Incident electron beam, surface analysis

A variation on depth profiling that can be performed by modern scanning Auger instruments (see Sect. 2.2.6) is to program the incident electron beam to jump from one pre-selected position on a surface to each of many others in turn, with multiplexing at each position. This is called multiple point analysis. Sets of elemental maps acquired after each sputtering step or each period of continuous sputtering can be related to each other in a computer frame-store system to derive a three-dimensional analysis of a selected micro volume. 

Because many environmental particles are poor electrical conductors, charging produced by the incident electron beam is a major analytical concern in EMP and SAM (11,14). Because EMP analysis is not very surface specific, the sample surface is normally coated with a thin film of a low Z conducting material such as C. The surface specificity of the Auger technique generally precludes the use of surface coatings, and electrical charging of the particles may be exceedingly difficult to overcome experimentally (11, 14). 

Electron microprobes permit chemical microanalysis as well as SEM and BSE detection, often referred to as analytical electron microscopy (AEM), or electron probe microanalysis (EPMA)56 57. This is because another product of the surface interaction with an incident electron beam is X-ray photons which have wavelengths and energies dependent on element identity and on the electron shell causing the emission. Analysis of these photons can give a local chemical analysis of the surface. Resolution of 1 pm is attainable. Two types of X-ray spectrometer can be employed ... 

Because AES is a method of surface analysis its information depth should be considered. Figure 4 contains the relevant information. The incident electron beam striking the specimen penetrates to a... 

AES Smallest volume of any analytieal technique, depth profiling analysis Surface may be damaged by the incident electron beam Watts and Wolstenholme... 

Conventional chemical analysis techniques such as wet analysis or atomic absorption (AA) can provide a relatively accurate chemical composition for studying the Ca/P ratio and stoichiometric composition of HA. Energy dispersive spectroscopy (EDS), typically in conjunction with scanning electron microscopy (SEM) or transmission electron microscopy (TEM) has also been used extensively to analyze chemical composition, both quickly and directly. This technique allows the incident electron beam to be moved from one surface region to another for mapping the localized surface elemental composition and their distribution. However, low concentrations of elements are not measured accurately when using EDS. 

The simplest diffraction measurement is the determination of the surface or overlayer unit mesh size and shape. This can be performed by inspection of the diffraction pattern at any energy of the incident beam (see Figure 4). The determination is simplest if the electron beam is incident normal to the surface, because the symmetry of the pattern is then preserved. The diffraction pattern determines only the size and shape of the unit mesh. The positions of atoms in the surface cannot be determined from visual inspection of the diffraction pattern, but must be obtained from an analysis of the intensities of the diffracted beams. Generally, the intensity in a diffracted beam is measured as a fimction of the incident-beam energy at several diffraction geometries. These intensity-versus-energy curves are then compared to model calculations. ... 

Quantitative Analysis. In its basic form, AES provides compositional information on a relatively large area ( 1 mm2 ) of surface, using a broad-focussed electron beam probe. Sufficient signal may be obtained in this way with a low incident electron flux, thus avoiding potential electron-induced modifications of the surface. 

The sample surface is bombarded with an incident high energy electron beam, and the action of this beam produces electron changes in the target atoms the net result is the ejection of Auger electrons, which are the characteristics of the element. Because of the small depth and small spot size of analysis, this process is most often used for chemical analysis of microscopic surface features. 

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