Introduction to glow discharge analytical spectrometry

Introduction

This Glow Discharge Laboratory of optical emission spectroscopy is part of The Spectroscopy Net, a free and open resource originally designed by Dr Richard Payling of Surface Analytical, for Spectroscopists everywhere and everyone interested in Spectroscopy

Glow Discharge Spectroscopy (GDS) provides a rapid, direct bulk analysis and depth profiling analysis of solids: metals, powders, polymers, glasses and ceramics. Glow Discharge Optical Emission Spectrometry (GD-OES) comprises a glow discharge source and one or more optical spectrometers.

The principle of operation is fairly easy to understand. In a glow discharge, cathodic sputtering is used to remove material layer by layer from the sample surface. The atoms removed from the migrate into the plasma where they are excited through collisions with electrons or metastable carrier gas atoms. The characteristic spectrum emitted by this excited atoms is measured by the spectrometer.

Find out more about glow discharges on our web pages.

A Glow Discharge Optical Emission Spectrometer (GD-OES) comprises a glow discharge source and one or more optical spectrometers. A schematic layout is given to the right. The spectrometer displayed here uses a concalve grating in the Rowland circle or Paschen-Runge configuration and photomultiplier tubes for the light detection.

The use of solid state detetectors, CCD's and photo diode array's have become a common alternative for PM tubes. These detectors allow the acquisition of the entire spectrum, or at least a large portion of it, but are usually slower then PM tubes and therefore not suitable for very short acquisition times used in thin film analysis.

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Glow discharge; Example of application

After calibration, glow discharge optical emission spectoscopy can provide a quantitative depth profile (QDP) or compositional depth profile (CDP) of materials. A typical example, an eight years old computer hard disc is shown at the left. Both results are displayed with courtesy of Max Aeberhard, EMPA Thun, CH.

The upper graphic shows the outer layers of the computer hard disk: a carbon film, followed by Cobalt and Chromium layers. At a depth of 75 nm the Nickel-Phosphate layer starts.

The lower graphics shows the same analysis on the µ-meter scale. Here we can distinguish the 5µm thick NiP treatment on the aluminium-magnisium alloy.

The expampel demonstrates the ability of GDOES to analysis in a single run both nano-meter thin layers, µm thick layers and the bulk-material.

The field of application of glow discharge optical emssion spectrometry is fairly wide :

Bulk analysis by GD Spectrometry of :
- Steel (low and high alloyed, leaded, re-sulphurised)
- Tungsten
- Magnesium
- Iron (as-cast, chilled-cast, grey, ductile)
- Zinc
- Aluminium (high Si, composites)
- Copper, Brass, Bronze (leaded),
- Low Melting Point Alloys, Solders (Sn, Pb)
- Nickel, Titanium, Cobalt
- Carbides
- Powdered Metals
- Elements not in solid solution

Composition Depth Profile by analytical GD Spectrometry
- Galvanizing (EG, Hot Dip, Galvalume, Galvanneal, Galfan, Zinc Nickel)
- Clad (Aluminium)
- Oxide layers, corrosion studies
- Plating (Sn, Cr, Cd, Ni, Cu)
- Organic coatings
- Thermochemical treatments (Carburising, Nitriding, Carbonitriding)
- Electrochemical treatments
- Semiconductors
- Glass/Ceramics
- Hard coatings made by PVD/CVD

It has been shown to be a successful analytical tool, whenever the elemental composition of the first few nanometers up to 100 µm of solid material must be investigated. Conductive material can be analysed using a direct current glow discharge, non-conducting materials require radio-frequency excitation of the plasma.

GD-OES has various acronyms: GD-AES, GDOS, SDL, GDA, GDS. The GD source (or GD lamp) used in most commercially available GDOES insturments are based on the original design of the Grimm source.
Topics of interest include: density, coating mass,Sputtering, Emission Yield, self-absorption, Calibration, spectral interferences, and RF (radio frequency) versus DC (direct current).

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