Complex Plasma Processes Research Articles

Analysis of the synergetic effect of process parameters of hydrogenated amorphous carbon deposition in plasma-enhanced chemical vapor deposition using machine learning

The synergetic effects of process parameters on hydrogenated amorphous carbon (a-C:H) films properties were quantitatively analyzed in plasma-enhanced chemical vapor deposition. Predictive models, created from experimental datasets and machine learning, indicate a synergetic effect between the H2 ratio and ion energy impacting on the substrate. At a H2 ratio of 75 %, etch rates of 200 nm/min were predicted, regardless of radio frequency (RF) bias. These rates decreased to 120–140 nm/min at lower H2 ratios, primarily depending on RF bias. Quadrupole mass spectrometry and Raman scattering were used to investigate the underlying mechanisms. High H2 ratios led to a greater presence of low-mass hydrogen-rich molecules in the plasma. The C2H3 radical intensity at a 75 % H2 ratio was 15 times greater than that at 0 % at an RF bias of 50 W, and similar trends were observed for other low-mass neutrals. The synergetic effects of H2 ratio and RF bias decreased the film's hydrogen content. Machine learning and those diagnostics revealed that ion bombardment induces dehydrogenation of a-C:H, influenced by hydrogen-rich species. This study demonstrates that machine learning can uncover complex plasma processes and optimize material synthesis.

Dynamic mode decomposition for data-driven analysis and reduced-order modeling of E × B plasmas: II. Dynamics forecasting

In part I of the article, we demonstrated that a variant of the dynamic mode decomposition (DMD) algorithm based on variable projection optimization, called optimized DMD (OPT-DMD), enables a robust identification of the dominant spatiotemporally coherent modes underlying the data across various test cases representing different physical parameters in an E × B simulation configuration. We emphasized that the OPT-DMD significantly improves the analysis of complex plasma processes, revealing information that cannot be derived using conventionally employed analyses such as the fast Fourier transform. As the OPT-DMD can be constrained to produce stable reduced-order models (ROMs) by construction, in this paper, we extend the application of the OPT-DMD and investigate the capabilities of the linear ROM from this algorithm toward forecasting in time of the plasma dynamics in configurations representative of the radial-azimuthal and axial-azimuthal cross-sections of a Hall thruster and over a range of simulation parameters in each test case. The predictive capacity of the OPT-DMD ROM is assessed primarily in terms of short-term dynamics forecast or, in other words, for large ratios of training-to-test data. However, the utility of the ROM for long-term dynamics forecasting is also presented for an example case in the radial-azimuthal configuration. The model’s predictive performance is heterogeneous across various test cases. Nonetheless, a remarkable predictiveness is observed in the test cases that do not exhibit highly transient behaviors. Moreover, in all investigated cases, the error between the ground-truth and the reconstructed data from the OPT-DMD ROM remains bounded over time within both the training and the test window. As a result, despite its limitation in terms of generalized applicability to all plasma conditions, the OPT-DMD is proven as a reliable method to develop low computational cost and highly predictive data-driven ROMs in systems with a quasi-periodic global evolution of the plasma state.

The (3+1)-dimensional generalized mKdV-ZK equation for ion-acoustic waves in quantum plasmas as well as its non-resonant multiwave solution* *Project supported by the National Natural Science Foundation of China (Grant No. 11975143), the Natural Science Foundation of Shandong Province of China (Grant No. ZR2018MA017), the Taishan Scholars Program of Shandong Province, China (Grant No. ts20190936), and the Shandong University of Science

The quantum hydrodynamic model for ion-acoustic waves in plasmas is studied. First, we design a new disturbance expansion to describe the ion fluid velocity and electric field potential. It should be emphasized that the piecewise function perturbation form is new with great difference from the previous perturbation. Then, based on the piecewise function perturbation, a (3+1)-dimensional generalized modified Korteweg–de Vries Zakharov–Kuznetsov (mKdV-ZK) equation is derived for the first time, which is an extended form of the classical mKdV equation and the ZK equation. The (3+1)-dimensional generalized time-space fractional mKdV-ZK equation is constructed using the semi-inverse method and the fractional variational principle. Obviously, it is more accurate to depict some complex plasma processes and phenomena. Further, the conservation laws of the generalized time-space fractional mKdV-ZK equation are discussed. Finally, using the multi-exponential function method, the non-resonant multiwave solutions are constructed, and the characteristics of ion-acoustic waves are well described.

Characteristics of the Ionospheric Mid-Latitude Trough Measured by Topside Sounders in 1960–70s

The ionospheric mid-latitude trough (IMT) is the electron density depletion phenomenon in the F region during nighttime. It has been suggested that the IMT is the result of complex plasma processes coupled to the magnetosphere. In order to statistically investigate the characteristics of the IMT, we analyze topside sounding data from Alouette and ISIS satellites in 1960s and 1970s. The IMT position is almost constant for seasons and solar activities whereas the IMT depth ratio and the IMT feature are stronger and clearer in the winter hemisphere under solar minimum condition. We also calculated transition heights at which the densities of oxygen ions and hydrogen/helium ions are equal. Transition heights are generally higher in daytime and lower in nighttime, but the opposite aspects are seen in the IMT region. Utilizing the Incoherent Scatter Radar (ISR) electron temperature measurements, we find that the electron temperature in the IMT region is enhanced at night during winter. The increase of electron temperature may cause fast transport of the ionospheric plasma to the magnetosphere via ambipolar diffusion, resulting in the IMT depletion. This mechanism of the IMT may work in addition to the simply prolonged recombination of ions proposed by the traditional stagnation model.

Effect of dust particle polarization on scattering processes in complex plasmas

Screened interaction potentials in dusty plasmas taking into account the polarization of dust particles have been obtained. On the basis of screened potentials scattering processes for ion-dust particle and dust particle-dust particle pairs have been studied. In particular, the scattering cross section is considered. The scattering processes for which the dust grain polarization is unimportant have been found. The effect of zero angle dust particle-dust particle scattering is predicted.

Diffusive shock acceleration at laser-driven shocks: studying cosmic-ray accelerators in the laboratory

The non-thermal particle spectra responsible for the emission from many astrophysical systems are thought to originate from shocks via a first order Fermi process otherwise known as diffusive shock acceleration. The same mechanism is also widely believed to be responsible for the production of high energy cosmic rays. With the growing interest in collisionless shock physics in laser produced plasmas, the possibility of reproducing and detecting shock acceleration in controlled laboratory experiments should be considered. The various experimental constraints that must be satisfied are reviewed. It is demonstrated that several currently operating laser facilities may fulfil the necessary criteria to confirm the occurrence of diffusive shock acceleration of electrons at laser produced shocks. Successful reproduction of Fermi acceleration in the laboratory could open a range of possibilities, providing insight into the complex plasma processes that occur near astrophysical sources of cosmic rays.

Modelling of plasma etching process using radial basis function network and genetic algorithm

Computer prediction models are crucial to control complex plasma processes. A new plasma model was constructed by using a radial basis function network (RBFN) and genetic algorithm (GA). The GA was used to search for an optimized set of training factors. This technique was evaluated with the plasma etching data. The etching of silica thin film was conducted in an inductively coupled plasma. The etch responses modelled include aluminum (Al) etch rate, silica etch rate, Al selectivity, silica profile angle, and silica sidewall roughness. For comparison, conventional RBFN models as well as four types of statistical regression models were constructed . Compared to conventional RBFN models, GA-RBFN models exhibited improved predictions of more than 20% for Al etch rate, Al selectivity, and silica sidewall roughness. For the remaining two etch responses, both GA-RBFN and RBFN models were almost comparable. Compared to statistical regression models, GA-RBFN demonstrated improved predictions for nearly all etch responses. The improvement was even more than 35% for the Al selectivity and silica sidewall roughness. The comparisons revealed that the presented method can be effectively used to construct improved prediction models for plasma control.

Complex plasmas IV: Theoretical approaches to complex plasmas and their application

This paper completes a series of reviews devoted to the physics of complex plasmas, in which one of the components (dust) is in a crystalline or liquid state, while the others (electron, ions, and neutral atoms) are in a gaseous state. This review is devoted to the theoretical approaches used to describe complex plasmas so far. The main theoretical developments have been concentrated in the gaseous and weakly nonideal states of complex plasmas. Here, we describe the achievements in the new kinetic and new hydrodynamic approaches to complex plasmas. At present, only generalizations of the van der Waals approach for complex plasmas have been used to describe phase transitions and plasma condensation in complex plasmas. Here, criteria for transitions are described and compared with the existing experimental observations. Theoretical and numerical results for nonlinear structures, such as dust layers, dust voids, dust sheaths, and dust convective vortices, obtained by solving the stationary balance equations, are also discussed and compared with state-of-the-art experiments. At present, experiments in this field are progressing very fast, while theory is not advancing at the same rate of development. To further develop new theoretical models, one can use the elementary physical processes in complex plasmas described in the previous parts of the review. However, the detailed comparison of theory and experiments also needs more detailed experimental diagnostics of the phenomena observed. In the concluding part of our review, the trends in experiment and theory, as well as some existing applications, including industrial, environmental, and astrophysical ones, are described.

Prediction of plasma etching using a polynomial neural network

A polynomial neural network (PNN) is first applied to construct a predictive model of plasma etch processes. The process was characterized by a full-factorial experiment, and two attributes modeled are an etch rate and a dc bias. The training and test data for the etch rate consisted of 32 and 15 patterns, respectively. For the dc bias, the training and test data were composed of 34 and 17 patterns, respectively. Prediction performance of PNN was optimized with a variation in the number of input factors and in the polynomial type. For each data type, 27 cases were evaluated. The root mean squared prediction accuracy is 8.49 nm/min and 5.43 V for the optimized etch rate and dc-bias models, respectively. For comparison, backpropagation neural network (BPNN) and three types of statistical regression models were constructed. Five training factors involved in training the BPNN were experimentally optimized. Compared to other models, the PNN model demonstrated an improvement of more than 30% and 80% in modeling the etch rate and dc bias, respectively. By the demonstrated high prediction ability, the PNN can be effectively used to model and control complex plasma processes.

Complex plasmas: III. Experiments on strong coupling and long-range correlations

This paper continues a series of review papers devoted to the physics of complex plasmas, in which one of the components (dust) is in a crystalline or liquid state, while the others (electron, ions, and neutral atoms) are in a gaseous state. This review is devoted to the experimental investigations of new phenomena incomplex plasmas. The experiments are explained using estimates based on the theory of elementary processes in complex plasmas, including the new phenomena considered in the previous parts of the review. The paper describes (i) the experiments on multilayer plasma crystals, including the study of their structure and phase transitions; (ii) the experiments on dust monolayer crystals; (iii) the experiments on plasma clusters formed by small number of dust grains; (iv) the experiments on dust ion-sound waves, dust acoustic waves, dust lattice waves, and dust shear waves; (v) the experiments on shock waves; (vi) the experiments on the ionization instabilities and the creation of dust voids and dust clumps; and (vii) the experiments on Mach cones excited either by fast grains or laser radiation.

Complex plasmas: II. Elementary processes in complex plasmas

This paper continues a series of review papers devoted to the physics of complex plasmas. The review contains a description of elementary processes in complex plasmas. The elementary processes are described in the simplest way and in a form useful for applications and estimates for existing experiments. The paper describes (i) the processes related to the charging process, including the dust charge fluctuations, absorption of plasma particles on dust grains, and friction of ions in collisions with grains; (ii) the forces acting on dust grains in complex plasmas, including ion drag force, thermophoretic force, and dust friction on neutral gas; (iii) dust-dust noncollective interactions, including the attraction related to the nonlinearity of screening and the shadow attraction related to ion and neutral particle bombardment; and (iv) dust-dust collective attraction in the absence and presence of an ion flow and a strong magnetic field. First experiments on the detection of dust-dust interactions are described. Elementary processes in complex plasmas can be used to explain existing experiments on dust-plasma crystals, dust clusters, and waves and instabilities in complex plasmas, as well as to discuss the trends in theoretical research, laboratory experiments, and in astrophysical and industrial applications.

Complex plasmas: I. complex plasmas as unusual state of matter

This paper opens a series of review papers devoted to the physics of the so-called complex plasmas. The review contains a description of new physical phenomena met in dusty plasmas and complex plasmas. The term complex plasma is used for a state where some components (dust) are in crystal or liquid state, while the others (electron, ions, and neutral atoms) are in gaseous state. Experimental and the theoretical investigations of such a complex state of matter are presented. It is emphasized that (i) complex plasma represents quite an unusual state of matter, (ii) the system of dust grain cannot be considered as a Coulomb system, because it always has long-range and nonscreened interactions between the grains both of repulsive and attractive type and because the shielding of the grains is always nonlinear, (iii) the interactions between the particles in complex plasmas and in usual matter differ substantially due to strong plasma absorption on grains and the necessity to support the plasma density by an external source of ionization, (iv) the usual concept of free energy is not applicable to complex plasmas, because it is actually an open system, and (v) the collective nature of pair dust interaction is one of the most important new features of complex plasmas. The theory of elementary processes in complex plasmas, including these new phenomena, can be used to explain existing experiments on dust-plasma crystals, dust clusters, and waves and instabilities in complex plasmas.

Reaction of carboxylic groups containing organic layers with the surface of a ZnSe internal reflection element during high-energy processes

The aim of this FTIR study was to get information about the reactivity of plasma-deposited organic layers on a ZnSe-ATR probe by means of modification experiments using organic layers with carboxyl groups as model compounds. The model layers prepared on ZnSe internal reflection elements were exposed both to a low pressure plasma in a reactor (generation of ultraviolet radiation: λ=100–400 nm) and to a ultraviolet radiation outside of the reactor (lamp with λ max=254 nm) for the simulation of partial steps of complex plasma processes. The reaction behaviour was investigated without irradiation under ambient conditions as well. The reactions of the carboxyl groups of the model substances allow conclusions to the behaviour of the carboxyl groups often produced during plasma deposition processes and whose chemical reactions to zinc carboxylates on ZnSe probes. However, the individual reaction time is very strong dependent on the corresponding conditions of the processes.

The Viking Program

Sweden's first satellite, Viking, was successfully launched on February 22, 1986, at 01:44 UT. It was carried into orbit with the French remote sensing satellite SPOT by an Ariane 1 booster, launched from French Guiana. Viking's perigee boost motor placed this relatively small satellite (∼520 kg) into a final 817 km×13,527 km polar orbit, where it is conducting scientific observations of complex plasma processes in the magnetosphere and ionosphere of the earth. The satellite carries experiments to measure electric fields, magnetic fields, charged particles, waves, and auroral images.

Optics in laser-produced plasmas

The concepts of linear optics are applied to the physics of energy coupling into laser-produced plasmas currently of interest in laser-initiated fusion research. The effects of polarization and angle of incidence upon reflection and refraction are recalled. Singular behavior occurs in the region of the plasma where the real part of the dielectric constant vanishes, the so-called ’’critical density’’ region. The effective generation of Langmuir waves at the surfaces of bounded plasmas through the optical resonance of obliquely incident electromagnetic radiation is heuristically discussed. Enhanced electric fields, enhanced ’’collisionless’’ damping, harmonic generation, the presence of energetic electrons, and density profile modifications which result near the critical density from the excitation of these plasma waves are described. While more complex plasma processes may occur, they are not within the scope of this review.