Depth profile analysis

We know from the Boltzmann distribution that most atoms are in the ground state even at the temperatures found in GFs. Therefore, most analytical AFS uses ground-state resonance fluorescence transitions because these have the greatest transition probability and thus result in the highest 

The calculation of fluorescence yields for AFS are similar to those for molecular fluorescence (Chapter 5). Ingle and Crouch present an extensive discussion of theory of atomic fluorescence. Given the limited commercial applications of AFS, the theory will not be covered here. It is sufficient to understand that for a resonance transition and low analyte concentration, the fluorescence signal is proportional to the analyte concentration and to the intensity of the source. This assumption is valid for sources that do not alter the population of the analyte states. Intense laser sources can deplete the population of lower energy states, including the state from which excitation occurs. This condition is called saturation and is discussed under applications of AFS in Section 7.6.3. 

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Besides the conventional Grimm-type dc source, which has dominated the GD-OES scene for approximately 30 years, other discharge sources are well known. Among those are various boosted sources which use either an additional electrode to achieve a secondary discharge, or a magnetic field or microwave power to enhance the efficiency of excitation, and thus analytical capability none of these sources has, however, yet been applied to surface or depth-profile analysis. 

The introduction of rfpowered sources has extended the capability of GD-OES to non-conductors, and several rf sources of different design have become commercially available. This is of the greatest importance for surface and depth-profile analysis, because there exists a multitude of technically and industrially important non-conductive coating materials (e. g. painted coatings and glasses) which are extremely difficult to analyze by any other technique. 

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 of ten comprise widely different types of material which means that a common reference element is not available. 

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]. 

Chw Discharge Optical Emission Spectroscopy (CD-OES) 229 Tab. 4.2. Some typical applications of GD-OES depth-profile analysis. 

H. Oechsner, Thin film and depth profile analysis, Springer-Verlag, Heidelberg 1984. 4-264 O. Dersch, M. Laube, E. Rauch,... 

Scheme V. Representation of the catalytic p-type Si photocathode for Ht evolution prepared by derivatizing the surface first with Reagent III followed by deposition of approximately an equimolar amount of Pd(0) by electrochemical deposition. The Auger/depth profile analysis for Pd, Si, C, and O is typical of such interfaces (49) for coverages of approximately 10 8 mol PQ2 /cm2.

H. Oechsner, in Thin Film and Depth Profile Analysis, H. Oechsner (Ed.), Springer, Berlin,... 

Dynamic SIMS is used for depth profile analysis of mainly inorganic samples. The objective is to measure the distribution of a certain compound as a function of depth. At best the resolution in this direction is < 1 nm, that is, considerably better than the lateral resolution. Depth profiling of semiconductors is used, for example, to monitor trace level elements or to measure the sharpness of the interface between two layers of different composition. For glass it is of interest to investigate slow processes such as corrosion, and small particle analyses include environmental samples contaminated by radioisotopes and isotope characterization in extraterrestrial dust. 

Elemental Depth Profiling Analysis. An important aspect of a surface specific analysis is that it does not directly contrast the surface composition with that of the... 

SIMS and SNMS are versatile analytical techniques for the compositional characterization of solid surfaces and interfaces in materials research.92-94 As one of the most important applications, both surface analytical techniques allow depth profile analysis (concentration profile as a function of the depth analyzed) to be performed in materials science and the semiconductor industry with excellent depth resolution in the low nm range. For depth profiling in materials science, dynamic SIMS and SNMS using high primary ion beam doses are applied. Both techniques permit the analysis of light elements such as H, , C and N, which are difficult to measure with other analytical techniques. 

A depth profile analysis of trace and matrix elements (B, Na, Ni, Fe, Mg, V, A1 and C) in a 26p.m Si layer on a SiC substrate measured by GDMS, yielded impurity profiles, for example, with constant Ni contamination in the Si layer and enrichment at the interface layer.45 However, with respect to depth profiling of thin layers using dc GDMS with a depth resolution between 50 and 500 nm, this technique plays a subordinate role compared to the commercially available and cheaper GD-OES (glow discharge optical emission spectrometry). 

A quite different application of GDMS is the measurement of hydrogen and deuterium concentration, including depth profile analysis, e.g. in a gold electroplated layer on a CuSn substrate as described in reference.116 The relative sensitivity coefficient of hydrogen was evaluated by measurements of titanium standard reference material. 

The development of surface analytical techniques such as LA-ICP-MS, GDMS and SIMS focuses on improvements to sensitivity and detection limits in order to obtain precise and accurate analytical data. With respect to surface analytical investigations, an improvement of spatial and depth resolution is required, e.g., by the establishment of a near field effect or the apphcation of fs lasers in LA-ICP-MS. There is a need for the improvement of analytical techniques in the (j,m and nm range, in depth profiling analysis and especially in imaging mass spectrometry techniques to perform surface analyses faster and provide more accurate data on different materials to produce quantitative 3D elemental, isotopic and molecular distribution patterns of increased areas of interest with high spatial and depth resolution over an acceptable analysis time. 

Instruments based on GD-MS coupling have been employed most commonly for the quantitative analysis of trace and ultratrace amounts in high-purity materials. However, it has been demonstrated that, as in GD-OES, quantitative depth profile analysis by GD-MS is possible [33]. At present, a GD-MS prototype which allows the depth quantification of thin layers on conducting or insulating materials is being developed for commercial purposes [34]. 

M. Vazquez Pelaez, J. M. Costa-Fernandez, R. Pereiro, N. Bordel and A. Sanz-Medel, Quantitative depth profile analysis by direct current glow discharge time of flight mass spectrometry, J. Anal. At. Spectrom., 18,2003, 864-871. 

RBS depth profile analysis of the A- and D-treated samples are shown in Figure 3, where lines correspond at the maximal energy from which the C, O, and Si atoms at the surface are detected. 

Auger electron spectroscopy (AES) is particularly suited for surface analysis (depth 0.5-1 nm). AES depth profile analysis was employed to determine the thickness and composition of surface reaction layers formed under test conditions in the Reichert wear apparatus in the presence of four different ZDDPs additives at different applied loads (Schumacher et al., 1980). Using elemental sensitivity factors the concentration of the four elements (S, P, O, C) was determined at three locations corresponding to a depth of 1.8, 4.3, and 17 nm. No significant correlation between wear behavior and carbon or oxygen content of the reaction layer was observed. A steady state sulfur concentration is reached after a very short friction path. Contrary to the behavior of sulfur, phosphorus concentration in the presence of ZDDPs increases steadily with friction path, and no plateau value is reached. 

The quantity of interest is the elemental concentration as a function of depth. The detected number of events are related to the concentration while their energies with respect to the maximum energy (surface energy) gives the depth information. These are discussed further in the depth profile analysis procedure described later. 

J.A. Sawicki, J. Roth, L.M. Howe, Thermal release of tritium implanted in graphite studied by T(d,a)n nuclear reaction depth profiling analysis, J. Nucl. Mater. 162-164 (1989) 1019... 

Surface chemical and isotopic analysis of very small areas depth profile analysis... 

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