helium metastable creation, quenching by water impurities and destruction upon surface impact, is developed. The model shows a very good agreement if one assumes that surfaces mainly control secondary electron emission, affecting the local heating at the plasma boundary sheath. This, in turn, changes the rate for He(23S1) and He2(a3Σu +) conversion. The helium metastable induced desorption of adsorbed water causes a decay of the metastable density along the plasma channel due to quenching.

Morphology and Wettability of Polystyrene film on QCM Sensor Caused by Oxygen Plasma with DC Bias

Surface wettability and morphology are important aspects of developing QCM sensors for chemical sensors and biosensors. The functionality and morphology of the sensitive coating on the sensor surface affect the sensor sensitivity and selectivity. Various materials and methods to improve the functionality and morphology of the sensor surface have been developed, one of which is plasma. Species and parameters of the generated plasma play an important role in the treatment. One parameter is the DC-bias voltage of RF plasma, which plays an important role in plasma generation and bombardment on the surface being modified. This work shows the effect of the DC bias of the 2 MHz RF plasma with oxygen species. Surface functionality and morphology modification of the polystyrene film on the QCM sensor was done by surface modification using oxygen plasma. Partial etching of the polystyrene film occurred when treated with oxygen plasma generated using 2 MHz with negative DC-bias. CII, CO+, and CO were detected during the exposure of oxygen plasma on polystyrene film, indicating a chemical reaction between the plasma species, oxygen plasma, and the polystyrene. Observation by optical microscope and SEM showed the occurrence of partial etching on the polystyrene film. As there is no masking, the partial etching occurred randomly. The average size of the etched film increased with the DC bias voltage, but the percentage of the etched area was not. The oxygen plasma converts the polystyrene film from hydrophobic to hydrophilic, but hydrophobicity increases with the DC bias voltage. HIGHLIGHTS Polystyrene surface treatment using oxygen plasma can be used to alter the surface hydrophobicity. The DC bias plays important role in the surface modification. High DCV bias gives stronger plasma energy which may etched the polystyrene film. Observation using SEM shows part of the polystyrene film was etched as the DC bias voltage increase. The amount of the etched surface can be measured using optical microscope and processed using ImageJ. Increasing the DC bias result in a wider etched film. GRAPHICAL ABSTRACT

Effect of gas injection pattern on magnetically expanding rf plasma source

Abstract Argon gas is injected from a back plate having either a radial center hole or shower-patterned eight holes into a 13.3-cm-diameter and 25-cm-long radio frequency (rf) plasma source attached to a 43.7-cm-diameter and 65cm-long diffusion chamber under an expanding magnetic field, which resembles the magnetic nozzle rf plasma thruster. The source has a double-turn loop antenna powered by a 13.56 MHz rf generator at a maximum power level of ~2.8 kW in low-pressure argon, providing a plasma density of about 1018 m−3 in the source. A high plasma density and a slightly low electron temperature are obtained for the shower-pattered case in both the source tube and the diffusion chamber, compared with the center hole case, suggesting that the neutral density profile significantly affects the plasma density profile. This result will provide an improvement in the thruster performance by the gas injection pattern.

Effects of Substrate and Annealing Conditions on the Ferroelectric Properties of Non-Doped HfO2 Deposited by RF Plasma Sputter.

In this study, the effect of annealing and substrate conditions on the ferroelectricity of undoped hafnium oxide (HfO2) was analyzed. Hafnium oxide was deposited on various substrates such as platinum, titanium nitride, and silicon (Pt, TiN, Si) through RF magnetron sputtering. Annealing was performed in a nitrogen atmosphere at temperatures ranging from 400 to 600 °C, and the process lasted anywhere from 1 to 30 min. As a result, it was confirmed that the orthorhombic phase, the main cause of ferroelectricity, was dominant after a post-anneal at 600 °C for 30 min. Additionally, it was observed that interface mixing between hafnium oxide and the substrate may degrade ferroelectricity. Accordingly, the highest remanent polarization, measured at 14.24 μC/cm2, was observed with the Pt electrode. This finding was further corroborated by piezo force microscopy and endurance tests, with the results being significant compared to previously reported values. This analysis demonstrates that optimizing substrate and annealing conditions, rather than doping, can enhance the ferroelectricity of hafnium oxide, laying the foundation for the future development of ferroelectric-based transistors.

Influence of pre-treatment with non-thermal atmospheric pressure plasma on bond strength of TP340 titanium-PEEK direct bonding

Direct bonding of a TP340 titanium to PEEK by hot pressing via pre-treatment of non-thermal atmospheric pressure plasma jet has been demonstrated. The plasma irradiation effect on the bonding surface on the bond strength after hot pressing was investigated. The tensile shear strength of TP340-PEEK joined by hot pressing after plasma pre-treatment was measured by comparing specimens bonded using conventional hot pressing and those bonded using adhesives. The plasma treatment to the TP340 side resulted in the formation of TiO2, which is chemically fed to oxide formation due to the irradiation of oxygen radicals generated by the plasma, resulting in a bond strength of less than 1 MPa, similar to the bond strength of the untreated specimens. The plasma irradiation effect on the PEEK side on the bond strength of TP340-PEEK bonded samples was also investigated. The bonding strength was increased by plasma irradiation to PEEK. As the plasma irradiation time was increased, the bonding strength gradually increased to 9.2 MPa, which is about 19 times higher than the bonding strength without plasma irradiation. These results suggest that oxygen radicals in the atmospheric pressure RF plasma jet produced oxygen-containing surface functional groups on the PEEK surface, which increased the strength of the TP340-PEEK direct joining.

Investigation of microstructure, tribological, and corrosion behaviors of TiN@AISI 316 CN processed by rf plasma

In order to enhance the tribological and corrosion properties of AISI 316 substrate, rf plasma carbonitriding was involved to form a carbonitrided layer on AISI 316 base substrate. The deposition of TiN top layer was executed engaging titanium nitride target in argon plasma atmosphere. All the deposition working parameters were reserved to be constant excepting the deposition period that was altered from 4 to 20 min. The effect of TiN top layer thickness on the structure, tribological and electrochemical characteristics of TiN@AISI 316 CN duplex treatment was explored. It was demonstrated, that the duplex coating enhanced the hardness (max: 1740 HV0.025 for TiN (1325nm)@AISI 316 CN sample) and improved the wear and corrosion resistance. Besides, the coefficient of friction is reduced to 0.12–0.2 for TiN@AISI 316 CN duplex treated samples compared with untreated AISI 316 which recorded a value of 0.47.

Laser induced fluorescence using frequency modulated light.

The small signal-to-noise ratio (SNR) of conventional laser induced fluorescence (LIF) measurements using a continuous wave laser, either diode or dye, is typically overcome by amplitude modulating the laser at a specific frequency and then using lock-in amplification to extract the signal from measurement noise. Here, we present LIF measurements of the neutral helium velocity distribution function in an rf plasma using frequency modulated (FM) laser injection. A pulse train of 100% amplitude modulation is generated synthetically with a random sequence of pulse lengths. The FM signal then drives an acoustic optic modulator placed in the path of the injection beam in an LIF measurement. The signal from a fast photomultiplier tube is digitized and cross-correlated with the known modulation signal. The resultant FM-based LIF signal outperforms a conventional lock-in-based LIF measurement on the same plasma in terms of SNR and precision.

Investigation of amorphous-SiC thin film deposition by RF magnetron sputtering for optical applications

Silicon carbide (SiC) is a rapidly emerging material for photonic applications, thanks to its exceptional optical properties. To be used as a waveguide, SiC thin films must be deposited directly on silica at low temperature. Amorphous SiC films were deposited by RF magnetron sputtering using a single source of high-purity polycrystalline SiC. A systematic study of the chemical, structural and optical properties of the films was carried out, using a combination of XRD, XPS, SIMS, spectroscopic ellipsometry, Raman spectroscopy and UV–Vis absorption spectroscopy. The aim was to link deposition conditions to film properties. By exploring a three-parameter space (RF power, substrate temperature, pressure), we have demonstrated that RF power is the main parameter which controls the entire deposition process and film properties. By simply adjusting the RF plasma power between 150 and 450 W, it is possible to adjust the refractive index at a wavelength of 1.5 μm in the range 2.50–2.75 and vary the bandgap from 2.5 to 1.7 eV. This is attributed to a slight variation in film composition, particularly in terms of Si/C ratio and C–C bond concentration.

Development of RF plasma generator based on Class E Power Amplifier

Abstract The plasma produced at 2 MHz radio frequency has substantial energy release stability and can be used to modify a material’s surface. The objective of this research is to create a plasma generator operating at a radio frequency of 2 MHz and examine how voltage affects the plasma spectrum that is generated. An AD9833 frequency generator, a class E amplifier, and impedance matching are used in the design of a 2 MHz radio frequency plasma generator as power amplifiers to create plasma. Variations in voltage, gas, and gas flow have been used. The input voltage variations used were 10V and 15V from the power supply. Two different media, free air (atmospheric gas) and argon gas flow, are used for plasma generation. In free air, the plasma formed is Nitrogen plasma with a wavelength of 300 to 380 nm. Argon gas flow is given a variation of Argon gas rate of 2 LPM, and 3 LPM. The plasma formed is Argon plasma with a wavelength of 700 to 850 nm.

Producing two-dimensional dust clouds and clusters using a movable electrode for complex plasma and fundamental physics experiments.

We report a Bidirectional Electrode Control Arm Assembly (BECAA) for precisely manipulating dust clouds levitated above the powered electrode in RF plasmas. The reported techniques allow the creation of perfectly 2D dust layers by eliminating off-plane particles by moving the electrode from outside the plasma chamber without altering the plasma conditions. The tilting and moving of electrodes using BECAA also allows the precise and repeatable elimination of dust particles one by one to achieve any desired number of grains N without trial and error. Simultaneously acquired top and side view images of dust clusters show that they are perfectly planar or 2D. A demonstration of clusters with N = 1-28 without changing the plasma conditions is presented to show the utility of BECAA for complex plasma and statistical physics experimental design. Demonstration videos and 3D printable part files are available for easy reproduction and adaptation of this new method to repeatably produce 2D clusters in existing RF plasma chambers.

Electric field and higher harmonics of RF plasma slit jet measured by antennas and VI probes

The cold atmospheric plasma jets change their character when interacting with the different surfaces. Since such interaction is the primary area of plasma jet applications, it is essential to monitor the process. The non-linearity of the RF plasma slit jet (PSJ) was analyzed using the VI probes and a novel method, the non-intrusive antenna measurements. Regardless of the experimental setup and gas mixture (Ar, Ar/O2, Ar/N2), the PSJ frequency spectrum consisted of the following main features: dominant fundamental frequency peak, relatively strong odd harmonics, and significantly weaker even harmonics. The lowest degree of non-linearity was recorded for the Ar PSJ ignited against a grounded target. Admixing a molecular gas increased the discharge non-linearity. It was attributed to the enhancement of secondary electron emission from the dielectric surfaces. In addition to the non-linearity analysis, the antenna spectra were for the first time used to determine the semi-quantitative values of the PSJ-radiated electric field. The electric fields decreased by a factor of 2 after the admixing of nitrogen and oxygen molecular gases. Out of the studied targets, the highest electric fields were observed when plasma impinged on the grounded targets, followed by the floating target (2× lower) and the PSJ ignited in the open space configuration (4× lower than in the grounded target configuration).

H− Ion Source RF Plasma Testing with 13 MHz and 27 MHz at the Spallation Neutron Source (SNS)1

The baseline RF-driven H− ion source configuration at the Spallation Neutron Source (SNS) facility uses a continuous wave (CW) 600 W 13 MHz RF system to ignite and maintain a low-density plasma inside the ion source vacuum chamber. After the continuous low-density 13 MHz plasma has been established, a pulsed (typical 1 ms pulse width and 60 Hz pulse repetition rate) 80 kW 2 MHz RF system is used to increase the plasma density to produce the pulsed H− ion beam. Incremental upgrades and improvements to the SNS ion source systems have resulted in the ability to reliably operate an H− ion source for SNS neutron production run cycles that can last up to four months. Conditions inside the SNS H− ion source evolve throughout a four-month run cycle due to changes in impurity levels, sputtering, and erosion. As the internal ion source conditions change during the run cycle, there can also be changes in plasma stability and the 13 MHz RF power level required to ignite the plasma. This paper presents the preliminary results of testing performed on the SNS Ion Source Test Stand (ISTS) system where we looked at plasma ignition and plasma stability using 27 MHz RF in place of the baseline 13 MHz RF system.

Ammonia synthesis by plasma catalysis in an atmospheric RF helium plasma

The in-plasma-catalytic synthesis of ammonia from nitrogen and hydrogen admixed to a helium RF plasma is studied with infrared absorption spectroscopy, optical emission spectroscopy, and through chemical kinetics modeling. Sandblasted glass is used as support for iron, platinum, and copper catalysts up to a surface temperature of 150 ∘C . It is shown that the optimum ammonia production occurs at very small N2/(N2+H2) ratios with an increase of concentration with plasma power. The global kinetic modelling agrees well with the data for a variation of the N2+H2 admixture and the absorbed plasma power. The introduction of the catalyst enhances ammonia production by up to a factor of 2. Based on the comparison with the modelling, this is linked to a change in the electron kinetics due to the presence of the catalyst. It is postulated that introducing the catalyst increases the reduced electric field because it reduces the secondary electron emission coefficient. As a result, the dissociation of N2 is stimulated, thereby enhancing the NH3 formation. These experiments show that the impact of the catalyst on the plasma performance in noble gas-diluted RF plasmas can be more important than the impact of the plasma on any catalytic surface process.

High-Power RF Plasma Systems for the Elevated Pressure Applications: Single-Torch Versus Multitorch Design

High-Power RF Plasma Systems for the Elevated Pressure Applications: Single-Torch Versus Multitorch Design

Electron power absorption in CF4 capacitively coupled RF plasmas operated in the striation mode

The electron power absorption mechanisms in electronegative capacitively coupled plasmas in CF4 are investigated using particle-in-cell/Monte Carlo collisions simulations at a pressure of p= 60 Pa, a driving frequency of f= 13.56 MHz for voltage amplitudes in the interval of ϕ0= 100 − 300 V, where pronounced self-organized density variations, i.e. striations, develop. The calculations are based on the Boltzmann term analysis, a computational diagnostic method capable of providing a complete spatio-temporal description of electron power absorption. The discharge undergoes an electron power absorption mode transition from the drift-ambipolar- to the striation-mode at φ 0 = 180 V. Although Ohmic power absorption is found to be the dominant electron power absorption mechanism in the parameter range considered, the electron power absorption mode transition can be inferred from the behaviour of the spatio-temporally averaged ambipolar power absorption as a function of the voltage amplitude. Furthermore, it is shown, that as a consequence of the presence of striations, the temporal modulation of the electron density leads to a temporal modulation of the ambipolar electric field, which is responsible for the striated structures of various physical quantities related to electrons, such as the electron temperature and the ionization source function.

Controlled synthesis of NO in an atmospheric pressure plasma by suppressing NO destruction channels by plasma catalysis

NO synthesis using plasma catalysis is analyzed in a parallel-plate atmospheric pressure RF plasma from N2/O2 admixed to helium exposed to Fe and Pt catalysts on a SiO2 support. The NO x species are measured by Fourier-transform infrared spectroscopy in a multi-pass cell. The trends in species densities can be well explained by air chemistry reactions, where NO’s progressive oxidation occurs with increasing oxygen admixture and ozone generation. The sequence can be controlled by the state of the surface that preferentially quenches O3 and allows for an optimum NO production. The maximum production of NO is found at 70% N2/(N2+O2) mixture ratio at 120 ∘C using sandblasted glass, with a conversion rate of 0.085%.

Modelling a thrust imparted by a highly ionized magnetic nozzle rf plasma thruster

Influence of the local-ionization-induced neutral depletion on the thrust imparted by the magnetic nozzle plasma thruster is discussed by simply considering reduction of the neutral density due to the ionization in the thruster model combining the global source model and the one-dimensional magnetic nozzle model. When increasing the rf power, it is shown that the increase rate of the plasma density is reduced, while the electron temperature continues to increase due to a decrease in the neutral density. Since the major components of the thrust are originated from the electron pressures in the source and in the magnetic nozzle, the increase in the electron temperature contributes to the increase in the thrust in addition to the gradual density increase by the rf power. The model qualitatively predicts the reduction of the thruster efficiency by the neutral depletion for the high-power condition, compared with the constant neutral density model.

Trapping and actively transporting single particles of arbitrary properties in low-pressure rf plasmas with and without a magnetic field

We report the experimental realization of optical trapping and controlled manipulations of single particles of arbitrary properties, e.g., nano- to micrometer in size, transparent spheres to strongly light absorbing nonspherical particles, in low-pressure rf plasmas. First, we show optical trapping and transport of single particles in an unmagnetized rf plasma. Then, we show similar observations in a weakly magnetized rf plasma. This is the first demonstration of actively transporting (pushing and pulling) light-absorbing, nonspherical single particles in plasmas. The result suggests that optically trapped, actively controlled, single plasma dust particles (not limited to those externally sampled spheres) could be an in situ micro-probe for dusty plasma and magnetized dusty plasma diagnostics.