Electromagnetic lens

Modem EMs use electromagnetic lenses, shift devices and spectrometers. However, electrostatic devices have always been used as electron beam accelerators and are increasingly being used for other tasks, e.g. as the objective lens (LVSEM, [10]). 

As the beam travels down the column, a number of electromagnetic lenses are used to guide the beam to the sample [44], The condenser lenses are part of the illumination system and are used to deliver electrons from the electron gun crossover to the sample. The condenser lenses determine the beam current reaching the sample. The objective, or final, lens determines the final spot size of the beam. A set of scanning coils are also present in the instrument column to scan the beam in a raster pattern over an area of the sample. At each point, data is collected and the points are combined to form the image. More detail on the data collection is given in the image formation section. 

E. G. Johnson and A. O. Nier. Angular Aberrations in Sector Shaped Electromagnetic Lenses for Focusing Beams of Charged Particles. Phys. Rev., 91(1953) 10-17. 

The first electron microscope was built in Germany in 1931 by Knoll and Ruska (Ref 2). Its principles were based on previous works of L. de Broglie (1924), Busch(1926) and others. The first electron microscopes gave images inferior to those obtained by optical microscopes, but by 1934 a quite satisfactory instrument was obtained by B. von Borries, E. Ruska and M. Knoll. Commercial production of electron microscopes was begun in 1939 by Si emens and Halske, AG, Berlin. These instruments (the total number built was about 30) used electromagnetic.lenses... 

The lenses used m electron microscopes act on the beams of electrons in much the same way that ordinary glass lenses act on beams of light. Most electron microscopes have electromagnetic lenses, although electrostatic lenses also can be used. Application of a uniform axial magnetic field causes the electrons to travel in a spiral path and return to the axis as shown schematically in Fig. I. Except for the spiral part of the motion, this behavior is just like that of a simple glass lens, and the equations for... 

A block diagram of a scanning-type electron microscope is given in Fig. 5. Major elements of the instrument include the electromagnetic lenses that are used to form the electron probe, the scan coils that sweep the beam over the sample, the detector that collects the secondary electrons, and the amplifying means where the secondary electrons are amplified and fed to the cathode ray tube for display. Since the cathode ray tube is scanned in synchronization with the electron beam, the resulting picture corresponds to the area of the sample being examined. 

In electron microscopy, a beam of electrons is focused using electromagnetic lenses. The specimen is mounted within a vacuum so that the electrons are not absorbed by atoms in the air. 

In contrast with light microscopy where optical lenses focus a beam of light, in electron microscopy electromagnetic lenses focus a beam of electrons. Because electrons are absorbed by atoms in the air, the specimen has to be mounted in a vacuum within an evacuated tube. The resolution of the electron microscope with biological materials is at best 0.10 nm. 

In a SEM equipment, the electrons which result from the emission from a filament located in the electron gun are accelerated, with the help of a voltage ranging from 1 to 30keV (see Figure 4.10) [8,52], The electron emission event takes place in a vacuum milieu ranging from 10-4 to 10-10 Torr. Then, the accelerated electrons are directed to the specimen by a series of electromagnetic lenses in the electron column (see Figure 4.10) [8,52],... 

The basic principles that govern electron microscopy are analogous to optical microscopy. Whereas optical microscopes use light and optical lenses to illuminate and magnify the sample, electron microscopes utilize high-energy electrons and electromagnetic lenses. There are two types of lenses in electron microscopes (see... 

The principles ofTEM and HRTEM have been discussed in several textbooks [6, 7]. A TEM column can be described using a ray diagram as shown in Figure 10.1, which is very similar to that for an optical microscope. The most important components in an electron microscope are the electron source (normally called the electron gun) and a group of electromagnetic lenses. 

The basic construction of a modern transmission electron microscope is shown schematically in Figure 2.2. It consists of an electron gun and an assembly of electromagnetic lenses, all within a column which is evacuated to about 10 Torr (= 2.7 x 10 Pa). The beam of electrons produced by the electron gun is accelerated by a high voltage and then focused onto... 

Several electromagnetic lenses the first lenses (or first lens in the case of a field emission microscope), known as ""condensers", reduce the size of the source. The last, so-called objective", lens, positioned near the sample, focuses the probe onto the specimen. 

As with optical lenses, electromagnetic lenses have aberrations (chromatic aberration, spherical aberration, electron diffraction limit, astigmatism, etc.) each entailing an enlargement of the electron probe expressed by the diameter of a circle of least confusion. Under standard operating conditions, when the astigmatism has been adjusted, only spherical aberration plays a significant role. The expression of the probe diameter becomes ... 

When crossing a sample, an electron beam may be partially adsorbed and partially deflected. Via the use of electromagnetic lenses, a certain fraction of these electrons, and of those that have not been deflected, can be recombined to form an image. The use of transmission electron microscopy is based on controlling the electrons involved in image formation. The transmission electron microscope thus offers an image of the sample that depends on the electron-matter interactions. 

A simplified diagram of a SEM is shown in Figure 1. An electron gun emits electrons (by thermionic or field emission) which are accelerated down the column by a large potential energy (typically 1-30 keV). Electromagnetic lenses and mechanical apertures are used to demagnify and focus the electrons to form an electron probe of small diameter and high current density. 

Although there is no inherent phase problem in the HREM technique it is replaced by an "instrumental" phase problem arising from the relatively imperfect nature of electromagnetic lenses. Spherical aberration is the chief limitation, and its effect can be coupled with that of deviation from the Qatissian focus into a phase factor which can be considered to act upon the diffracted amplitudes before re-combination by the lens to produce the initial image amplitude, namely ... 

Focusing of the electron beam use of a Wehnelt or electromagnetic lenses... 

---