Abstract The impact of an inhomogeneous magnetic field generated by supplementary direct current (DC) coils on the uniformity of capacitively coupled discharges is examined utilizing a two-dimensional implicit particle-in-cell/Monte Carlo model. Typically, at low pressure, the radial density distribution of plasma is characterized by a high density at the center of the chamber and a lower density near the periphery. This results in non-uniform radial plasma density profiles and large ion impact angles at the electrode. We find that placing a DC coil above the chamber produces a non-uniform static magnetic field, which facilitates the transport of plasma species toward the electrode periphery, resulting in a more uniform plasma density distribution. Nonetheless, this approach leads to a decrease in central density and adversely affects the ion incident angles near the chamber’s center. Consequently, this compromise undermines both the efficiency and uniformity of processes occurring in the central region of the chamber. To overcome these limitations, we propose innovative coil configurations, specifically dual direct current (DC) coils comprising an inner and an outer coil. The outer coil, situated above the chamber, has a larger radius, while the inner coil, positioned either above or below the chamber, has a smaller radius. Additionally, the currents in the inner and outer coils flow in opposite directions. Our findings indicate that the outer coil predominantly governs the density distribution across the entire electrode surface, while the inner coil allows for precise adjustment of the plasma density near the discharge center. Therefore, by adjusting the currents of the outer and inner coils, significant improvements can be achieved in both the uniformity of plasma density and the vertical alignment of the ion angles above the electrode. These factors are critical for the fabrication of high aspect ratio microelectronic structures.

Abstract Motivated by the need for precise control of plasma parameters of atmospheric non-equilibrium plasma, we present an approach for the transient measurement of the electric field for discharge termination (i.e. the discharge termination fields, DTFs) based on the electric field induced second harmonic generation method. This approach utilizes a custom-designed electrode structure and introduces the integration system as the connection between the electric field and the response signals according to the superposition theorem of electric fields. Considering the electric field profile, the effect of the undesired electric field outside the copper foils on second harmonic generation is eliminated, leading to more accurate DTF measurement results. The DTFs for positive discharges decrease with increasing voltage amplitude or gap height, while the DTFs for negative discharges display the opposite trend, both of which are closely related to the discharge magnitude. Moreover, the higher measurement accuracy of the proposed method is further demonstrated in the experimental measurement of electric field evolution. The presented quantitative measurement results can be used to validate high-fidelity modeling predictions and facilitate a deeper understanding of the physical mechanisms of discharge extinction.

Abstract We report a comparison of inferred electron density (ne) in a He capacitively-coupled plasma, deduced from Laser-Collision Induced Fluorescence measurements, with values computed using a hybrid simulation framework based on Particle-in-Cell / Monte Carlo Collisions simulations and a fluid model for excited He atoms. The studies were carried out for gas pressures between 50mTorr and 1000mTorr and peak-to-peak radio-frequency (13.56 MHz) voltages between 150V and 350 V, in a highly symmetric source equipped with plane-parallel electrodes. A good agreement is found between the experimental and modeling results for ne except at the lowest operating voltages and gas pressures. The (effective) electron temperature (Te) values derived by the two methods agree as well reasonably within the plasma
bulk. The simulation results are used to compare the density distributions of He+ and
various He excited levels and their major populating and de-populating channels at
100 mTorr and 1000 mTorr.

Abstract The exhaust plume of a 1 J ablative pulsed plasma thruster (APPT) fed with non-volatile liquid propellant (perfluoropolyether—PFPE) was studied experimentally with electric probes. A Faraday cup and retarding potential analyzer were used with a time-of-flight approach to determine the shape and ion content of the plume, including the energy and velocity distributions of individual ion species. It was found that the plume was composed mainly of fluorine ions, with charge-states up to 6+. Other constituents of PFPE (carbon and oxygen) were detected in very low quantities. The results for liquid PFPE were similar to those obtained earlier for solid polytetrafluoroethylene (Teflon), with the exception of a larger contribution of highly ionized fluorine species. The plume measurements allowed calculation of efficiency factors and comparison with thruster efficiency. The obtained distributions of ions as well as the large relative abundance of high charge-state ions were indicative of a lack of thermal equilibrium in the plasma of APPT.

Abstract In an atmospheric DC glow discharge with liquid anode, the plasma attachment under certain conditions self-organize into coherent patterns at the anode. Optical emission spectroscopy revealed that attachment emission consists primarily of the second positive system of nitrogen N2(C-B) whose excitation energy is low and sensitive to the change of electron energy distribution. Besides the electrons, negative ions can also accumulate in the anode sheath and affect the local space charge. It has been conjectured that these negative ions play a role in pattern formation at the anode surface. In this work, the role of oxygen negative ions was explored. It was found that the establishment of anode patterns requires at least a 7 % volume fraction of oxygen in the ambient gas. Results showed that at least in this work, O2- is the dominant negative ion species and has a density ~10^13 cm^-3. While the presence of oxygen appears crucial to pattern formation, this study indicated that the mere presence of the negative ions itself was not sufficient for pattern formation, suggesting a more complex mechanism involving electronegative species must be present. In fact, it was found that even when as many as 67 % of negative ions in the plasma were detached, no obvious geometry changes were observed in the self-organized pattern.

Abstract The ionic liquid electrospray thruster (ILET), a promising technology for space electric propulsion, has been gaining attention due to its theoretical capability to operate without an external neutralizer, which is a key component for traditional ion thrusters. Some experiments have shown that the cation-to-anion mass ratio significantly affects the self-neutralization of thruster plumes, but further depth research is still needed. Therefore, numerical simulations of ion emission process both in uni-thruster and bi-thruster modes of ILETs are conducted using particle-in-cell method, to investigate the effect of the cation-to-anion mass ratio in ionic liquid on plume neutralization characteristics. In bi-thruster mode, the anions and cations are emitted simultaneously, while in uni-thruster mode, only anions or cations are emitted under the action of electric field force. Plume neutralization characteristics of four ionic liquid propellants, with different cation-to-anion mass ratio, in bi-thruster mode are obtained and compared to the uni-thruster mode. The results show that the plume profile morphology is significantly dependent on operating mode of the thruster and the cation-to-anion mass ratio of ionic liquid. Unlike the circular profile in uni-thruster mode, the plume in bi-thruster mode exhibit distortion with vertical stratification. The contours of the high-potential region in bi-thruster mode also distort, showing stratification and a V-shape, respectively, influenced by the mass ratio. Ion beam neutralization in bi-thruster mode is primarily achieved through the horizontal displacement of anion and cation beams, as well as their interactions. The cation-to-anion mass ratio influences the oscillation effects of the mass center in anion and cation clouds during neutralization, which in turn affects the neutralization effect. Ionic liquids with a significant mass ratio difference between cations and anions necessitate a longer time and greater distance for neutralization under identical conditions. These findings are instrumental for understanding the plume neutralization process and advancing the design of ILETs.

Abstract In recent years, magnetized inductively coupled plasma (MICP) has been proposed as an improved version of inductively coupled plasma to meet the increasing production process requirements. However, due to the more complex structure of the plasma system, numerical simulations face challenges such as modeling difficulty, model convergence issues, and long computation times. In this paper, a deep neural network (DNN) with a multi-hidden layer structure is developed based on deep learning technology to serve as an assisted modeling tool of traditional fluid simulations. This approach aims to study the discharge characteristics and plasma chemistry of argon-oxygen MICP more efficiently. The simulation data from the fluid model is used to train the neural network. The well-trained DNN can efficiently and accurately predict the target plasma characteristics under new discharge parameters, such as electron density, ionization rate, and particle reaction rate. The effectiveness of the DNN is verified by comparing its predictions with fluid simulation results. Compared to the traditional fluid simulation, which takes thousands of seconds, the trained DNN can produce highly consistent prediction results only within 1 s. The computing efficiency is significantly improved. The prediction results of the DNN demonstrate that by adjusting discharge parameters such as magnetic field coil current, power, and O2 ratio, the plasma discharge characteristics and plasma chemistry can be modulated, including charged particle density, ionization rate, chemical reaction rate, and plasma radial uniformity. Additionally, the DNN shows good generalization ability across multiple input and output scales, providing a paradigm for its application in other plasma research fields. This study indicates that the DNN is a feasible method for studying MICP discharge characteristics and offers a novel and effective computational tool for future plasma discharge research based on deep learning methods.

Abstract Electron-assisted etching of poly-Si and SiO2 is performed via a grid system in inductively coupled CF4 plasma. The feasibility of electron-assisted etching is discussed with a focus on the low-surface damage of the etching. The etch rate increases with electron beam energy, which indicates that the electrons assist the surface etching process. To verify this, etching of poly-Si and SiO2 is performed in several plasma conditions, which leads to differences in etch rate that depend on the presence or absence of radicals and electron beams. Poly-Si and SiO2 are not etched without radicals of CF4 plasma, but they are etched when such radicals are present. When the electron beam and radicals exist simultaneously, the etch rate increases more dramatically than in the case of a CF4 plasma without an electron beam, demonstrating that the electron beam assists the etching process. Optical emission spectroscopy is employed to verify the F radical does not affect the etch rate increase. The surface roughness is measured after electron-assisted etching and compared with the surface roughness after ion-assisted etching.

Abstract Electron-impact ionization cross sections of atoms and molecules are essential for plasma modelling. However, experimentally determining the absolute cross sections is not easy, and ab initio calculations become computationally prohibitive as molecular complexity increases. Existing AI-based prediction methods suffer from limited data availability and poor generalization. To address these issues, we propose DeepCSNet, a deep learning approach designed to predict electron-impact ionization cross sections using limited training data. We present two configurations of DeepCSNet: one tailored for specific molecules and another for various molecules. Both configurations can typically achieve a relative L2 error less than 5%. The present numerical results, focusing on electron-impact doubly differential ionization cross sections, demonstrate DeepCSNet's generalization ability, predicting cross sections across a wide range of energies and incident angles. Additionally, DeepCSNet shows promising results in predicting cross sections for molecules not included in the training set, even large molecules with more than 10 constituent atoms, highlighting its potential for practical applications.

Abstract Understanding the rapid dynamics of the primary streamer is crucial for comprehending the nanosecond pulsed discharge process. To reveal the fast primary streamer process, this study introduces a newly developed quadruple emICCD camera system capable of capturing a sequence of four discharge images in single pulse, coupled with self-customized software for data analysis. A nanosecond pulse power with its FWHM of 10.5 ns was applied to a coaxial reactor, focusing on the dynamics of the primary streamer. Our research clarifies the spatiotemporal variations of the primary streamer’s properties and examines their relation with inner electrode diameter (i.d. 0.2–2.0 mm). Results showed that in a pulse-powered coaxial electrode, there are three stages in the primary streamer process and that i.d. serves as an important factor influencing the formation and propagation of streamers. Interestingly, we found that streamer head velocity, streamer width, and streamer area for individual streamers remain constant prior to streamer channels reaching the outer electrode. Furthermore, we also observed an initial increase followed by a decrease in both streamer head velocity and streamer width with increasing i.d values. This study sheds light on the fundamental properties of the primary streamer during nanosecond pulsed discharge, contributing valuable insights for future plasma applications.