Beam-specimen interactions

Beam voltage (kV) drain including chromatic aberration (nm)  

When high energy electrons bombard a solid, x-rays are produced, some at specific energies characteristic of the elements present in the solid. This process is not efficient, with only one electron in 10 or 10 producing a detectable characteristic x-ray [62, 631. The efficiency falls further for low atomic number elements. This makes x-rays a poor signal to use for simple 

The electrons cannot produce the characteristic x-rays if they do not have sufficient energy. Thus the x-rays do not come from the whole interaction volume, but from the region where the electron has not lost too much energy. The size of this region will depend on the beam energy, the x-ray energy and the composition of the material. Problems of this sort make quantitative microanalysis difficult (see refs 23-26, 119-122 in Chapter 2). 

For secondary electrons the STEM acts as a high performance SEM. A very thin sample has no low resolution backgroimd due to secondaries produced by backscattered electrons, but secondaries come from both surfaces. This means that topographic information from both sides of the specimen is superimposed on the image, making interpretation more complicated. If the specimen is thicker, but still transmits the electrons in a spread beam, there will be a large low resolution signal from the back surface of the specimen. In 

The energy spectrum of electrons emitted by a specimen in an electron beam has two maxima. One is at high energy where most of the back-scattered electrons are and the other, in the low energy region, contains the secondary electrons. The number emitted divided by the number of incident electrons is the secondary emission 

The basic difference between the SEM and the STEM is in the interaction volume (Section 2.3) which is much smaller in the STEM where very thin specimens are used. High energy electrons can penetrate several micrometers into a thick 

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The infonuation that can be extracted from inorganic samples depends mainly on tlie electron beam/specimen interaction and instrumental parameters [1], in contrast to organic and biological materials, where it depends strongly on specimen preparation. 

Ideally the EDS should only receive the X-rays from beam-specimen interaction volume, but it is not possible to prevent radiation from the microscope stage and... 

With the microscope aberrations eliminated, the exit wave function can be directly understood only in terms of the electron beam - specimen interaction. 

Electron beam specimen interactions give rise to secondary electrons throughout the total interaction volume but only those that are generated close to the surface will leave the sample and contribute to the signal. The depth is about 1 nm for metals and 10 nm for most insulating (low z) materials. 

Figure 2 The primary electron beam-specimen interaction in a scanning electron microscope.

Max Knoll, in 1935, succeeded in obtaining the first SEM image of silicon still showing electron channeling contrast [71]. Following this invention, Manfred von Ardenne, in 1937, worked on the physical principles of the SEM and beam specimen interactions [72, 73]. In 1965, Professor Sir... 

Fig. 8 Schematic of electron beam interaction with a sample and the electron beam interaction volumes for electron-specimen interactions.

Image formation in a transmission electron microscope can be considered as a two-step process. In the first step, the electron beam is interacting with the specimen. This interaction is very strong compared to X-ray or neutron scattering and causes multiple scattering events. In order to understand this process, the classical particle description of the electron is not adequate, and the quantum mechanical wave formalism has to be used. Thus, assuming the... 

When an electron beam is interacting with a specimen surface, different reactions can occur. The various signals arising from the specimen s surface... 

Scanning electron microscopy (SEM) is a useful technique for the analysis of plastic surfaces. It involves a finely collimated beam of electrons that sweeps across the surface of the specimen being analyzed. The beam is focused into a small probe that scans across the surface of a specimen. The beam s interactions with the material results in the emission of electrons and photons as the electrons penetrate the surface. The emitted particles are collected with the appropriate detector to yield information about the surface. The final product of the electron beam collision with the sample surface topology is an image (Fig. 10.18). 

TEM is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through it. An image is formed from the electrons transmitted through the specimen, magnified and focused by an objective lens and appears on an imaging screen. A fluorescent screen in most TEMs is detected by a sensor such as a CCD camera. 

Backscattered (high-energy) electrons are elastically scattered beam electrons. The electrcms are deflected back out of the specimen interaction volume. 

Primary beam electrons interact with electrons within the specimen atoms, knocking them free secondary electrons, SE, low energy of usually 0-200 eV). 

HRTEM exploits three different interactions of electron beam-specimen unscattered electrons (transmitted beam), elastically scattered electrons (diffracted beam) and inelastically scattered electrons. Different types of images are obtained in HRTEM. As a result, diffraction patterns are shown because of the scattered electrons. If the unscattered beam is selected, we obtain the Bright Field Image. Dark Field Images are obtained if beams are selected by the objective aperture. 

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