Recalibration-free measurement of layer thicknesses by glow discharge optical emission spectrometry (GD-OES) –based on nitrocarburized samples

M. Wilke(1), M. Analytis(2), V. Breternitz(1), Ch. Knedlik(1), G. Teichert(3)
1 TU Ilmenau, Department of Materials for Electrical Engineering
2 Spectruma-Analytik GmbH, Hof/ Saale, Germany
3 Institute for Materials Research and Testing at the Bauhaus-University Weimar

Glow discharge spectrometry has been broadly used for quantitative depth-profile analysis not only in the field of research but also in the area of quality assurance and the process accompanying analysis. Because of the short measurement times, high reproducibility, low analysis costs and the possibility to measure curved surfaces with an adapter, GD-OES becomes a method in the routine analysis of nitrided and nitrocarburized samples. The decreasing transparency as a result of contaminations on the lens between the glow discharge source and the optical spectrometer demands a periodical recalibration of the instrument. Furthermore in the available quantification algorithms the electronic and lattice structures in nitrides and oxides as well as a possible porosity causes deviations in the calculated depth which can extend 10%.
The present work has pointed out a possible way for recalibration free measurements of the compound layer thickness by using several characteristic curve points in the depth profile of the elements Fe, C and N. Calibration graphs were recorded by plotting the sputtering time of those points versus the known layer thickness of calibration samples. Linear correlations were found for the inflection, turning  and maximum points of the Fe-, C- and  N-profiles. By doing so, it is not necessary to define one point of the profile which represents the thickness of the compound layer but it is possible to use all points which change continuously with the increasing layer. This also improves the accuracy of the method. The results indicate that the sputtering time determination of the selected curve points is independent of the decreasing transparency and the changing background, so that recalibration-free measure­ments with an accuracy better than 6% can be performed. Once recorded, those calibration graphs can be adapted for any discharge parameters because of the linear correlation between sputtering rate, voltage and current. Furthermore the suitability for other applications especially of thicknesses notedly below 1 µm shall be demonstrated.

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Signal Enhancement in GD-MS Analysis of Pure Metals by Application of Argon – Helium Mixtures as Discharge Gas

Britta Lange, Tamara Gusarova, Ralf Matschat, Heinrich Kipphardt and Ulrich Panne

Federal Institute for Materials Research and Testing (BAM), Richard-Willstätter-Straße 11, 12489 Berlin

Glow Discharge Mass Spectrometry (GD-MS), with its low limits of determination, is an excellent technique for fast multi-element analysis of solid ultra-high purity metals. In addition to metallic impurities also non metals can be determined. But the sensitivity for these elements can be limited because of their high 1th ionisation potentials. Elements whose 1th ionisation potentials are in the range or higher than that of the discharge gas cannot effectively be ionised. The common discharge gas used in GD-MS is Argon with a 1th ionisation potential of 15.75 eV.
To improve the sensitivity for elements with high ionisation potentials we investigated the influence of different parameters at our GD mass spectrometer, an Element GD (Thermo Fisher Scientific). The investigated parameters were discharge current and voltage, discharge gas flow and, additionally, the discharge gas composition which had the greatest effect on sensitivity of the measured intensities. We used Helium-Argon mixtures because of the very high ionisation ability of He, especially in terms of the high energy level of its metastable states. We tested different Ar-He compositions and studied the effect on the peak intensity of various impurities, especially of non metals. The investigated matrix was pure copper material. With Ar-He mixtures, we achieved signal enhancements up to 4000% in comparison with Ar alone used as discharge gas. We used pressed powder samples doped with definite mass fractions of trace analytes for calibration. In this way, we were able to establish traceable calibration curves for different elements down to the µg kg-1 range with high sensitivity and very good linearity which was not possible when Argon alone was used as discharge gas.

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Model of microsecond pulsed glow discharge for mass spectrometry

M. Voronov, V. Hoffmann,
Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden e.V.  (IFW Dresden), Helmholtzstr. 20, 01069 Dresden, Germany. e-mail: VoronovMV@mail.ru

A. Ganeev, Lumex Ltd., Moskovsky pr.19, 190005, Saint-Petersburg, Russia; Saint-Petersburg State University, Department of Chemistry, Universitetsky pr.26, 198504, St.Petergoff, Saint-Petersburg, Russia.

A new comprehensive model of microsecond pulsed glow discharge is presented. The model includes simulation of the discharge pulse phase and afterglow (see fig. 1). The simulation of the discharge pulse is based on the Monte-Carlo simulation of particles movement and a self developed method of electrical field computation based on phenomenological laws of plasma behavior. The last method is especially developed for simulation of the electrical field in microsecond time range. The simulation of afterglow is based on solution of continuity equations. At that results of the discharge are used as input parameters for afterglow model. All basic processes forming the pulsed discharge and afterglow are included in the model.
The model is suitable for both hollow cathode and Grimm type sources. A satisfactory agreement between model results and different experimental and theoretical data is demonstrated. Processes of sample sputtering, ionization and transportation in microsecond pulsed glow discharge and it’s afterglow are studied. It is shown an important role of Penning ionization of the sample atoms in afterglow. The model results are applied for optimization of parameters of a Time-of-Flight mass-spectrometer with hollow cathode ion source and for investigation of processes in a fast flow Grimm type ion source.

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MODELLING OF GDOES DEPTH PROFILES OF METAL (Cr,Ti) MULTILAYER COATINGS

R. Escobar Galindo and J.M. Albella;
Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, E-28049 Madrid (Spain); e-mail: rescobar@icmm.csic.es

The broadening effects found in the depth profiles of abrupt periodic multilayers obtained by Glow Discharge Optical Emission Spectroscopy (GDOES) have been modelled by assuming that the resulting absolute concentration profiles can be described by Gaussian functions of constant area (equal to that of the cross section of the individual layers) and decreasing amplitude. The full width at half maximum (FWHM) of these functions has been assumed to increase with depth z according to a power law of the type: FWHM (z) = a + bz^c, using for a and c, values estimated from previous works. In this model, the parameter b, defining the broadening and degradation of the layer interfaces, is obtained by fitting the experimental profiles with the theoretical spectra. We have found a close correlation between b and the erosion rate of the material layers. In general, the model properly describes the depth profiles of multilayer structures made of alternating Ti and Cr layers in the nanometre range, and can be an useful tool to explain the profiles of more complex systems.

The graphic shows a) GDOES experimental depth profile of 10 x (70 nm Ti / 150 nm Cr) multilayer, b) Simulated depth profile using the nominal individual thickenesses.

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