Alan Garscadden

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

A “reference cell” for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

Friction and wear of diamond and diamondlike carbon coatings

Abstract Reciprocating sliding friction experiments were conducted with a natural diamond flat, diamond films, and low- and high-density diamondlike carbon (DLC) films in contact with pin specimens of natural diamond and silicon nitride (Si 3 N 4 ) both in humid air and in dry nitrogen. The results indicated that for natural diamond pin contacts the diamond films and the natural diamond flat were not susceptible to moisture but that moisture could increase both the coefficients of friction and wear factors of the DLC films. The coefficients of friction and wear factors of the diamond films were generally similar to those of the natural diamond flat both in humid air and in dry nitrogen. In dry nitrogen the coefficients of friction of the high-density DLC films in contact with pin specimens of both diamond and Si 3 N 4 were generally low (about 0.02) and similar to those of the natural diamond flat and the diamond films. The wear factors of the materials in contact with both natural diamond and Si 3 N 4 were generally in the ascending order of natural diamond flat, diamond film, high-density DLC film, and low-density DLC film. Moisture in the environment increased the coefficients of friction for Si 3 N 4 pins in contact with all the materials (natural diamond flat, diamond films, and DLC films). This increase in friction is due to the silicon oxides film produced on the surface of Si 3 N 4 pins in humid air.

Friction and wear of plasma-deposited diamond films

Reciprocating sliding friction experiments in humid air and in dry nitrogen and unidirectional sliding friction experiments in ultrahigh vacuum were conducted with a natural diamond pin in contact with microwave‐plasma‐deposited diamond films. Diamond films with a surface roughness (R rms) ranging from 15 to 160 nm were produced by microwave‐plasma‐assisted chemical vapor deposition. In humid air and in dry nitrogen, abrasion occurred when the diamond pin made grooves in the surfaces of diamond films, and thus, the initial coefficients of friction increased with increasing initial surface roughness. The equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. In vacuum the friction for diamond films contacting a diamond pin arose primarily from adhesion between the sliding surfaces. In these cases, the initial and equilibrium coefficients of friction were independent of the initial surface roughness of the diamond films. The equilibrium coefficients of f...

Synthesis and characterization of fine grain diamond films

A fine grain diamond film has been developed by microwave plasma assisted chemical vapor deposition. Various analytical techniques, including Rutherford backscattering, proton recoil analysis, Raman spectroscopy, and X‐ray diffraction, were utilized to characterize the diamond films. The grain size of the film was determined from bright and dark field electron micrographs, and found to be 200–1000 A. The films exhibited good optical transmission between 2.5 and 10 μm, with a calculated absorption coefficient of 490 cm−1. The friction coefficients of this film were found to be 0.035 and 0.030 at dry nitrogen and humid air environments, respectively, and the films had low wear rates.

Ion chemistry in octafluorocyclobutane, c-C4F8

Abstract Cross-sections for electron impact ionization of octafluorocyclobutane (c-C4F8) have been measured from 10 to 200 eV by Fourier transform mass spectrometry. No parent ion is observed, and over half of the dissociative ionization yields C2F4+ and C3F5+. Eleven other fluorocarbon cations are produced with smaller cross-sections, giving a total ionization cross-section of (1.6±0.2)×10−15 cm2 between 80 and 200 eV. Only CF2+ and C2F3+ react further with the parent molecule to yield C3F5+ as the primary product. No evidence of cationic polymerization was found. F− and C4F8− are formed by electron attachment at energies below 10 eV, but neither reacts further with c-C4F8.

Rydberg state Stark spectroscopic measurement of electric‐field profile in a glow discharge

The electric‐field profile in the cathode‐fall region of a low‐pressure helium gas discharge has been measured by high‐level Rydberg state Stark spectroscopy using optogalvanic detection. The Stark manifolds of triplet atomic helium Rydberg states with principal quantum numbers n=15 up to 20 have been used to measure the electric‐field vector using both Δmj=0 and ‖Δmj‖=1 laser polarizations. The absolute accuracy of the electric‐field measurement was found to be 5% or better. The ‖Δmj‖=1 polarization Stark spectra exhibit pseudohydrogenic behavior in Stark splitting, as well as in the manifold intensity distribution. The Stark splitting of states corresponding to 1>4 was almost hydrogenic for all the measured principal quantum‐number states and the intensity distribution became approximately hydrogenic for n≥16 at electric fields ≤700 V/cm.

DISPERSION AND STABILITY OF MOVING STRIATIONS.

Experiments are reported on the waves of ionization which can be excited in the low‐ and medium‐pressure discharge in argon, neon, and a mercury‐argon mixture. The glow to arc transition and the change from a forward wave to a backward wave of ionization were studied in detail using a space‐time display which immediately reveals the dispersion of the waves. Evidence is also presented for the transition from a convective to an absolute instability. A comparison is made with the Pekarek‐Krejci theory of moving striations and using Derflers stability criteria, calculations are presented to show that this theory predicts both forward and backward waves which can be stable, convectively unstable, or absolutely unstable.

Wave Nature of Moving Striations

Abstract : The results are presented of time and space resolved measurements on the response of the positive column of a low-pressure glow discharge to external perturbations. From these the profile, structure and the backward wave nature of moving striations are clearly indicated. A new method of solution to the equation originally presented by Pekarek (Proceedings of Sixth International Conference on Ionization Phenomena in Gases, v.2, p. 133) is shown to contain all of the experimental results including the dispersion of a pulse disturbance. A stability criterion is derived; especially useful is that the onset of striations in a discharge can be calculated from a quadratic equation. (Author)

Optical diagnostics of positive column dc discharge by Rydberg state spectroscopy

High resolution measurements of the Stark broadening of very high Rydberg 1P states in helium provide a resolution in electric field of approximately 1 V cm−1. The tunable laser‐induced optogalvanic signal experiments were performed in a low pressure, low current discharge. The high sensitivity permits, for the first time, the net electric field to be obtained by a spectroscopic technique. The relative metastable density profile and indirectly, the electron density profile are also obtained from the magnitudes of the optogalvanic signals.

INFLUENCE OF XENON ON CO2 LASER PLASMAS

Measurements are reported showing the influence of xenon on the discharge properties of CO2 lasers. Small admixtures of xenon lower the electron temperature considerably and it is possible to operate the CO2 laser without nitrogen and with little decrease in output power for low flow rates or sealed‐off operation. The influence of added CO is reported briefly. It is proposed that direct electronic excitation to the vibrational levels of CO and consequent vibrational energy transfer from the CO (v = 1) to the CO2 upper laser level is the most important mechanism when N2 is absent.