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1. Universidad Autónoma de Nuevo León Facultad de Ingeniería Mecánica y Eléctrica Microscopía Electrónica de Barrido Dr. Marco Antonio Garza Navarro
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4. Principios de funcionamiento Total cross section for scattering from the specimen Number of scattering events per unit distance that the electron travels through the specimen Probability of scattering from the specimen of thickness t
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7. Principios de funcionamiento Electrones secundarios Influence of accelerating voltage and specimen atomic number on the primary excitation volume: (a) low atomic number and (b) high atomic number. 1 kV 20 kV
9. Principios de funcionamiento Dispersión elástica de electrones Elastic scattering of electrons can occur in one of two ways, both of which involve Coulomb forces. The electron may interact with the electron cloud, resulting in a small angular deviation. This interaction is namely diffraction which is particularly important at low-scattering angles. Alternatively, if an electron penetrates the electron cloud and approaches the nucleus, it will be strongly attracted and may be scattered through a larger angle that can approach 180°. Then this interaction can result in the emission of backscattered electrons An isolated atom can scatter a high-energy electron by two mechanisms. Coulombic interaction within the electron cloud results in low-angle scattering; Coulombic attraction by the nucleus causes higher angle scattering (and perhaps complete backscatter when θ > 90°). The potential within the electron cloud is always positive.
11. Principios de funcionamiento The detection of backscattered electrons (BSEs) is a valuable method of producing an image in SEM, which provide both compositional and topographic information of a given sample. A BSE is defined as one which has undergone a single or multiple scattering events and which escapes from the surface with an energy greater than 50 eV. The elastic collision between an electron and the specimen atomic nucleus causes the electron to bounce back with wide-angle directional change. Roughly 10–50% of the beam electrons are backscattered toward their source, and on an average these electrons retain 60–80% of their initial energy. Elements with higher atomic numbers have more positive charges on the nucleus, and as a result, more electrons are backscattered, causing the resulting backscattered signal to be higher. An isolated atom can scatter a high-energy electron by two mechanisms. Coulombic interaction within the electron cloud results in low-angle scattering; Coulombic attraction by the nucleus causes higher angle scattering (and perhaps complete backscatter when θ > 90°). The potential within the electron cloud is always positive. Electrones retrodispersados
13. Principios de funcionamiento The variation of the logarithm of the screened relativistic Rutherford cross section with a given scattering angle, describing the change in cross section for electrons scattered at angles > θ (A) for different elements at 100 keV and (B) for scattering from Cu at different accelerating voltages. Scattering events/electron/atom/m 2
15. El microscopio electrónico de barrido Schematic drawing of a conventional SEM. The evacuated microscope column (inside the bold dashed frame) contains the electron gun, electromagnetic lenses, electromagnetic defl ection coils, apertures, the specimen stage, and the detectors. The electronics console houses the power supplies for the acceleration voltage and the electromagnetic lenses, the scan generator, amplifiers for the signals, and monitors for display and recording of images. Modern SEMs are controlled by a PC. A, anode; BSE, backscattered electrons; C, cathode; ConA, condenser aperture; ConL, condenser lens; CL, cathodoluminescence; Defl . X, pair of beam deflection coils in the X direction; Defl . Y, pair of beam deflection coils in the Y direction; Det., detectors; DF-D, dark-field detector; O, specimen; OA, objective aperture; OL, objective lens; PC, personal computer; SE, secondary electrons; STEM, scanning transmission electron microscope signal; W, Wehnelt cylinder; X-ray, X-ray signal.
19. El microscopio electrónico de barrido Concepto de resolución Criterio de Rayleigh: Una fuente puntual de luz que pasa por una lente no dará como resultado una punto luminoso como imagen, aun en ausencia de aberraciones ópticas; en cambio se observara un patrón de difracción de los rayos de luz. En este patrón de difracción es observado un punto luminoso en el centro del patrón (disco de Airy) (a). Rayleigh estableció que si el máximo del patrón de una fuente de luz intersecta el primer mínimo de otra fuente, entonces el promedio del perfil de intensidades exhibirá una deflexión en medio a aproximadamente el 80% de la intensidad máxima (c). Bajo estas circunstancias, la distancia entre las dos fuentes puntuales es definida como la resolución teórica de la lente. Abbe´s equation
21. El microscopio electrónico de barrido Lentes condensadoras The electron beam will diverge after passing through the anode plate from the emission source. By using the condenser lens, the electron beam is converged and collimated into a relatively parallel stream. A magnetic lens generally consists of two rotationally symmetric iron pole pieces in which there is a copper winding providing magnetic field. There is a hole in the center of pole pieces that allows the electron beam to pass through. A lens-gap separates the two pole pieces, at which the magnetic field affects (focuses) the electron beam. The position of the focal point can be controlled by adjusting the condenser lens current. A condenser aperture, generally, is associated with the condenser lens, and the focal point of the electron beam is above the aperture. As appropriate aperture size is chosen, many of the inhomogeneous and scattered electrons are excluded.
23. El microscopio electrónico de barrido Lentes objetivas Objective lens configurations: (a) asymmetric pinhole lens, which has large lens aberration; (b) symmetric immersion lens, in which small specimen can be observed with small lens aberration; and (c) snorkel lens, where the magnetic field extends to the specimen providing small lens aberration on large specimen
24. El microscopio electrónico de barrido Lentes objetivas An appropriate choice of lens demagnification and aperture size results in a reduction of the diameter of electron beam on the specimen surface (spot size), and enhances the image resolution. 30 μ m 7.5 μ m
26. El microscopio electrónico de barrido Detección de señales To detect electrons in SEM three different principles are commonly used. One principle is based on the conversion of signal electrons to photons by a scintillation material. Then, the photons are converted into an electric signal by a photomultiplier, which is proportional to the number of electrons impinging on the scintillator. The generated SE are collected by a positively biased collector grid, then they pass the grid and are accelerated by about 10 kV to the conductive coated scintillator. The scintillation material converts electrons to photons, which are guided by a metal-coated quartz glass to the photocathode of a photomultiplier where photoelectrons are generated and amplified by a factor of about 10 6 . Everhart–Thornley detector, ETD (scintillator–photomultiplier combination) <−50 V SE are not collected Lithium activated glass, P-47 powder, YAG and YAP single crystals
27. El microscopio electrónico de barrido Detección de señales Three types of different detectors: (a) conventional (below-lens) scanning electron microscope with below-lens ET detector; (b) condenser/objective (in-lens) scanning electron microscope with above-lens ET detector; and (c) high resolution (near-lens) scanning electron microscope with above-lens ET detector. TLD ETD ETD
28. El microscopio electrónico de barrido Detección de señales Backscattered electrons solid state detector, BSED The second principle is based on the conversion of electrons to electron hole pairs by a semiconductor, which can be separated before recombination causing an external charge collection current. This current is proportional to the number of electrons impinging on the semiconductor. While the principle of scintillation detection is used for secondary, backscattered, and transmitted electrons (in case of thin specimens), the semiconductor detector is mostly used for backscattered electrons only.
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