Session DT1: DPP/GEC Joint Session: Low Temperature Plasmas I

Damage-free Neutral Beam Etching, Deposition and Surface Modification Processes for Novel Nano-scale Devices

For the past 30 years, plasma process technology has led in the efforts to shrink the pattern size of ultralarge-scale integrated (ULSI) devices. However, inherent problems in the plasma processes, such as charge build-up and UV photon radiation, limit the process performance for nanoscale devices. To overcome these problems and fabricate nanoscale devices in practice, we have proposed damage-free neutral-beam process. In this presentation, I describe the issues of plasma processes and the demanded atomic layer process for future nanoscale devices. I also introduce our developed damage-free etching, structure-designable deposition of super low-$k $SiOC film and low-temperature oxidation (thin SiO$_{2}$, GeO$_{2})$ processes using neutral beams and discuss the actual applications of neutral beam processing for future nanoscale devices (such as, Fin-MOSFET, and Quantum Dot Solar Cell). Neutral beams can perform atomically damage-free etching, deposition and surface modification. Then, the neutral beam process can precisely control the atomic layer chemical reaction and defect generation. This technique is a promising candidate for the nano-fabrication technology in future nanoscale devices.   

Electric double layers and their applications to astrophysical objects and electric propulsion

Electric double layers, abrupt potential drops within a plasma, exist in the plasma environment of the Earth and the stars, can cause phenomena as diverse as aurorae, or electromagnetic radiation from rotating neutron stars and may also play an important role in supplying and accelerating plasma in coronal funnels at the surface of the Sun. In the laboratory, both current-driven and current-free double layers have been generated and studied in a variety of experimental devices. The class of current-free double layers which form in low pressure magnetically expanding plasmas (e.g. from Helicon sources) for a variety of gases and geometries exhibit interesting electron and ions dynamics properties. The latter are also observed in particle-in-cell simulations. Application of expanding plasmas to the field of electric propulsion is receiving increasing interest: in addition to thruster performance assessment and optimisation, direct measurements of thrust combined with spatial mapping of the expanding plasma provides some information on momentum flux imparted from an expanding plasma and on plasma detachment from a magnetic field.   

Effect of non-Maxwellian Electron Energy Distributions on Langmuir Probe Measurements and Heat Transmission in Tokamak Divertor Sheaths

Tokamak scrape-off-layers have long been analyzed and modeled with a single-fluid approach. Recent research indicates that kinetic effects in the SOL may be significant enough to challenge this approach [1]. Classical Langmuir probe interpretation is expected to be sensitive to the tail of the electron distribution and could yield results characterized by the tail population. The assessment of derived quantities, such as the sheath heat transmission coefficient, would also be affected. Non-local probe theory indicates a new interpretation method making calculation of the distribution function in the tokamak edge possible [2]. This method is applied to the divertor Langmuir probes of NSTX and non-Maxwellian distributions are found to result from the analysis. The inferred distributions contain a cool, bulk population and a hot electron tail fraction (typical $T_{hot}/T_{cool}\approx 4$) corroborating previous experiments and modeling. Determination of the sheath heat transmission coefficient with $\gamma \approx 2.5$ supports this observation. The two Langmuir interpretation methods are compared using OEDGE interpretative modeling. This fluid modeling indicates that the plasma collisionality is marginal in the NSTX SOL ($\nu_{e}^* < 20$) making kinetic effect probable. Candidate processes involved are 1) electron-neutral interactions, 2) non-local/gradient effects at the target plate and 3) turbulent fluctuations. The inferred distributions are compared to calculations of electron-neutral interactions, fluctuation-induced effects and PIC simulations of non-local effects.   

Helicon mysteries: fitting a plane wave into a cylinder

Since the first reports in the 1960s, the dispersion of helicon waves in a plasma cylinder has been difficult to describe theoretically for axial wavelengths that are greater than the plasma radius. About 10 years ago, Breizman and Arefiev showed how radial density gradients make the plasma column similar to a coaxial cable, allowing the helicon waves to propagate below the cut-off frequency. The resulting dispersion relation is similar to that of a plane wave propagating parallel to the magnetic field. A few years later, Degeling et. al. presented experimental evidence demonstrating such a plane wave dispersion for a broad range of axial wave numbers. The reason lies in the decoupling of the Hall and electron inertial terms in the dispersion, the former describing the electromagnetic propagation and the latter the electrostatic propagation. Combining the experimental and theoretical results has recently thrown further light on this phenomenon that is applicable to both space and laboratory situations. Radially Localized Helicon Modes in Nonuniform Plasma, Boris N. Breizman and Alexey V. Arefiev, Phys. Rev. Letts. 84, 3863 (2000). Transitions from electrostatic to electromagnetic whistler wave excitation, A. W. Degeling, G. G. Borg and R. W. Boswell, Phys. Plasmas, \textbf{11}, 2144, (2004)   

Radio-Frequency Plasma Probes

The utility of radio-frequency probes for diagnosing low-temperature plasmas has recently been extended through the use of a network analyzer to measure the small-signal, complex probe impedance Z$_{ac}$ as a function of applied frequency and dc bias. To interpret the results, account must be taken of the gas pressure, the plasma density, the applied frequency, and the applied magnetic field if any. In this talk four different models are presented for use in different regimes. At high gas pressure, Re(Z$_{ac})$ is shown to give n$_{e0}$/$\nu $, where n$_{e0}$ is the ambient electron density and $\nu $ is the electron-neutral collision frequency. At low pressure Re(Z$_{ac})$ gives not only n$_{e0}$ but also n$_{e}$(r) within the sheath immediately outside the probe, the plasma potential, and the electron energy distribution and temperature. Magnetized plasmas can be treated by adding an external inductance in series with the probe and operating above the upper hybrid frequency; alternatively, rf voltage can be applied between two closely-spaced planar electrodes oriented either parallel or perpendicular to the field. As will be shown, rf probes not only provide more information than Langmuir probes, but the data is easier to analyze and generally less affected by noise. Additional advantages include the following: decreased sensitivity to secondary electron emission, ions, plasma flow, and high-energy beams; clear and unequivocal determination of n$_{e0}$, even in magnetized plasmas; direct utility at high pressure; multiple checks on the results; and the ability to operate in reactive and depositing gases.   

Dynamics and pattern formation during microwave breakdown at atmospheric pressure

A self-organized array of plasma filaments moving towards the source has been recently observed in microwave breakdown experiments in the millimeter range at MIT (Y. Hidaka et al., Phys. Rev. Lett. 100, 035003 (2008)). These filaments are qualitatively different from the well-known filaments observed in laser breakdown, and develop transverse to the propagation direction, along the direction of the electric field polarization. A model coupling Maxwell's equations with a simple description of the plasma dynamics has been developed and has been shown to reproduce very well the experimental observations (J.P. Boeuf et al., Phys. Rev. Lett. 104, 015002 (2010)). The propagation of the plasma toward the source is due to an ionisation-diffusion mechanism and the self-organized filamentary structure is associated with the scattered field pattern. The filaments develop in the direction of the incident field due to field enhancement by polarization at their tip and form an array with a spatial period on the order of one quarter wave length. The physics and dynamics of the filamentary plasma array will be discussed in a first part, on the basis of comparisons between model and experimental results. In a second part other aspects of microwave breakdown at atmospheric pressure will be presented, such as the development of microwave streamers (that can absorb very efficiently the microwave energy under specific, resonant conditions), the formation of complex nets of plasma filaments during breakdown in an under-critical field (breakdown is initiated next to a metallic initiator and propagates, due to thermal instabilities, in a region where the microwave field is below the critical field). The context of applications of this study (plasma aided combustion and flow control, breakdown next to an antenna) will be presented briefly.