Optical column, electron

An electron-optical column capable of forming a beam rangii in diameter from nm to nm and carrying a current ranging from pA to jA... 

These complicated systems are controlled by means of a computer, which in real time monitors the various operating parameters of the column and stage position and which in addition, transfers primary pattern data directly to the electron deflection system. The rate at which the data can be transmitted to the electron optical column ultimately governs the modulation rate of the electron beam machine, i.e., flash time. Modulation rates in excess of 100 MHz have been achieved, and it is conceivable that much higher modulation rates will be attained in the future. 

With shaped beams the source for the electron optical column is an image of a square aperture that is formed in the plane of a second square... 

Figure 1 Schematic of the electron optical column and detectors of a dedicated STEM...

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. 

Figure 15.1 shows a schematic diagram of an EBL exposure system, which consists of three main subsystems, namely, the electron source (gun), electron optical column (heam-forming system), and exposure stage. A computer is used to control the various machine subsystems and transfer pattern information to the beam deflection coils. 

Ignoring electron diffusion effects within the specimen, SEM resolution is determined by the diameter of the electron beam which scans the specimen. The minimum useable beam diameter, and therefore the optimum resolution, is determined by a number of instrument parameters, which include the brightness of the electron gun, the SE collection efficiency, and the aberrations of the final focusing lens in the electron optical column. This very critical lens is normally referred to as the objective lens. While aberrations are associated with the condenser lenses, their magnitude is much smaller than those of the objective lens, and they can be ignored. 

Magnetic materials and samples too large to fit in the top stage of the instrument can be examined in the bottom chamber,where a conventional "pinhole" lens and SE detector arrangement is used. A schematic diagram of the complete electron optical column is shown in Fig. 8. 

The method relies on the ability to provide a vacuum differential across the objective aperture in the electron optical column. This is in principle possible in all SEMs, but in practice easier to achieve in some microscopes than others. Image formation requires a BSE detector rather than the more commonly used SE detector. While any backscattered electron detector can be used, the high resolution potential of the SEM can only be maintained by employing a high efficiency detector, such as the Robinson detector used here. 

An alternative technique is offered by the Charge-Free Anti-contamination System (CFAS) which was used to produce the series of micrographs shown in Figs. 16-21. In this system, the specimen chamber of the SEM is isolated from the main instrument vacuum pumps. A mechanical roughing pump with controlled leak is used to evaluate the specimen chamber. The objective aperture requires to be mounted in the electron optical column, in such a way that a pressure differential can be maintained across it. Thus, the electron gun and path between the electron source and the... 

The scanning electron microscope consists of an electron-optical column mounted on a vacuum chamber (Figure 1). 

In Transmission Electron Microscopy (TEM), a very high energy monoenergetic electron beam (100 to 400 keV) passes through a thin specimen (less than 1000 nm) of diameter less than 3 mm (necessary to fit in the electron optics column). A series of post specimen lenses transmits the emerging electrons, with spatial magnification up to 1,000,000, to a detector (fluorescent screen or video camera) viewed in real time. 

In addition, the speed with which the electrons pass through the electron optic column in TEM is typically much larger than in SEM, since the former operates at higher accelerating voltages, which can vary depending on the analysis conditions between 80 and 200 kV in a conventional TEM. 

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