Download Solutions of Chapter 4 4.1. Compare light microscopy, transmission

Solutions of Chapter 4
4.1.
Compare light microscopy, transmission electron and scanning electron microscopy in
terms of optical arrangement, illumination source, working environment, imaging
formation mechanism and specimen preparation; define their similarities and
differences.
Light microscopy
TEM
SEM
Optical
Light source →
arrangement collector lens→
condenser lens →
specimen (on stage) →
objective lens → eye
piece
Electron gun →
accelerator → 1st
condenser lens → 2nd
condenser lens →
specimen → objective
lens → intermediate
lens → projector lens
fluorescent screen →
Electron gun →1st
electron lens→2nd
condenser lens→3rd
condenser lens →
specimen → detector
→ amplifier →display
screen
Illumination
source
High energy electron
beams (over 100 kV)
High energy beam
electron beam (1 ~ 40
kV)
Working
Ambient, vacuum or
environment liquid
High Vacuum
High Vacuum
Imaging
formation
mechanism
Transmitted or
reflected visible light
from a specimen.
Transmitted and/or
diffracted Electron
beam from a specimen
Secondary electrons or
backscattered electrons
emitted from a
specimen.
Specimen
preparation
For reflection: cutting,
grinding, polishing
and etching.
For transmission:
microtomy
Pre-thinning, and final
thinning (electrolytic,
ion milling or
ultra-microtomy)
do be aware of
problems such as
surface charging,
dehydration, coating.
visible light (3 types):
TEM and light microscope have similarities in the overall setup and structure. The
main differences are the illumination sources, working environment, number of lens
and apertures. Both TEM and SEM employ electron guns as their illumination source
and vacuum as working environment. SEM is significantly different from TEM in
image formation mechanism.
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4.2.
Given a specimen with surface fracture features with high differences up to 50 μm, how
do you select SEM operation parameters to ensure the whole field is in focus?
The depth of the field Df should be greater than 50 μm. We should select a small
aperture size, a long working distance and a low mangification. Table 4.1 in the text
indicate that M = 100 × and α f = 3 × 10−2 that will provide a depth of field 67μm.
4.3.
Estimate the effective magnifications of an SEM image showing on your computer
screen, when the probe size varies from 1 nm to 1 μm.
SEM image resolution is primarily controlled by the probe size of electron beam. The
effective magnification for showing image details equal to the probe size on computer
screen with pixel size of about 100 μm should be
100 μ m
100 μ m
≤ M eff ≤
;
that is 100 ≤ M eff ≤ 10000 .
1μ m
1nm
4.4. How can you obtain SEM images with the best resolution by adjusting operation
parameters?
The best resolution will result from optimizing the probe size, probe current density,
and eliminate astigmatism. High resolution of SEM requires reduce probe size. The
probe current should maintain at certain level to ensure the sufficient signal/noise ratio.
Unfortunately, reduction of probe size is accompanied by probe current reduction. To
obtain the best resolution, we should select a short work distance, a medium or small
size of final aperture, a high acceleration voltage, and correct astigmatism. Also, we
should adjust the probe current, which is often ignored during SEM operation.
4.5. When we find that noise level is too high in a SEM image, how can we reduce the noise
level by adjusting operational parameters?
High noise level often results from weak signal electrons during probe scanning.
Slow scanning rates can effectively reduce the noise level. Also, increasing probe
current can increase intensity of signal electrons in probe without reduce scanning rate.
4.6. Can backscattered electrons generate the same levels of resolution as the secondary
electrons? Why?
Materials Characterization Yang Leng © 2008 John Wiley & Sons (Asia) Pte Ltd
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Backscattering electrons cannot generate the same levels of resolution as the secondary
electrons. BSEs with higher energy than SEs can escape from a larger volume under
a specimen surface than SEs as shown in Fig. 4.10. Thus, the lateral resolution of
BSEs is worse than that of SEs.
4.7. In a SEM image with compositional contrast as shown in Fig. 4.16b, which areas
contain the heavy metallic elements--bright or dark? Explain.
The bright areas contain the heavy metallic elements. The compositional contrast is
generated by backscattered electrons. The number of backscattered electrons increases
with increasing atomic number.
4.8. Is it possible to eliminate surface charging of a polymer sample without conductive
coating? Explain.
It is possible to reduce the surface charging by reducing acceleration voltage of
electron beam. Field emission gun can ensure high brightness of beam with
acceleration voltage as low as 1 ~ 5 kV.
Materials Characterization Yang Leng © 2008 John Wiley & Sons (Asia) Pte Ltd
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