MY4200/MY5200 Questions and Problems
Chapter 1:
1. What are the advantages of the SEM over optical microscopy?
2. Identify imaging capabilities of the SEM.
3. Briefly describe the capability of the SEM in elemental analysis of the samples.
4. What signals are generated during electron beam-specimen interactions? What signals are
used in SEM, TEM, Auger Spectroscopy and EPMA? (not discussed in the textbook)
5. What type of samples can be analyzed under the SEM instrument?
6. What is the resolution (lateral and depth) of the SEM technique?
7. What is the preferential form of the specimen for electron probe x-ray microanalysis? How
accuracy of elemental analysis degrades if the sample is rough or in the form of particles?
8. What is the limit of detection in EPMA?
9. What is the analytical resolution of EPMA?
Chapter 2:
10. Three major electron beam parameters
include electron probe size (dp), electron
probe current (ip), and electron beam
convergence angle (p). Which of these
parameters is important in setting: a)
highest resolution image, b) best quality
image, or c) best depth of field image.
Should this parameter be small or large?
11. Define: a) quality of the image, b)
resolution of the image, c) depth of focus,
and d) magnification.
12. On the schematic drawing shown on the
right, identify basic components of the
electron column of the scanning electron
microscope. In one sentence describe the
function of (each) component.
13. Why there is a vacuum in the electron
column?
14. What are the practical ways to: a) increase the depth of field; b) improve the image quality; c)
increase signal current for x-ray microanalysis; d) enhance resolution on images of bulk
samples; e) enhance resolution on images of nanomaterials?
15. List three components of tungsten hairpin electron gun and describe the function of each
component.
16. There is a collector situated in front of the scintillator in the Everhart-Thornley detector. Let us
assume that this collector is under a voltage of i) +200 V, ii) 0 V, and iii) –100 V. Explain the
differences, if any, in collection of signals by the detector for three above-listed SEM working
conditions.
17. Draw a hypothetical correlation between beam current and tungsten filament current. Mark the
optimal operating current and explain what would happen if the filament current is i) much
larger and ii) much smaller than the optimal operating current.
18. List three different sources of electrons and specify major differences between them and
conditions of their operation.
19. Draw a schematic of conventional self-biased thermionic electron gun and explain its function.
20. Define focal length of the lens and explain how it is controlled/changed.
21. Three aberrations in the SEM instrument include spherical aberration, aperture diffraction and
chromatic aberration. Briefly explain causes for these aberrations (1-2 sentences) and how
they are reduced or avoided.
22. Briefly explain the astigmation effect, its cause and how it is corrected/eliminated.
23. Explain what energy spread for electron source is and why this parameter is important.
24. Define the focal length for a lens. Can you change it? If yes, what parameter do you control?
25. Everhart-Thornley detector has a collector under voltage situated in front of the scintillator.
Explain what the function of this collector is.
26. The filament made of lanthanum hexaboride (LaB6) can produce 5-10 times brighter electron
beam than tungsten (W) filament does. Also lifetime of the LaB6 filament is 10 times longer
than for the W filament. Explain why we cannot install the LaB6 filament in the JEOL JSM35C SEM instrument, used in our laboratory sessions.
27. Calculate the maximum theoretical brightness for thermionic (tungsten) emitter operated at 5
kV accelerating voltage and a temperature of 2700K.
28. (MY5200) Explain difference(s) between cold field emitters and Schottky emitters.
29. (MY5200) Why cold field emission requires that the cathode surface is atomically smooth?
30. (MY5200) Clarify advantages of field emission scanning electron microscopes over
conventional scanning electron microscopes.
31. (MY5200) When will you use FE-SEM instead of JEOL6400 in your research?
32. The schematic on the left illustrates the ray traces in a
two-lens SEM column with a single condenser lens
and objective lens; each one is set to certain strength.
Imagine that a focal length of the condenser lens is
reduced by 30%. A) Draw the new ray traces in the
SEM system that will correspond to this shorter focal
length (you can draw over the existing schematic
with a color pen/marker/pencil!). B) Explain the
consequences of stronger condenser lens on the
parameters of electron probe as well as imaging of
the sample in terms of image quality, resolution, and
depth of field.
33. See problem #27: diameter of the emitted electron
beam (do) is reduced by 50%.
34. See problem #27: diameter of the aperture is
increased by 50%.
35. See problem #27: working distance is increased two
times.
36. See problem #27: aperture is moved by 20% to the
left (de-centered).
37. Draw the ray diagrams for a) strong condenser lens,
b) weak condenser lens and indicate which one has the highest beam current at the specimen
and which one has the smallest probe diameter.
38. A tungsten filament produces an electron beam with a diameter of 50 m and brightness 105
A/(cm2 sr). This electron beam passes through a condenser lens with a focal length of 1 inch.
The distance between the beam crossover produced by the gun to the center of the condenser
lens is 10 inches. After passing through the lens, the beam current, measured with a
picoammeter, is 500 pA. Calculate the convergence angle of the probe produced by the
electron gun and condenser lens, ignoring all possible aberration effects that could be caused
by lens defects.
39. Calculate the maximum theoretical brightness for thermionic (tungsten) emitter operated at 5
kV accelerating voltage and a temperature of 2700K.
Chapter 3:
40. When elastic scattering takes place? What happens with energy and trajectory of the electron
during its elastic scattering with atoms of the specimen? What signals are generated during
elastic scattering? What are the primary factors that affect probability of elastic scattering?
41. When inelastic scattering takes place? What happens with energy and trajectory of the electron
during its inelastic scattering with atoms of the specimen? What signals are generated during
inelastic scattering? What are the primary factors that affect probability of inelastic scattering?
42. The distance between scattering events is known as the “mean free path” (). What properties
of the sample affect the mean free path? Does  depend on the parameters of the electron
probe?
43. Beam electrons undergo elastic and inelastic scattering during penetration through a specimen.
Elastic scattering generates backscattered electrons that are used in topographic and
compositional imaging of the sample. This elastic scattering changes for targets made of
different elements and depends on the accelerating voltage that is used. Does the image
resolution increase or decrease with increasing atomic number of the target? Explain. Will you
increase or decrease the accelerating voltage if you would like to improve the resolution of
your image? Justify your decision.
44. Explain what is the “interaction volume.” Briefly discuss the influence of beam and specimen
parameters on the interaction volume.
45. Define backscattered coefficient (). Discuss the effect of atomic number (Z) on backscattered
coefficient. How do we take advantage of the  vs. Z correlation?
46. What is the “compositional image” (also known as Z contrast or atomic number contrast).
47. (MY5200) How secondary electrons are produced? Specify the energy, range and escape
depth of secondary electrons.
48. (MY5200) Define the secondary electron coefficient (). How different it is from the
backscattered coefficient ()? Does it depend on atomic number of the target?
49. A CD attached to the textbook contains a Monte Carlo simulation (demo) program that allows
to analyze electron scattering. Using this program, perform the following experiments:
A)
B)
C)
D)
Using iron as the target, determine the 90% beam broadening for incident beam
energy of 20 keV and for sample thicknesses of 5, 10, 20, 50, 100, 200, and 500 nm
using 500 trajectories per run. Plot the results as the beam broadening versus
sample thickness.
Determine the beam broadening for a gold foil with a thickness of 200 angstrom
when using the incident beam energy: 2, 5, 10, 15, 20, 30, 50, and 100 keV. Use
1000 trajectories per run. Plot the results.
Using 1000 trajectories compute the backscattering coefficient for silver at energies
1, 5, 10, 20 and 35 keV. Plot the backscattering coefficient versus beam energy.
Using 1000 trajectories compute the backscattering coefficient for all elements
listed in the program at 20 keV. Plot the backscattering coefficient versus atomic
E)
number. (Note that the experiment #E can be conducted together with experiment
D.)
Using 1000 trajectories and beam energy of 20 keV plot the interaction volumes for
all elements listed in the program. From the plots measure (estimate) radius (or
diameter) of the interaction volume (you can neglect 5-10% of trajectories at
peripheries of the interaction volume). Plot estimated size of the interaction volume
versus atomic number.
50. Compare backscattered coefficient values determined in task #46 with the backscattered
coefficient values calculated from the following empirical equation:
  0.0254  0.016Z  1.86 104 Z 2  8.3 107 Z 3
Chapter 4:
51. What are advantages of digital imaging over the analog imaging?
52. Define magnification in SEM. How is it controlled?
53. What should be the size of the pixel on a recorded image in relation to the diameter of the
probe used? Is this beneficial to use the largest number of the pixels on the recorded image?
54. For a given choice of magnification, images are considered to be in sharpest focus if the signal
measured when the beam is addressed to a given picture element comes only from that picture
element. Overlap of information from adjacent picture elements must eventually occur as the
magnification is increased, because of the finite size of the interaction volume. This overlap of
pixel information is manifested as blurring in the image, and increasing magnification beyond
this point results in “hollow magnification.”
Consider electron beam with energy of i) 20 keV and ii) 5 keV is focused to 100 nm on
specimens of aluminum and gold (flat surface set normal to the incident beam) and determine
limiting magnification you can use to avoid the blurring effect by calculating the effective
signal-producing area (deff). Compare deff values for Al and Au. Discuss whether the diameter
of the probe has dominant or negligible effect on the resolution. What is the depth of field for
the calculated limiting magnification if the final aperture size is 240 m and the working
distance is 39mm?
Assume that you need to collect signal from the region emitting 90% of the backscattered
electrons. Also assume that your scan matrix is 512x512 and display on CRT is 10x10 cm.
55. Define the depth of field. What are the approaches in controlling the depth of field. Briefly
explain.
56. A variation in topography in your sample is about 200 m. You would like to have all surface
features in focus. What maximum magnification can be used to satisfy your need (to
accomplish depth of field at 200 m level). The smallest final aperture that you can use in the
SEM instrument is 30 m, and the largest working distance is 48 mm. Assume that the size of
CRT is 10 cm and you would like to record picture with 1024x1024 pixels.
57. (MY5200) Explain the phenomenon of image distortion.
58. (MY5200) Briefly discuss the principles of operation of the E-T detector.
59. (MY5200) Briefly discuss the principles of operation of the solid state diode (BSE) detector.
60. When will you prefer the use of the BSE detector over the E-T detector?
61. (MY5200) Why BSE detector is higher geometrical efficiency than E-T detector?
62. (MY5200) Why the BSE detector is not efficient at collecting secondary electrons as well as
backscattered electrons with energy less than 2-5 keV?
63. (MY5200) What parameters are important in describing the location and performance of
detectors?
64. Define contrast. What are the three components of contrast and how do we use them?
65. Explain (briefly!) the origins of topographical contrast obtained using secondary electrons.
66. The threshold equation defines a minimum current necessary to observe a certain level of
contrast. What are the simple ways to reduce this minimum current limit without sacrificing
the contrast level or/and improving the signal collection efficiency.
67. In a lunar melt rock brought from the moon two minerals were identified: trolite (FeS) and
phosphide (FeNi)3P. Using the threshold equation, calculate the minimum probe current that
needs to be produced to prepare the compositional image of the surface of polished lunar melt
rock using backscattered electrons. Assume the dwell time to be 0.1 ms, and efficiency of
signal production and collection to be 25%. Discuss whether these minerals produce
compositional image with sufficient contrast.
68. (MY5200) Explain the principles of gamma processing used in signal amplification. What is
the purpose of this processing.
Chapter 5 (incomplete):A
69. Briefly explain the major approaches in high-resolution imaging.
70.
Chapter 6 (incomplete):A
71. Explain two basic processes (at atomic level) that are responsible for the formation of x-rays
during electron beam interaction with a specimen. Draw an imaginary x-ray spectrum
representing accumulation of these two distinctly different-type of signals.
A
Additional questions and problems will be added during the course.
72. Imagine that you can solidify helium (He24) and analyze it under an electron microprobe. How
many of the characteristic x-rays will be emitted by helium? Assume that the high voltage
supply is set at 5kV.
73.
Chapter 7 (incomplete):A
74. List three major types of artificial peaks that can appear on the EDS x-ray spectrum and
briefly explain causes for their formation.
75. Explain in what situation and/or for what sample the use of a wavelength dispersive x-ray
spectrometer is more appropriate than the use of energy dispersive x-ray spectrometers.
76.
Chapter 8 (incomplete):A
77. The following is the EDS spectrum of a sample of unknown composition. Identify the
elements in composition of this sample and corresponding peaks.
Next, you would like to determine the concentration of identified elements in the sample. List
all steps required in this quantitative analysis (but you are not asked to perform calculations!)
and explain why you need it.
78. Other EDS or WDS spectra with characteristic peaks that need to be identify.
79.
Chapter 9 (incomplete):A
80. During x-ray microanalysis of samples with multi-elemental composition, identification of
elements with a concentration less than 5wt% is difficult due to weak and noisy peaks. What
to do during the collection of x-ray spectrum to enhance the visibility and clarity of such
peaks?
81. xxx
82. xxx
Chapter 10 (incomplete):A
83. xxx
84.
Chapters 11-15 (incomplete):A
85. xxxx
86.