Scanning Electron Microscopy - Gbhenterprises.com

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Test M604: Scanning electron microscopy Fig. 9: Basic design of an electron gun. An electric heating current heats up the filament on the negative potential (cathode) opposite the anode. The relevant accelerating voltage accelerates the emitted electrons towards the anode where they pass through a gap to enter the microscope column. The filament is situated in a Wehnelt cylinder so that the electrons can be focused. The potential of the Wehnelt cylinder is slightly more negative than that of the filament. The Wehnelt cylinder focuses the electrons by emitting them from one point. This point, also referred to as virtual electron emitter or as cross-over, can be shifted by a variable bias resistance. The Wehnelt cylinder does not only adjust the diameter of the cross-over but the number of electrons leaving the cathode (emission current). 2.3.1.2 Lens system Magnetic lenses and various apertures focus the electron beam. When an electron with the charge e and the velocity v reaches a magnetizing field of the intensity B, force F acts on the electron such that the force vector F is perpendicular to the velocity vector v and the magnetizing field vector B. F = e( B ∧ v) (7) Fig.10: Force vectors of a charge moving in the magnetizing field 8
Test M604: Scanning electron microscopy The magnetizing field of an electromagnetic lens can be divided into an axial and a radial part. The axial part, running in parallel to the direction of movement of the electron, does not influence the electron. The radial part, however, forces the electron to take a helix-path by the force (B rad e v). Thus, due to such circular component the velocity vector is influenced by the axial magnetizing field (B ax e v zirk ). As a result the radius of the helix-path is becoming ever smaller. The electromagnetic lenses of an SEM produce an image reduced in diameter of the cross-over in the gun on the surface of the specimen. Two condenser lenses (Fig. 8) reduce the diameter of the electron beam (the diameter of the electron beam is also referred to as "probe size") from d 0 to d 2 . The higher the lens current, the smaller the diameter (Fig. 11). Fig. 11: Schematic illustration of the probe a) low, b) high lens current The smaller the probe size, the smaller the portion of electrons reaching the specimen since not all electrons leaving lens 1 can pass through lens 2. (Fig. 11): α 2 < α1 . Increasing noise results, limiting the resolving power of the SEM. The third lens, i.e. the objective lens, focuses the probe towards the specimen. Lens holes which are not absolutely symmetrical mechanically, whose magnetizing fields are inhomogeneous, and whose pole piece holes are contaminated, and contaminated apertures in particular, will result in an elliptical probe producing "axial astigmatism". The surface of the specimen cannot be brought into focus accurately since an elliptical probe will produce a distorted image of specimen structures during the focusing process. A corrective magnetic field, required to recover the rotational symmetry of the probe, is to be produced by a stigmator. A stigmator consists of 2 times 4 coils arranged centrically towards the optical axis. 2.3.1.3 Scanning system / magnification Beam deflection coils in a scanning generator (Fig. 7) scan the specimen surface by means of the primary electron beam for a certain period of time; beam deflection coils are installed in the pole piece duct of the objective lens. Simultaneously a cathode ray scans the screen of a monitor. Due to the principle of scanning, an SEM lineagraph consists of many spots. The beam deflection coil can be used to produce horizontal and vertical deflections by means of the electron beam. Horizontal deflection generates a line whose position is determined by vertical deflection. 9
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Scanning Electron Microscopy - Gbhenterprises.com

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