Review
Pulsed glow discharges for analytical applications

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Abstract

Non equilibrium plasmas such as glow discharges have become a commonly used tool in direct surface and interface analysis of solid materials. The application of pulsed glow discharges to material analysis has been studied by several research groups over the last 20 years. Two European projects, EMDPA and GLADNET currently work on the analytical applications of glow discharges, giving a particular attention to pulsed discharges. This review demonstrates the advantages of pulsed discharge operation by showing how the specific excitation and ionisation processes observed during the plasma ignition phase and the afterglow can be used for analytical applications.

In the first part of the review the dominant physical processes occurring during the plasma ignition and the afterglow of a pulsed plasma are reviewed. For both phases, the evolution of the population of electrons and sputtered atoms is discussed and related to the excitation and ionisation processes. In view of the complexity of the processes occurring and the variety of experimental conditions presented in the published papers, we have made some effort to point-out and compare the relevant features of the various experimental set-ups used.

In the second part of this review, analytical applications of pulsed discharges for both mass spectrometry and optical emission techniques are presented and discussed. In particular the importance of time resolved signal acquisition is pointed out. The question of why pulsed discharges have not yet been introduced in routine analysis despite their obvious advantages over the continuous mode is discussed. Finally the first exciting results of the application of a pulsed glow discharge to surface and interface analysis of polymer multi-layers are shown.

Introduction

Glow discharge (GD), coupled to optical emission (GDOES) or mass spectrometry (GDMS), is now an established technique for direct analysis of solid samples [1], [2]. In particular, when a Grimm type source [3] is used or a modified design based on Grimm's geometry [4], [5], [6], [7] glow discharge analytical instruments are appreciated for their superb depth resolution [8], [9]. The relative ease of use and straight forwardness of data interpretation[10] are also appreciated by the analysts. Commercial GD spectrometers are employing DC sources for conductive materials and RF sources for both conductive and non-conductive samples [11] and cover a large range of applications [12]. GDOES instruments are used for bulk analysis of homogeneous samples as well as for surface, depth profile and interface analysis. GDMS instruments employing high resolution sector field spectrometers offer detection limits in the low ppb range [13] and beyond but; due to the sequential ion detection, the field of application of these instruments is restricted to bulk analysis and possibly depth profile analysis of rather thick layers in the micrometer range [14], [15].

Pulsing the glow discharge offers a number of advantages [16]. One is to provide an additional way of controlling the plasma by choosing the pulse parameters, such as pulse length and period, in order to select optimal discharge conditions (Fig. 1). Another interesting property is that the instantaneous power, responsible for the sputtering, excitation and ionisation yields can be chosen nearly independent from the average power responsible for thermal stress on the sample, by varying the duty cycle of the applied pulses. Finally within a single pulse different discharge processes take place at different times allowing selective measurements if a time resolved acquisition is available [17].

The first optical glow discharge spectrometers based on the Grimm source, were commercialized by RSV in the 1980s. They used a pulsed (400 Hz) dc power supply. In these instruments, the average discharge power was adapted to the analytical needs by varying the pulse duty cycle at fixed instantaneous excitation voltage. Later, when surface and interface analysis became the most important field of applications of GDOES spectrometry, continuous sources were used. Today many high-end instruments are again equipped with optional pulsed sources, but to our knowledge, none of the commercially available instruments exploits the intrinsic advantages of modulated excitation and time resolved data acquisition (except for a locally available system provided by Horiba in Japan [18]).

By comparison, in spark-OES and Atomic Absorption Spectroscopy [19], considerable improvements of the analytical performance have been achieved using time resolved detection and data analysis techniques.

Section snippets

Physical aspects

This review deals with the analytical applications of pulsed glow discharges. Understanding the analytical benefit of pulsing a glow discharge requires some insight into the underlying physical processes. First we will deal with the time characteristics of the main processes occurring in a glow discharge and give some information on the dynamics of the sheaths. We will then focus on the ignition phase and the afterglow.

Analytical applications

Glow discharges are used as a versatile tool in analytical sciences. They are applied to the direct analysis of homogeneous solids, generally conducting materials. Using radio frequency excitation of the discharge, non-conducting materials can also be analyzed, though the lack of adequate certified reference materials often does not allow a precise quantification for non-conductors. For this “bulk” application the most important factors are the limits of detection, the time and ease of analysis

Conclusion

In this review we have tried to demonstrate that glow discharges may well be exciting plasmas [141], but pulsed glow discharges are even more exciting. The temporal sequence of ignition, plateau and afterglow gives access to more information than just stable operation. During short pulses, transient conditions can be obtained, which are not accessible to continuous operation.

In the first part we have concentrated on the physical aspects of the pulsed operation of a glow discharge, giving some

Acknowledgement

The authors are member of the Technological Research Team ERT 2000, CUFR JFC Albi, FR and gratefully acknowledge financial support from the European Community through the FP6 contracts STREP-NMP no. 032202 (EMDPA) and MRTN-CT-2006-035459 (GLADNET). We would like to thank L. Pitchford (Laplace, Toulouse, Fr); A. Martin (Shiva Technologies, Tournefeuille, Fr), L. Thérèse (CURF JFC, Albi, Fr) and P. Chapon (Horiba Jobin Yvon, Lonjumeau, Fr) for stimulating discussions during the preparation phase

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