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factors affecting the depth of field for SEM

factors affecting the depth of field for SEM. Lei Zhang physics and Astronomy. Introduction. definition of DOF(depth of field) the distance between the nearest and farthest objects in a scene that appear acceptably sharp in one image ---(SEM large DOF). Introduction.

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factors affecting the depth of field for SEM

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  1. factors affecting the depth of field for SEM Lei Zhang physics and Astronomy

  2. Introduction • definition of DOF(depth of field) • the distance between the nearest and farthest objects in a scene that appear acceptably sharp in one image ---(SEM large DOF)

  3. Introduction • In a conventional equipped SEM, DOF is mainly changed by these two independent variables: • the final aperture size (radius R) • the working distance (WD mm) controlled by software

  4. the final aperture size (radius R) • the working distance (WD mm)

  5. Introduction • difference between DOF and DOF' • depth of field: refers to space defined on the specimen • depth of focus:refers to space in the region of an image plane (TEM) • But SEM use difference way to produce image from TEM, only need consider DOF

  6. Determination of DOF • DOF is not affected by the nature of the sample • But the requirements for DOFare determined by the roughness of the specimen in the field of view and the degree of tilt on the specimen stage

  7. Determination of DOF • M (magnification) • d (the effective signal emission diameter) • 'r' (the resolution capabilities of the human eye when viewing a VDU or a micrograph at 30cm: r = 0.3mm) • a (the semi-angle of the electron probe) • semi-angle of the probe is dependent on: • R (the radius of the final aperture in a normal SEM) • WD (the working distance in mm)

  8. Maths Determination of DOF • The equation relating these variables is: D = [(r x 10^3) - d/10^3]/a • Where, • a = R/(WD x 10^3)

  9. qualitative effects • the depth of field is higher when the emission disc is smaller • the depth of field is higher when the final aperture is smaller • the depth of field is higher when the working distance is longer • the depth of field is higher when the SEM is at lower magnifications

  10. example • the effective emission disc is 50 nm in diameter and the resolution capability of the eye is 0.3 mm Table 1. Depth of Field at 10 mm working distance

  11. example Table 2. Depth of Field at 25 mm working distance

  12. Enhancing the Depth of Field • increasing the working distance • decreasing the size of the final aperture • But largest WD and smallest R are not best choice! (side effects)

  13. side effects • increasing the working distance produces: • increased depth of field • lower attainable limits for low magnification • some loss of resolution • a possible decrease of signal strength • astigmatism will worsen at long WD • some serious distortions in the image

  14. side effects • decreasing the size of the finalaperture produces: • an increased depth of field • a decrease in probe current • a possible improvement in the proberesolution • a change in astigmatism (needs to be corrected again

  15. Conclusion • In most cases, a combination of changes in working distance and aperture size will be enough to provide the necessary range requirements for adjustments to the depth of field. • not perfect, but good enough for your sample

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