Kenneth R. Hess

Optical investigations of excitation processes responsible for ionized sputtered species in a low pressure, low current, coaxial geometry glow discharge

Abstract Investigations of a low pressure (0.3–2.0 torr, 30–270 Pa), low current (1–4 mA), coaxial geometry glow discharge were undertaken to investigate the role of a charge exchange mechanism in the ionization of sputtered cathode material for argon and neon discharges. Emission from three copper ion transitions served as a probe of the charge exchange process. The three transitions monitored were the 221.81 and 224.26 nm transitions which are not accessible to a charge exchange reaction, and the 224.70 nm transition which may undergo charge exchange with an argon ion level. A significant enhancement of the 224.70 nm transition relative to the other two would provide evidence that a charge exchange process is occurring. The results of these investigations did not show a significant enhancement of the 224.70 nm transition, indicating that under these discharge conditions charge exchange is not a significant contributor to the observed ionization of copper sputtered from the cathode surface. In addition, the behavior of each transition was observed with variations in discharge pressure and with the addition of methane to the discharge. All three transitions behaved in an identical manner, with signal peaks occurring at the same pressure as the peak argon metastable population for a pure argon discharge, and signals that decrease linearly for the 10% methane/argon mixture. These studies indicate a common mechanism of ion formation for all three transitions, providing evidence that charge exchange is not a significant contributor to discharge ionization of copper at the low currents employed in these coaxial geometry glow discharges.

Glow Discharge Mass Spectrometry

The glow discharge is a simple, yet effective device which has found application in atomic absorption, atomic emission, and elemental mass spectrometry. It takes the form of a low-pressure gas discharge wherein rare gas ions are attracted to a sample cathode and efficiently sputter atoms from the surface. Figure 1 shows the essential components of the source. The discharge has been powered by dc, pulsed dc, and rf supplies, usually operating in a low power regime, with optimum currents and voltages determined by such factors as source configuration, discharge gas, and pressure. Nearly all the discharge voltage is dropped across the cathode dark space, serving to accelerate the argon ions onto the cathode, which is formed directly from the sample or compacted from a sample/matrix mixture. The glow discharge approach is simple, inexpensive, and shows relatively few matrix effects. Sensitivity in the ppb range is seen for most elements. A disadvantage of the technique is that the gas discharge produces certain background species which can create interferences for some elements, if alternative isotopes are not available.

Investigations of a Metastable Dependence on the Ionization of Sputtered Species in Neon Glow Discharges

Optical investigations of a low-pressure (0.3–4.0 Torr), low-current (1–4 mA), coaxial geometry glow discharge operating with neon as the fill gas are described. Studies were designed to experimentally illustrate the role of neon metastable atoms in the population of selected excited-state ion levels of copper atoms sputtered from a brass cathode. Methane was employed as a quenching agent to reduce the neon metastable population, and ion emission signals from a variety of copper ion transitions showed a decrease in intensity corresponding to the introduction of methane to the plasma. In addition, with variations in discharge pressure, a correlation between the number of neon metastables and the strength of the ion emission signals was observed. These results provide evidence that Penning ionization is an important mechanism for the ionization of sputtered atoms in neon glow discharges, similar to the results obtained for an argon system. Finally, a brief comparison of the neon and argon systems was made which showed the neon discharge gas to be more efficient at populating the monitored copper ion levels. This is most likely due to the higher energy of the neon metastables, which permits the direct population of these ion levels from the copper ground state.

Influence of Solution-Deposited Anions on Glow Discharge Relative Ion Yields

When standard addition methods were employed for quantification in glow discharge mass spectrometry (GDMS), relative ion yields for some, but not all, elements were observed to be greater when deposited as a solution residue than as a corresponding solid of the same composition. The origin of these effects was investigated and determined to result from the deposition of anions present in the solution standards. The enhancements could be duplicated from a solid by pretreatment with a solution containing the anion. The mechanism by which the enhancements occur is not well understood, but we believe it involves a form of reactive etching followed by collisional dissociation of the sputtered molecular species into metal ions. Preliminary research suggests that it may be possible to compensate for changes in relative ion yield with solution treatment by employing an appropriate internal reference standard, or to match closely the matrices of the sample and standard in such a manner that quantification by standard addition is possible.

Laser-Based Methods

The development of cost-effective laser systems has generated a host of laser hyphenated techniques that have been introduced into the analytical laboratory. These hyphenated techniques take advantage of the laser’s ability to deliver a high photon flux, high photon energies, and a narrow, tunable photon wavelength range to optimize specific atomization/excitation/ioniza-tion processes in an analytical procedure. A laser system coupled to a glow discharge is one such hybrid technique that offers unique opportunities for both diagnostic and analytical investigations. This chapter will serve as an introduction and overview of several reported methodologies that have advantageously combined laser systems and glow discharges.

Investigation into the Analytical Utility of Plasma Etching in Reactive Glow Discharge Plasmas

Tetrafluoromethane (CF4, 1.01% by weight) was added to the argon support gas of a hollow cathode glow discharge to investigate the analytical utility of etch atomization. When a conducting copper cathode was analyzed, the sputtering rate (as measured by weight loss) was reduced by a factor of five compared to operation with pure argon. Copper atomic absorbance and copper atomic emission intensity were also reduced by factors of seven and two, respectively. When a nonconducting sample was analyzed, the stainless steel ring that held the sample acted as an auxiliary cathode, supporting the discharge processes. Radical fluoride species formed in this discharge reacted with the nonconducting substrate (silica) to produce volatile SiF4 that spontaneously evolved into the gas phase, carrying with it copper and uranium. This approach is analogous to plasma etching, a well-established technique for semiconductor processing. Atomic emission data were obtained with a pure argon discharge and an argon/CF4 discharge. With the addition of CF4, a 30% enhancement was observed for uranium in glass and a 50% enhancement for copper in glass. Scanning electron microscopy (SEM) was used to support the supposition that etching of the silica matrix on the inner surface of the hollow cathode contributed to this enhancement.

A Novel Approach to Resonance Ionization Mass Spectrometry Employing a Glow Discharge Atom Source

Abstract This paper reports on preliminary efforts to design and implement a novel combination of a laser, a glow discharge atom source, and a magnetic sector mass spectrometer to be used for resonance ionization of the discharge sputtered material. In this experimental design, a glow discharge source has been constructed and interfaced to a magnetic sector mass spectrometer. The laser is then focused in a spatial region between the exit orifice of the glow discharge source and the extraction optics of the mass spectrometer. Laser interaction in the low pressure region external to the discharge plasma is believed to provide several advantages for resonance ionization over previous designs in which the laser was allowed to interact within the discharge plasma. Preliminary results show resonance ionization to improve the isotopic ratios of iron in an aluminum matrix (iron-58 content of 396 μg g −1 ) relative to those observed directly from the glow discharge which are subject to isobaric overlaps.

Study of laser resonance ionization mass spectrometry using a glow discharge source

The mass spectra of a metal alloy sample consisting of Al, Cu and Fe were studied using both glow discharge mass spectrometry (GDMS) and resonance ionization mass spectrometry (RIMS). Particular emphasis was placed on the reduction of isobaric interferences and discrimination between those ions formed by the discharge and those formed by the laser radiation.