Beam-sample interaction area

The interaction of the primary electrons with the specimen results in the emission of secondary electrons (SE), i.e., inelastically scattered electrons that escape from the sample and reach the respective detector if emitted per definition from a depth less than about 50 nm equivalent of solid phase (Figure 21.2). The resolutirm of the SE is determined by the quality of the focus of the primary beam and the area of the sample surface from which they are emitted. 

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. 

In electron microscopy a sample is bombarded with a finely focused beam of monochromatic electrons. Products of the interaction of the incident electron beam with the sample are detected. If the sample is sufficiently thin—up to 200 nm thickness—the beam is transmitted after interacting with the sample, leading to the technique of transmission electron microscopy (TEM). TEM is used to probe the existence of defects in crystals and phase distributions. Scanning TEM instruments have been recently developed to obtain images over a wider area and to minimize sample degradation from the high-intensity beams. 

Linked with its qualities, assessed above, as an imaging and structural tool, the STEM assumes prime importance when considered as a microanalytical instrument. As pointed out in the introduction, the interaction of the fine probe in STEM with, potentially, only a small volume of the sample suggests the possibility of microanalysis on a scale hitherto unattainable. Two main areas will be considered here -the emission of characteristic A -rays by the sample, and the loss of energy from the primary beam in traversing the latter. Ideally, a fully equipped analytical electron microscope will utilize both techniques, since, as a result of the relative positions of A"-ray detector and the energy loss spectrometer in the electron optical column, simultaneous measurements are possible. However, for the sake of convenience we will consider the methods separately. 

When the incident electrons strike the electrons in the outer shell of the atom, some of these latter electrons are ejected and x-rays are produced. These electrons have Iowa-energy compared to the incident electron beam and are called secondary electrons. Due to their low energy, those that reach the detector are generated from the sample s thin top layer. As shown in Figure 10.12, the interaction volume is small and comparable to the irradiated area by the incident beam. This affords higher resolution. 

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