Scanning Electron Microscopy Coursework Example | Topics and Well Written Essays - 2500 words

Scanning Electron Microscopy - Coursework Example Secondary Electron images (SE) Vs. Backscattered Electron images (BSE)Secondary electron images are formed from the low energy electrons that are formed near the surface of the sample (Johnson). The brightness is affected by the surface topology of the specimen. For backscattered electron images, higher energy electrons formed deeper in the material are used to form the image. The result of these images is less contrast due to surface topology and more contrast due to different chemical composition (Johnson). This explains the 3D nature of the SE image in comparison to the flat BSE image, and the higher contrast of the BSE image in comparison to the SE image.Secondary electrons have lower energy compared to backscattered electrons, and so, they interact with the outer regions of the specimen by inelastic collisions. Therefore, only the surface topology of the specimen is clearly defined. This is the reason why the fibers in the SE looked clumped.The contrast in the BSE image is becau se of the production of backscatter electrons produced due to collisions of high energy electrons of the specimen. Parts of the specimen with higher atomic number cause higher backscatter than the lighter atomic number elements, resulting in a greater contrast, enabling a better study of the chemical composition of the specimen.The greater edge highlight in the SE image is because raised surfaces yield more secondary electrons.... The greater edge highlight in the SE image is because raised surfaces yield more secondary electrons. Images of a tilted TEM grid are provided showing a large difference in depth of field (file names DOF 1, 2, 3). 3 Calculate the depth of field from the images provided. Explain how you arrived at your answer. Compare SEM figures with the depth of field that would be available from an optical microscope for the same magnification. Large depth of field is one of the most important characteristics of SEM. The sharpness of the images recorded at low magnifications depends more on depth of field available than on small beam size (Lyman 1990). We know that depth of field, Where, d = minimum resolution of SEM W = Working distance D = aperture size Accordingly, the depth of field from the given images is computed as follows: Taking the following assumptions, d = minimum resolution of SEM= 3.5 nm = 3.5 ?10-9 m W = Working distance = as given in image in mm ?10-3 m D = aperture size= 200?m = 2 00?10-6 m Depth of field for first image with WD=13.0 mm= 13.0?10-3 m = 0.455?10-6 m = 4.55?10-7 m Depth of field for second image with WD=14.3 mm= 14.3?10-3 m =0.5005?10-6 m = 5?10-7 m Depth of field for second image with WD=44.3 mm= 44.3?10-3 m =1.55?10-6 m Comparison of SEM figures with the depth of field that would be available from an optical microscope for the same magnification The depth of field of SEM can be as great as 300 times that of the optical microscope. At low magnifications, below 300 to 400X, the image formed by the SEM is inferior to that of the optical microscope (Abbaschian et al 2008). At the same magnification, the depth of field that would be