Richard F. Fernsler

Surface composition, chemistry, and structure of polystyrene modified by electron-beam-generated plasma.

Polystyrene (PS) surfaces were treated by electron-beam-generated plasmas in argon/oxygen, argon/nitrogen, and argon/sulfur hexafluoride environments. The resulting modifications of the polymer surface energy, morphology, and chemical composition were analyzed by a suite of complementary analytical techniques: contact angle goniometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and reflection electron energy loss spectroscopy (REELS). The plasma treatments produced only minimal increases in the surface roughness while introducing the expected chemical modifications: oxygen-based after Ar/O(2) plasma, oxygen- and nitrogen-based after Ar/N(2) plasma, and fluorine-based after Ar/SF(6) plasma. Fluorinated PS surfaces became hydrophobic and did not significantly change their properties over time. In contrast, polymer treated in Ar/O(2) and Ar/N(2) plasmas initially became hydrophilic but underwent hydrophobic recovery after 28 days of aging. The aromatic carbon chemistry in the top 1 nm of these aged surfaces clearly indicated that the hydrophobic recovery was produced by reorientation/diffusion of undamaged aromatic polymer fragments from the bulk rather than by contamination. Nondestructive depth profiles of aged plasma-treated PS films were reconstructed from parallel angle-resolved XPS (ARXPS) measurements using a maximum-entropy algorithm. The salient features of reconstructed profiles were confirmed by sputter profiles obtained with 200 eV Ar ions. Both types of depth profiles showed that the electron-beam-generated plasma modifications are confined to the topmost 3-4 nm of the polymer surface, while valence band measurements and unsaturated carbon signatures in ARXPS and REELS data indicated that much of the PS structure was preserved below 9 nm.

Models of lightning‐produced sprites and elves

Three different types of optical phenomena have been observed at high altitude above thunderstorms: an enhanced airglow (“elves”) at roughly ∼90 km; a reddish glow (“sprites”) from 50 to 90 km; and an upward moving, bluish emission (“jets”) below 40 km. A likely explanation for some or all of these phenomena is gas breakdown caused by the electromagnetic fields of lightning discharges. This paper examines the connection between these fields and breakdown at high altitude, using both analytic models and numerical simulations. Included in the calculations are the radiation fields from the lightning return stroke and the quasi-static fields from the continuing current. The different nature of the two fields is shown to produce two distinct types of breakdown, with characteristics similar to those of elves and sprites. Also mentioned is a third breakdown mechanism which may account for blue jets.

Theoretical overview of the large-area plasma processing system (LAPPS)

A large-area plasma processing system (LAPPS) is under development at the Naval Research Laboratory. In the LAPPS, the plasma is generated by a sheet electron beam with voltages and current densities of the order of kilovolts and tens of milliamps per cm2. The plasma dimensions are a metre square by a few centimetres thick. The beam is guided by a magnetic field of 50-300?G. Since an electron beam of this type efficiently ionizes any gas, high electron densities of n~1012-1013?cm-3 are easily generated at 30-100?mTorr background pressure. In addition to large area and high electron density, the LAPPS has advantages for plasma processing. These include independent control of ion and free radical fluxes to the surface, very high uniformity, very low electron temperature (Te, {<}1?eV, but can be controllably increased to a desired value) and a geometry that is well suited for many applications. This paper sketches an initial theoretical overview of issues in the LAPPS and compares aspects of the theory to a preliminary experiment.

Production of large-area plasmas by electron beams

An analysis is presented for the production of weakly ionized plasmas by electron beams, with an emphasis on the production of broad, planar plasmas capable of reflecting X-band microwaves. Considered first in the analysis is the ability of weakly ionized plasmas to absorb, emit and reflect electromagnetic radiation. Following that is a determination of the electron beam parameters needed to produce plasmas, based on considerations of beam ionization, range, and stability. The results of the analysis are then compared with a series of experiments performed using a sheet electron beam to produce plasmas up to 0.6 m square by 2 cm thick. The electron beam in the experiments was generated using a long hollow-cathode discharge operating in an enhanced-glow mode. That mode has only recently been recognized, and a brief analysis of it is given for completeness. The conclusion of the study is that electron beams can produce large-area, planar plasmas with high efficiency, minimal gas heating, low electron temper...

Breakdown of the neutral atmosphere in the D region due to lightning driven electromagnetic pulses

Electromagnetic pulses (EMP) driven by lightning can cause breakdown of the neutral atmosphere in the lower D-region. Using a computer simulation model, we study the dependence of the breakdown on the pulse strength, the orientation of the lightning discharge, the ambient plasma density, the ionization model, and the neutral density. For a discharge along a straight line the EMP is strongest in the plane perpendicular to the current so that for a given current, horizontal discharges will radiate the D-region more strongly than a vertical discharge. For horizontal currents, breakdown occurs for E100 > 20 V/m (I > 55 kA) in a low-density, nighttime ionosphere, where E100 is the amplitude of the pulse normalized to 100 km from the discharge and I is the discharge current. Vertical strokes require E100 > 50 V/m (I > 140 kA). Discharges with higher currents and fields form ionization patches which are larger in volume, larger in degree of ionization, and lower in altitude. The ionization is most sensitive to the pulse strength, pulse orientation, ambient plasma density, and neutral gas density at breakdown threshold. Higher ambient plasma densities reduce the ionization, but for large EMP, breakdown can occur even with high daytime densities. The breakdown increases the plasma density which acts to limit the EMP and ionization. This feedback reduces the sensitivity of the breakdown to the ionization model. Neutral density variations, such as caused by atmospheric gravity waves, can cause spatial variations in the ionization density.

Experimental investigations of the formation of a plasma mirror for high‐frequency microwave beam steering

The Naval Research Laboratory (NRL) has been studying the use of a magnetically confined plasma sheet as a reflector for high‐frequency (X‐band) microwaves for broadband radar applications [IEEE Trans. Plasma Sci. PS‐19, 1228 (1991)]. A planar sheet plasma (50 cm×60 cm×1 cm) is produced using a 2–10 kV fast rise time square wave voltage source and a linear hollow cathode. Reproducible plasma distributions with density ≥1.2×1012 cm−3 have been formed in a low‐pressure (100–500 mTorr of air) chamber located inside of a 100–300 G uniform magnetic field. One to ten pulse bursts of 20–1000 μs duration plasma sheets have been produced with pulse repetition frequencies of up to 10 kHz. Turn on and off times of the plasma are less than 10 μs each. The far‐field antenna pattern of microwaves reflected off the plasma sheet is similar to that from a metal plate at the same location [IEEE Trans. Plasma Sci PS‐20, 1036 (1992)]. Interferometer measurements show the critical surface to remain nearly stationary during th...

Experimental and theoretical evaluations of electron temperature in continuous electron beam generated plasmas

In this paper an experimental and theoretical evaluation of electron temperature in continuous, electron beam generated plasmas is presented. Spatial distributions of electron temperature and plasma density in pure and diluted argon were measured. The dependence of the electron temperature and plasma density on pressure, gas composition, hollow cathode voltage and magnetic field was investigated as well. It was observed that the electron temperature in argon was less than 1 eV and that a small addition of nitrogen reduced the electron temperature even more. The magnetic field, pressure and beam current did not strongly affect the electron temperature but greatly influenced the plasma density. The experimental findings are supported by analytical estimations of electron temperature in both noble and molecular gases.

Beam-generated plasmas for processing applications

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis ...

Argon metastables in a high density processing plasma

Absolute densities of metastable argon atoms (Paschen 1s5, 1s3) and the intermediate resonant state (1s4) were measured in a high density plasma etching environment. Excited species densities were measured ranging from 108 to 3×109 cm−3, depending on the particular atomic state. A straightforward reaction rate formalism consisting of only two competing electron-atom collision rates accurately predicts such densities. Because of the low densities of these long-lived excited state species, all excited argon species need to be considered only as energy loss channels in modeling high density (1011–1012 cm−3), low pressure (∼1 mTorr) plasma sources. Metastable production rates were also used to identify energy transfer mechanisms under etching conditions of Cl2/Ar mixtures and substrate biasing in the reactor.

Plasma acceleration by area expansion

As the area of a plasma increases, the plasma can accelerate smoothly from subsonic to supersonic velocity. The singularity which ordinarily occurs at the sonic velocity is resolved not by charge separation, as is the case for a sheath, but rather by a zero in the numerator at the same spatial position as the zero in the denominator, the sonic point. That is, at the sonic point, the acceleration due to expansion just cancels out the deceleration due to ion and electron neutral collisions. It turns out that, in this configuration, the plasma can accelerate to about three times the ion sound speed. The electron temperature is determined by the geometry, gas species, and, mostly, by the gas pressure. Applications to the production of a stream of neutrals for etching, and to space plasma propulsion are discussed.