Wehnelt cylinder

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have a repetition frequency... 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have repetition frequency about 800 Hz. They are used to deflect the electron beam horizontally and vertically over the exit window plane. The scanner can be equipped by two cathodes for maximum output. Then, the width of the exit window is more than double that of a standard unit with a single cathode. The exit window containing the 12-15 prn-thick titanium foil is relatively large to assure an effective cooling of the foil. 

The design of a SEM is shown in Fig. 2. It consists of the electronic gun (1) the Wehnelt cylinder (2) the anode (3) and beam alignment coils (4) on the top of the instrument. The condenser lenses (5) the aperture, and the objective lens (6) focus the beam onto the specimen that is mounted on the specimen holder (7). The latter one could be moved in X-, Y-, and Z-direc-tion within the specimen chamber. In addition, the sample could be moved by rotation. The arrangement to create the electronic beam is shown in Fig. 3. 

Fig. 2 The design of a secondary electron microscope where 1, electronic gun 2, Wehnelt cylinder 3, anode 4, beam alignment coils 5, condenser lenses 6, objective lens 7, specimen holder. (From Ref. " l)...

The electron source consists of a cathode (electron source in Eig. 1), an anode, and a Wehnelt cylinder. 

The cathode emits electrons that are accelerated towards the anode with a defined voltage, typically 50-30,000 V. There are basically two types of electrodes thermionic cathodes (tungsten or LaBs (lanthanum hexaboride)) and field emission cathodes. The Wehnelt cylinder controls the current density and brightness of the electron beam. Brightness is defined as current per unit area normal to the given direction, per unit solid angle, and a criterion for beam quality. 

The electrons emitted by the electron gun are focused on the specimen by the Wehnelt cylinder and two to three electromagnetic lenses. As the narrowly focused electron beam strikes the specimen surface, it has a focal diameter of 2-10 nm. A sweep generator is controlled in such a way that the electron beam scans the specimen surface line-by-line and point-by-point. The electron beam also sweeps across a cathode-ray tube. These two motions of the electron beam are synchronized so that each point on the specimen surface is depicted on the screen. The image on a second screen is recorded by a camera. 

In scanning electron microscopy, electrons emitted by an electron gun (primary electrons) are focused by means of Wehnelt cylinders and electromagnetic lenses onto the sample surface, which they scan line by line (Fig. 2). The secondary and back-scattered electrons emitted by the sample are captured by detectors and presented in the form of brightness modulation on, eg, a monitor. The secondary electrons come from a surface layer up to 10 nm thick, while the back-scattered electrons usually originate from deeper layers. 

As described in Sect. 12.2, the electron beam in electronic microscopy is generated in a cathode, heated by electric current. As shown, the electron beam is accelerated by an electrode system, the electron gun. The three cannon components are the filament (cathode), the cylinder Wehnelt, and the anode. 

As can be seen in Fig. 12.7, the cylinder Wehnelt causes the electrons that are ejected from the cathode to form a thin beam of electrons to the anode. 

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