I electron beam

Fig. I. Electron beam penetration in various materials as a function of electron beam energy.

The history of EM (for an overview see table Bl.17,1) can be interpreted as the development of two concepts the electron beam either illuminates a large area of tire sample ( flood-beam illumination , as in the typical transmission electron microscope (TEM) imaging using a spread-out beam) or just one point, i.e. focused to the smallest spot possible, which is then scaimed across the sample (scaiming transmission electron microscopy (STEM) or scaiming electron microscopy (SEM)). In both situations the electron beam is considered as a matter wave interacting with the sample and microscopy simply studies the interaction of the scattered electrons. 

EM instmments can be distinguished by the way the infonnation, i.e. the interacting electrons, is detected. Figure Bl.17.2 shows the typical situations for TEM, STEM, and SEM. For TEM the transmitted electron beam of the brightfield illumination is imaged simply as in an light microscope, using the objective and... 

As each mixture component elutes and appears in the ion source, it is normally ionized either by an electron beam (see Chapter 3, Electron Ionization ) or by a reagent gas (see Chapter I, Chemical Ionization ), and the resulting ions are analyzed by the mass spectrometer to give a mass spectmm (Figure 36.4). 

Cathodoluminescence (CL), i.e., the emission of light as the result of electron-beam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequendy this phenomenon led to the discovery of the electron. Currendy, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerfirl tools for the microcharacterization of the electronic propenies of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques include ... 

An electron gun produces and accelerates the electron beam, which is reduced in diameter (demagnified) by one or more electromagnetic electron lenses. Electromagnetic scanning coils move this small electron probe (i.e., the beam) across the specimen in a raster. Electron detectors beyond the specimen collect a signal that is used to modulate the intensity on a cathode-ray tube that is scanned in synchronism with the beam on the specimen. A schematic of the essential components in a dedicated STEM system is shown in Figure 2. 

The classical approach for determining the structures of crystalline materials is through diflfiaction methods, i.e.. X-ray, neutron-beam, and electron-beam techniques. Difiiaction data can be analyzed to yield the spatial arrangement of all the atoms in the crystal lattice. EXAFS provides a different approach to the analysis of atomic structure, based not on the diffraction of X rays by an array of atoms but rather upon the absorption of X rays by individual atoms in such an array. Herein lie the capabilities and limitations of EXAFS. 

An XPS spectrometer schematic is shown in Figure 7. The X-ray source is usually an Al- or Mg-coated anode struck by electrons from a high voltage (10—15 kV) Alka or Mgka radiation lines produced at energies of 1486.6 eV and 1256.6 eV, with line widths of about 1 eV. The X rays flood a large area (-I cm ). The beam s spot size can be improved to about lOO-jim diameter by focusing the electron beam... 

In principle, energy-analyzer systems can be designed such that their electron-optical properties do not limit the energy resolution attainable, i. e. their intrinsic energy resolution is much better than the energy width of the primary electron beam, which is of the order of approximately 1.5-2.5 eV for a tungsten hairpin cathode, approximately 1 eV for a LaBg cathode, approximately 0.7 eV for a Schottky field emitter, and 0.3-0.5 eV for a pure cold-field emitter. 

In addition to qualitative analysis of nearly all the elements of the periodic table, EEL spectra also enable determination of the concentration of a single element which is part of the transmitted volume and hence gives rise to a corresponding ionization edge. As in all comparable spectroscopic techniques, for quantification the net edge signal, which is related to the number N of excited atoms, must be extracted from the raw data measured. The net intensity 4 of the feth ionization shell of an individual element is directly connected to this number, N, multiplied by the partial cross-section of ionization ) and the intensity Iq of the incident electron beam, i.e. ... 

Data collection is mostly performed at normal incidence of the primary electron beam. Under these conditions usually several equivalent LEED spots exist because of the surface symmetry. By taking care that the I-V curves of equivalent spots are identical, normal incidence conditions can be adjusted to within a few tenths of a degree. 

Fig. 3.31. Distributions (i)/(Ee) dEe of electron energy (E ) for a low-pressure HF-plasma (suffix pi, Maxwellian with temperature = 80000 K) and an electron beam (suffix eb, simplified to Gaussian shape with 40 eV half-width) (ii) rTx (Ej) ofthe Ej dependent electron impact ionization cross-section for X=Ti...

Turnbull, D. (1980) in Laser and Electron Beam Processing of Materials, ed. White, C.W. and Peercy, P.S. (Academic Press, New York) p. I. 

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