Thin Layer Analysis

This form of analysis is a particular strength of MEIS in that it has the depth resolution and atomic sensitivity to characterize the integrity of such films, which are of immense importance in the semiconductor industry. 

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Secondary ion mass spectrometry (SIMS) and sputtered neutral mass spectrometry (SNMS) are today the most important mass spectrometric techniques for surface analysis, especially for thin layer analysis, for depth profiling, for the determination of contaminations and element distribution on a solid sample surface. 

Photoelectron spectroscopy of valence and core electrons in solids has been useful in the study of the surface properties of transition metals and other solid-phase materials. When photoelectron spectroscopy is performed on a solid sample, an additional step that must be considered is the escape of the resultant photoelectron from the bulk. The analysis can only be performed as deep as the electrons can escape from the bulk and then be detected. The escape depth is dependent upon the inelastic mean free path of the electrons, determined by electron-electron and electron-phonon collisions, which varies with photoelectron kinetic energy. The depth that can be probed is on the order of about 5-50 A, which makes this spectroscopy actually a surface-sensitive technique rather than a probe of the bulk properties of a material. Because photoelectron spectroscopy only probes such a thin layer, analysis of bulk materials, absorbed molecules, or thin films must be performed in ultrahigh vacuum (<10 torr) to prevent interference from contaminants that may adhere to the surface. 

From laminar thin-layer analysis [41,225,226] the value of Nur, corrected for stratification effects, is... 

Owing to the weU-known high sensitivity of optical emission spectrometry, detection limits reach from 10 g g for metals and C, S, P, B to 10 g g for elements such as H, O, N, considering instantaneous analysis. For quantitative analysis using calibration standards, accuracy 0.2 1 at.%, has been reported for thin layer analysis. For the bulk analysis one has to expect inferior accuracy in the 5—10% range, also for the general case of instantaneous in-depth analysis. 

Calibrations in surface and thin layer analysis have to be performed differently by measuring external standards of pure elements, analyzing dried residues of a standard element, or after spin coating of a support or wafer with a spiked solution. Internal standardization is not suitable for quantification as angle variation of the glancing incident beam does alter the fluorescence intensity. Peak fitting and quantification has to be carried out by using the fundamental parameter method as known for conventional XRF spectrometry. 

The applications of TXRF spectrometry are manifold and frequently more are added. Table 3 summarizes applications in different fields. Generally, there are two types of applications trace and microanalysis as well as surface and thin layer analysis. In both cases special care has to be taken so that sample preparation and/or analysis is done in very clean environments to avoid possible contamination. 

Heavy-ion induced X-ray emission applied to thick samples features several advantages, which are related to the small sample size assayed. As an example, with a 1 MeV u Kr + beam of 3mm diameter, the weight of graphite subjected to analysis would be less than 15 pg. Consequently problems associated with their target analysis (absorption of X-rays interference due to the substrate in the case of thin layer analysis) using X-ray methods are avoided. 

Hoffmann, V., Dorka, R., Wilken, L., Hodoroaba, V., Wetzig, K. (2003) Present possibilities of thin-layer analysis by GDOES. Surface and Interface Analysis, 35, 575-582. 

A rigid and final test for identifying an unknown can be made if an "authentic" sample of the compound is available for comparison. One can compare infrared and NMR spectra of the unknown compound with the spectra of the known compound. If the spectra match, peak for peak, then the identity is probably certain. Other physical and chemical properties can also be compared. If the compound is a solid, a convenient test is the mixture melting point (see Technique 9, Section 9.4). Thin-layer or gas-chromatographic comparisons may also be useful. For thin-layer analysis, however, it may be necessary to experiment with several different development solvents to reach a satisfactory conclusion about the identity of the substance in question. 

A solvent was found that would separate the mixture into four components (A-D). A column was run using this solvent, and 11 fractions of 15 mL each were collected. Thin-layer analysis of the various fractions showed that Fractions 1-3 contained Component A Fractions 4-7, Component B Fractions 8-9, Component C and Fractions 10-11, Component D. A small amount of cross-contamination was observed in Fractions 3,4,7, and 9. 

Thin-layer analysis of the hydrolysates requires removal of the mineral acid used for hydrolysis. Ion exchange resins are widely used, but some classical procedures are also very useful. After hydrolysis with 1 M sulfuric acid, the acid may be removed as barium sulfate by adding barium carbonate solution. Small amounts of 1 M hydrochloric acid can be removed in vacuo, and larger amounts of this acid can be conveniently removed from an aqueous hydrolysate by repeated washing with a 10% solution of di-n-octylmethylamine in chloroform etc. The hydrolysates plant of plant glycosides usually contain interfering compounds such as phenolics. These can be removed by extracting the hydrolysates with diethyl ether or ethyl acetate (26). 

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