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SEM Principles and Techniques
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SEM Principles and Techniques
Lecture 2
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1
Question
What are the two main types of electron guns used to generate an electron beam (e-beam) in SEM, and how do they differ?
Answer
Thermionic guns generate e-beam by providing thermal kinetic energy for electrons to escape the filament surface, operating at high temperatures (~1600-2800 K). Field emission (FE) guns generate e-beam by tunneling effects at much lower temperatures (~300 K), producing a brighter beam and requiring higher vacuum conditions.
2
Question
How do electromagnetic lenses and the deflection system contribute to SEM operation?
Answer
Electromagnetic lenses condense the e-beam into a nanoscale probe focused on the specimen surface. The deflection system scans this electron probe over the surface in a raster pattern for image acquisition.
3
Question
Explain the difference between secondary electrons (SEs) and backscattered electrons (BSEs) generated from electron scattering SEM.
Answer
Secondary electrons (SEs) are produced by inelastic scattering of primary electrons with sample atoms, typically with lower energy, providing topographic information. Backscattered electrons (BSEs) result from elastic scattering and have higher energy, carrying compositional information about the sample.
4
Question
Inelastic vs elastic scattering
Answer
PE = primary electrons from beam
5
Question
Evarhart-Thornley (E-T) detector vs BSE detector
Answer
• Evarhart-Thornley (E-T) detector is commonly used to detect secondary electrons (SEs). Usually placed at an low angle relative to the sample due to the lower energy state of the SEs. • Backscattered electrons (BSEs) have more energy and are detected using a detector in the path of the electrons (higher angle relative to the sample).
6
Question
What causes topographic contrast in secondary electron (SE) imaging mode in SEM?
Answer
Topographic contrast arises from two effects: the trajectory effect, where surface topography influences the escape direction of secondary electrons, and the electron number effect, where variations in surface geometry affect how many electrons escape.
7
Question
How is the magnification (M) in SEM defined mathematically, and what do the terms represent?
Answer
Magnification M = A / a, where 'A' is the size of the projector or digital sensor onto which the image is displayed, and 'a' is the scanned area on the specimen surface.
8
Question
SEM probe diameter (dp) formula
Answer
9
Question
Optimized probe diameter formula
Answer
10
Question
What are the working distance (WD) impacts on SEM?
Answer
11
Question
Advantages and disadvantages of increasing accelerating voltage in SEM.
Answer
12
Question
What is the impact of probe current on SEM image quality and resolution?
Answer
13
Question
Describe how sample charging can affect SEM images.
Answer
Sample charging can cause image distortion and anomalous voltage contrast, leading to warped or irregular appearance in SEM images.
14
Question
What is the relationship between accelerating voltage and the interaction volume in SEM?
Answer
Higher accelerating voltages increase the interaction volume of the electron beam with the sample, whereas lower voltages reduce the interaction volume, affecting resolution and signal generation.
15
Question
What types of signals are generated from the specimen surface during SEM imaging?
Answer
Signals include secondary electrons (SEs), backscattered electrons (BSEs), characteristic x-rays, and continuum (background) and fluorescent x-rays.
16
Question
How does accelerating voltage influence secondary and backscattered electron signals in SEM?
Answer
Higher accelerating voltages increase electron penetration, affecting signal types and image contrast; low voltages enhance surface sensitivity with fewer backscattered electrons.
17
Question
Explain how probe current and probe diameter relate to SEM imaging performance.
Answer
Increasing probe current generally increases probe diameter, yielding stronger signals but reduced resolution; lower probe current gives smaller probe diameters with higher resolution but lower signal intensity.
18
Question
Why is there a trade-off between probe current and image resolution in SEM?
Answer
Higher probe current increases signal but widens the electron beam, reducing resolution; lower probe current sharpens the beam and resolution but decreases the signal, causing noisier images.
19
Question
What is depth of field (Df) in SEM imaging and how is it calculated?
Answer
Depth of field is the range over which the sample appears in focus in SEM. where R is radius of aperture, α is tilt angle, P is pixel size, S is working distance, and M is magnification.
20
Question
What primary information does Backscattered Electron (BSE) imaging provide about a sample?
Answer
BSE imaging qualitatively informs us about compositional differences in a sample and is sensitive to the atomic number of the elements present.
21
Question
Scattering efficiency formula in BSE mode
Answer
ratio of the number of backscattered electrons (η_BSE) to the number of incident electrons (η_incident)