Bulletin of the American Physical Society
71st Annual Gaseous Electronics Conference
Volume 63, Number 10
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session HW4: Basic Plasma Physics Phenomena in Low-temperature Plasmas II |
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Chair: Alexandre Likhanskii, Applied Material Inc. Room: Oregon Convention Center A107-A109 |
Wednesday, November 7, 2018 9:30AM - 9:45AM |
HW4.00001: On the deviations of similarity laws in low-temperature discharges Yangyang Fu, Janez Krek, De-qi Wen, Peng Zhang, John P. Verboncoeur Similarity laws have powerful potential in parameter prediction in similar discharges. However, for different discharge conditions, the similarity laws may not be always valid, which narrows their applicability. In this work, factors causing the deviation of similarity laws in low-temperature discharges are investigated using different simulation models. First, using the spatially-averaged Kinetic Global Model framework, the effects of the nonlinear reaction processes (stepwise ionizations and three-body collisions) are evaluated. It is found that, when compared to modeling results, the similarity relations will overestimate the species densities when the nonlinear reaction processes are included, and agree well with them when the nonlinear reaction processes are excluded. Second, using a two-dimensional fluid model, the effects of the nonlinear processes and the electron energy distribution functions (EEDFs) on the scaling of the spatially-dependent species densities are studied in geometrically similar gaps. Third, using the PIC/MCC code, OOPD1, the evolutions of the EEDFs in similar discharges are also presented. The similar discharges can only be obtained with the correct EEDFs. [Preview Abstract] |
Wednesday, November 7, 2018 9:45AM - 10:00AM |
HW4.00002: Adiabatic expansion of electron gas interacting with a magnetic nozzle Kazunori Takahashi, Christine Charles, Rod Boswell, Akira Ando A specially constructed experiment shows the near perfect adiabatic expansion of an ideal electron gas resulting in a polytropic index greater than 1.4, approaching the adiabatic value of 5/3, when removing electric fields from the system, while the polytropic index close to unity is observed when the electrons are trapped by the electric fields. The measurements were made on collisionless electrons in an argon plasma expanding in a magnetic nozzle. The collision lengths of all electron collision processes are greater than the scale length of the expansion meaning the system cannot be in thermodynamic equilibrium, yet thermodynamic concepts can be used, with caution, in explaining the results. In particular, a Lorentz force, created by inhomogeneities in the radial plasma density, does work on the expanding magnetic field reducing the internal energy of the electron gas which behaves as an adiabatically expanding ideal gas. [Preview Abstract] |
Wednesday, November 7, 2018 10:00AM - 10:30AM |
HW4.00003: Electron beam generated plasma as a low T$_{\mathrm{e}}$ approach to atomic-precision processing Invited Speaker: Scott Walton The advantages of plasma-based materials processing techniques are numerous. The capability to rapidly and uniformly modify large (\textgreater 10$^{\mathrm{3}}$ cm$^{\mathrm{2}})$ areas with high precision is one reason plasmas are widely used in the materials and surface engineering communities. However, with the ever evolving demand for new materials and single nanometer-scale device dimensions across a variety of applications, some of the limitations of conventional plasma sources are becoming apparent. The lack of process control and excessive ion energies in the development of atomic layer processing strategies are examples. The Naval Research Laboratory (NRL) has developed a processing system based on an electron beam-generated plasma. Unlike conventional discharges produced by electric fields (DC, RF, microwave, etc.), ionization is driven by a high-energy (1-3 keV) electron beam, an approach that can overcome many of the problems associated with conventional plasma processing systems. Electron beam-generated plasmas are generally characterized by high charged particle densities (10$^{\mathrm{10}}$- 10$^{\mathrm{12}}$ cm$^{\mathrm{-3}})$, low electron temperatures (0.3 - 1.0 eV), and in reactive gas backgrounds, a relatively low radical production rate compared to discharges. These characteristics allow the ability to precisely control the flux of charged and reactive neutrals as well as ion energy at adjacent surfaces. This provides the potential for controllably etching, depositing, and/or engineering the surface chemistry with monolayer precision. An overview of NRL's research efforts in developing this technology will be presented, with a focus on source development and operation, plasma characterizations, and how the system can be advantageously applied to the processing of select systems. Examples include monolayer materials, such as graphene and MoS$_{\mathrm{2}}$, where erosion and damage is a major concern and the etching of semiconductor materials, such as Si, SiN and SiO$_{\mathrm{2}}$, where the focus is on etch rates and selectivity at low ion energy. [Preview Abstract] |
Wednesday, November 7, 2018 10:30AM - 10:45AM |
HW4.00004: Investigation of Electron-Neutral Collisions in Weakly Ionized Laser Plasma Using Two-Color Interferometry and Radar Scattering Christopher Limbach In weakly ionized plasmas, inelastic and elastic electron-neutral collisional phenomena play an important role in determining the plasma conductivity and EEDF, while also yielding electronic and vibrational excitation relevant to plasma chemistry. In a decaying plasma, such as those impulsively excited through electrical or laser discharge, electron-neutral collisions mediate the plasma decay process during the later stages. In this work, we investigate the role of electron-neutral collisions in a weakly-ionized, laser generated atmospheric pressure plasma. The collision rate is indirectly measured through 12 GHz radar scattering from the isolated plasma volume with heterodyne detection. Unique to our approach, the plasma density and neutral density are directly controlled for using complementary two-color interferometry measurements. Analysis of the dataset shows the collision rate decreases faster than the decay in plasma density. The results are discussed in relation to the effect of electron temperature and the influence of spatial inhomogeneity and plasma shielding on the radar measurements. [Preview Abstract] |
Wednesday, November 7, 2018 10:45AM - 11:00AM |
HW4.00005: An experimental study of population density distribution of H(n=2) fine structure by laser absorption spectroscopy S Nishiyama, T Naruse, K Sasaki Absorption spectrum of $\rm H_{\alpha}$ line consists of seven fine structure components. The relative intensity of each component depends on the population density distribution of the lower states, $2^2S_{1/2}, 2^2P_{1/2}$, and $2^2P_{3/2}$. The metastable $2S$ state should have larger population than $2P$ states. In most cases, however, observed spectrum can be understood that the $2S$ and $2P$ states are almost same population density. The cause of this phenomenon is explained as relaxation of $2S$ state by Stark mixing or collisional quenching. In this study, we observed absorption spectrum shape of $\rm H_{\alpha}$ line to evaluate the population density distribution of H(n=2) states. A laser beam oscillated by a tunable diode laser was pass through a hydrogen plasma generated by a ICP source. The intensity of the transmitted laser was measured and the synchronous component with the modulation of the rf power fed to the ICP source was detected by a lock-in amplifier. The observed absorption spectra were fitted to calculated spectrum by non-linear least square method in which the relative population density of H(n=2) states and temperature were free parameters. The observed spectra at 20 mTorr showed that the population density of $2S$ was twice than that of $2P$ states. [Preview Abstract] |
Wednesday, November 7, 2018 11:00AM - 11:15AM |
HW4.00006: Electron Temperature and Density Measurements of Continuous-Wave Microwave-Driven Free Space Plasma Adrian Lopez, Remington Reid, Erin Thornton An experimental setup to study continuous-wave microwave-driven free space plasma discharges was designed and constructed at the Air Force Research Laboratory at Kirtland AFB, NM. Free space plasma is generated by a multi-kW, 4.7 GHz microwave system at pressures ranging from 100 to 200 mTorr. A precision gas flow system controls the composition of the gas used to generate the plasma. Gas pressure, gas composition (mixtures of Ar, N$_{2}$, and O$_{2}$), and the power of the microwave beam are varied to study their effects on the stability, uniformity, location (relative to the beam’s geometric focus), and parameters of the plasma. Measurements of the electron temperature, electron energy distribution function and electron density of the free space plasma generated under various operational conditions will be presented. [Preview Abstract] |
Wednesday, November 7, 2018 11:15AM - 11:30AM |
HW4.00007: The General Theory of Ionization Instabilities in a Plasma Slab, Supported by Microwave Sergey Dvinin, Oleg Sinkevich, Vitaly Dovzhenko The development of ionization instability in an infinite plasma slab, maintained by a microwave with frequency $\omega_{\mathrm{0}}$, is investigated. The resonance (connected with surface wave excitation) [1] and kinetic (strata) [2] modes are considered. The angle between propagation vector and normal to slab boundary is assumed to be arbitrary. When the oblique incidence takes place, the wavelengths of the anti-Stokes and Stokes perturbations became different. Instability in an infinite system is determined by the position of branch points on the complex plane. The evolution of perturbations from a local source for an infinite plasma column (in the absence of reflection from the ends of a real plasma column) and in the presence of boundaries is calculated. Areas of parameters, when development of convective and absolute instabilities will take place, are determined. Phenomenological models of the nonlinear stage of instabilities development are proposed. The possibility of observing of this type of instability in low-pressure plasma reactors is discussed. For a spatially limited system, the results of the calculations show good agreement with experiment. $^{\mathrm{1}}$ S. Dvinin et al. Sov. Phys.: Fizika Plazmy, 9, 1983, 1297. $^{\mathrm{\mathbf{2}}}$ B. S. Kerner\textbf{, }V.V. Osipov\textbf{. }Autosolinons. Springer-science$+$Business Media B.V. 1994, 671 p. $^{\mathrm{3}}$ D. L. Bobroff, H. A. Haus. J. Apple. phys., 1967, \textbf{38}, ¹1, p. 390. [Preview Abstract] |
Wednesday, November 7, 2018 11:30AM - 11:45AM |
HW4.00008: Plasma-Based Electrically Small Antennas Abbas Semnani, Sergey Macheret, Dimitrios Peroulis The demand for wideband electrically small antennas (ESA) is rapidly growing because of (1) the need for compact multi-function devices and (2) the limited space available in many applications. In an ESA, the frequency dependent antenna impedance consists of a very small resistance and very large reactance, which makes impedance matching networks necessary. However, implementation of such a matching network over a wide frequency bandwidth using conventional passive elements is impossible due to the Gain-Bandwidth limitation. Therefore, non-Foster matching networks with a negative capacitor/inductor are usually employed for wideband matching. Although active and metamaterial-based non-Foster matching networks have been introduced, those are complicated, lossy, and cannot handle high power. Cold plasma is a medium that can exhibit tunable negative permittivity at sufficiently high electron density and at the same time can handle high power and temperature. We show that plasma augmentation can substantially increase the efficiency of small antennas over wide frequency range, and thus plasma-augmented high-power and wideband electrically small antennas can be a viable solution for efficiently transmitting high powers, particularly over low frequencies. [Preview Abstract] |
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