Stephen Brewer

PHYSICAL PROPERTIES OF A PLANAR MAGNETRON GLOW DISCHARGE

Abstract Continued work with a planar magnetron glow discharge plasma device which uses permanent magnets located behind a planar cathode to trap plasma electrons in circular orbits parallel to the cathode surface is described. This results in a significant reduction in plasma voltage, and stable operation is achieved over an extended range of pressures and cathode current densities. The plasma has the form of a ring extending from the cathode surface to several millimeters from the surface. The plasma geometry is determined by the locus of greatest magnetic field strength. Cathode current density values greater than 100 mA cm 2 are easily achieved for pressures in the range 0.0007 Torr (0.093 Pa) to over 2.5 Torr (330 Pa). Over this range, the plasma voltage is under 500 V. Spatially-resolved emission spectral data are presented showing the effects of plasma current and pressure on line intensities from plasma gas species as well as from cathodically-sputtered species. Data are presented for a pure Cu cathode and for a Zn-based alloy used as a standard reference material.

Preliminary studies of analytical applications of a novel magnetron glow discharge plasma

The application of a magnetron glow discharge plasma, previously described, to the analysis of six zinc-base NIST Standard Reference Materials for a minor element, aluminum, and three trace elements, magnesium, nickel, and manganese, is described. The wide range of argon pressures, from 0.0007 to more than 2.5 Torr, over which the plasma can be operated, allows selection of optimum pressure for determination of each element. Line-to-background ratios are examined as functions of pressure for each element determined at discharge currents ranging from 100 to 400 mA. Sensitivities of net line intensities to discharge current show wide variations, depending on the element and line studied. Analytical curves for aluminum, magnesium, manganese, and nickel are shown. Detection limits for manganese and nickel are 0.0060% and 0.00055%, respectively.

Sputtering Behavior of a Magnetron Glow Discharge Device

The sputtering characteristics of a previously described dc planar magnetron glow discharge device are described for six different alloys and metals. Low-pressure linear etch rates of Cu-alloyed Al, steel, high-purity Cu, leaded brass, Al-alloyed Zn, and high-purity Ag and Au are found to be higher than those achieved with other planar magnetron devices. Direct mass-loss measurements, made from 0.0007 to 4.0 Torr pressure of Ar support gas at plasma currents varying from 50 to 400 mA, show that mass loss rates are generally greatest for the highest-atomic-number species at the highest currents and lowest pressures. Comparison of mass loss rates in pure Ar and 1/1 Ar/CF4 at low pressure shows enhancement of mass loss rates for Al and Ag in the mixture, but reduction of mass loss rates for Cu, brass, Zn, and Au in the mixture. Addition of CF4 to the Ar support gas results in significant quenching of numerous spectral features at both high and low pressures.

Direct Analysis of Solid Powder Samples with Electrically Vaporized Thin-Film Sampling and ICP Detection

Solid powder samples deposited on Ag thin films are electrically vaporized by capacitive discharge. The aerosol produced by this method is introduced into the ICP. A low-volume vaporization chamber has been designed for these experiments. A magnetic field of a few kG normal to the electric field in the thin-film plasma is used to improve the plasma/sample interaction. A continuous flow of Ar or 60%/40% Ar/O2 through the chamber carries the aerosol from the capacitive discharge plasma into the ICP. Analytical data will be presented for a number of NIST reference materials emphasizing the determination of Mn, V, and Ni in biological and refractory inorganic matrices.

Sampling, Excitation, and Ionization Characteristics of a Planar Magnetron Glow-Discharge Device

Measurements of mass loss during sputtering, atomic emission intensities, and ionization characteristics are described for a coaxial magnetron glow-discharge device. By the use of the magnetic field from a coaxial magnet pair placed behind the cathode, stable plasma operation is achieved for all pressures from 0.0004 Torr (0.05 Pa) to over 2.5 Torr (330 Pa). Mass loss measurement values from pellets of Al, Cu, brass, Zn, and Au located in the region of greatest sputtering show much larger values at pressures which are lower than those usually used with glow-discharge excitation and ionization sources. Emission line ratio measurements at the lower pressures indicate that populations of low-energy electronic states are greater than equilibrium values. This observation suggests that the plasma is dominated by ionization processes rather than by recombination processes. Emission line ratio and mass spectrometric measurements also indicate that ionization is increased at the lower pressures.

Studies of Aerosols Generated by Electrically Vaporized Thin Films for ICP-AES

Aerosols generated by the electrical vaporization of samples placed on thin silver films for subsequent introduction into an inductively coupled plasma have been examined. Of the silver ablated during electrical discharges, less than 10% is trapped on 0.1-micrometer pore-size filters placed at the end of the transfer tube. Much larger masses of carbonaceous particulate material, up to 0.25 mg, are introduced into the ICP torch, possibly accounting for intense time-dependent band and continuum emission. Scanning electron microscopy of trapped particles of exploded film material shows evidence of melting. Particles of Mn and VC collected after film vaporization also show evidence of size reduction. The higher boiling point of VC may account for low emission intensities observed when larger particles of this material are taken as samples.

Application of a Magnetron Glow Discharge to Direct Solid Sampling for Mass Spectrometry

The behavior of a planar magnetron glow discharge used as a sampling device and ion source for mass spectrometry of conducting solids is examined. Low-pressure operation of the magnetron allows efficient sputtering and ion production from conducting samples. Interfacing the magnetron with the mass spectrometer is simplified by low-pressure magnetron operation. A special sample holder allows up to five sample plugs to be analyzed without the source chamber vacuum being broken. Sample ion current is not found to be a simple function of discharge current, but is a strong function of sample axial position relative to the mass spectrometer sampling cone. The mass spectrometer sampling cone orifice diameter may be varied from 0.5 to 5.0 mm. A cone orifice diameter of 2.0 mm yields optimum analytical conditions. Determination of Al at the 3–5% level in a series of NIST Zn-base alloy standards yields a straight-line analytical curve, suggesting the potential application of the device to high-precision analyses of alloys.

Surface Features of Conductors Eroded by Sputtering in a Magnetron Glow Discharge Plasma

Surface characteristics of four different metals and alloys sputtered in Ar in a plasma generated by a previously described dc planar magnetron are examined. The study of Al alloy, Cu, brass, and Au samples indicates qualitatively that erosion increases with decreasing support gas pressure, increasing discharge currents, and increasing atomic number of the sputtered elements. These observations are supported by quantitative studies of mass loss rates as functions of support gas pressure, discharge current, and sample atomic number. Microscopic examination reveals hillocks formed on sputtered surfaces. With increased mass loss, hillocks tend to overlap. No correlation between inclusions and hillock locations is observed. Silver samples, though relatively (99.9%) pure, behave anomalously, acquiring a Cu coating at low discharge support gas pressures.