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To maintain reproducible excitation conditions in the glow discharge source, the working conditions (e. g. argon pressure, dc-current or rf-power) are carefully controlled.
Because of the complex nature of the discharge conditions, GD-OES is a comparative analytical method and standard reference materials must be used to establish a unique relationship between the measured line intensities and the elemental concentration. In quantitative bulk analysis, which has been developed to very high standards, calibration is performed with a set of calibration samples of composition similar to the unknown samples. Normally, a major element is used as reference and the internal standard method is applied. This approach is not generally applicable in depth-profile analysis, because the different layers encountered in a depth profile often comprise widely different types of material which means that a common reference element is not available.
The quantification algorithm most commonly used in dc GD-OES depth profiling is based on the concept of emission yield [4.184], Raccording to the observation that the emitted light per sputtered mass unit (i. e. emission yield) is an almost matrix-independent constant for each element, if the source is operated under constant excitation conditions. In this approach the observed line intensity, ]&, is described by the concentration, c„ of element, i, in the sample, j, and by the sputtering rate qj:
^ik — Ci qj Rlk
226 J 4 Photon Detection
The sputtered mass, dm,, of element, i, during time increment, St, is described by:
The emission yield, R»;, defined as the radiation of the spectral line, fc, of an element, i, emitted per unit sputtered mass must be determined independently for each spectral line. The quantities q, and R& are derived from a variety of different standard bulk samples with different sputtering rates. In practice, both sputtering rates and excitation probability are influenced by the working conditions of the discharge. Systematic variation of the discharge voltage, Ug, and current, I, leads to the empirical intensity expression [4.185]:
where Õöå is an atomic- and instrument-dependent constant characteristic of the spectral line, k, of element, i, Ak is a matrix-independent constant characteristic of the spectral line, ê,/ê( Ug) is a polynomial of degree 1-3, also characteristic of the spectral line k, and Cq is a constant related to the probability of a sample atom being ejected during the sputter process. The most important reason for the success of the emission yield technique is that the total sputtered mass is easily determined by summing over all the elements present in each depth segment. The emission yield approach has proven to be extremely successful for a large and increasing number of applications and is currently implemented in commercial GD-OES depth-profiling instruments.
In contrast with the dc source, more variables are needed to describe the rf source, and most of these cannot be measured as accurately as necessary for analytical application. It has, however, been demonstrated that the concept of matrix-independent emission yields can continue to be used for quantitative depth-profile analysis with rf GD-OES, if the measurements are performed at constant discharge current and voltage and proper correction for variation of these two conditions are included in the quantification algorithm [4.186].
The primary information obtained in GD-OES depth profile measurements is the relative intensity, from the elemental detection channels, as a function of sputtering time. The intensity-time curves obtained for different elements can be converted into concentration-depth curves by applying eqs (4.21) to (4.23) and the sum normalization of all concentrations to 100%. The depth is determined from the sputtered mass, which is the quantity obtained by the emission-yield technique. The density of the composite material is calculated by an approach based on a weighted average of the density of pure elements. This method gives very accurate results for all types of metal alloy but tends to be less accurate for compounds which contains light and gaseous elements (oxides, nitrides, carbides, etc.). In general the accuracy, precision, and repeatability of quantitative GD-OES depth profile analyses are assured on
äïö = IfcSt/Rfc
Fig. 4.37. Depth (temporal) profile obtained on a multilayer coating produced by plasma vapor deposition (PVD) using optimized rf-glow discharge conditions. Layer thickness:
30x (300 nm CrN/300 nm CrAI)/100 nm Cr/steel [4.194].
the basis of inter-laboratory round-robin tests [4.187, 4.188], by use of a layered certified reference material [4.189], and evaluated by comparison with wet chemical analysis [4.188] or other surface techniques [4.190,4.191].
The information depth achieved by use of the GD technique is determined, in principle, by the depth of penetration of the incident ions, which is in the range of a few nanometers at the relatively low energies employed in the discharge. The practical depth resolution is, however, almost larger and determined by several effects which are introduced by the sputter process (preferential sputtering, atomic mixing), the sample properties (surface roughness, polycrystalline structure), and most seriously by the non-uni-form erosion of the sample material. Most of these effects are inherent also in other techniques which use sputtering for surface and depth-profile analysis. In practice, the depth resolution obtained on technical surfaces is roughly proportional to the sputtered depth, and usually deteriorates from the nanometer-range for near-surface layers to the micron-range at depths of several micrometers [4.192,4.193]. Excellent depth resolution is realized on multilayer coatings, cf. Fig. 4.37, as produced by vacuum evaporation or by plasma vapor deposition (PVD) [4.194], especially if the curved sputtering crater bottom is taken into consideration in quantification by an iterative deconvolution technique [4.195]. The lateral resolution of the GD-OES technique, on the other hand, is restricted by the size of the sputtered area of the sample surface (usually 4-8 mm diameter) and is much larger than with other surface techniques. The lateral and depth resolution of GD-OES are, however, usually both adequate for rapid quantitative determination of the elemental composition of technical surfaces.