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Electron Microscopy and Analysis Group Conference 2007 (EMAG 2007)
IOP Publishing
Journal of Physics: Conference Series 126 (2008) 012054
doi:10.1088/1742-6596/126/1/012054
Grain contrast imaging in FIB and SEM
S Canovic1, T Jonsson and M Halvarsson
Microscopy and Microanalysis, Department of Applied Physics, Chalmers University
of Technology, SE-412 96 Göteborg, Sweden
E-mail: [email protected]
Abstract. Grain contrast imaging can be performed with several techniques. In order to be able
to choose the most suitable one it is important to know which techniques are available and also
to be aware of the strengths and weaknesses of each technique. In this work, the grain contrast
imaging is performed with secondary electrons, backscattered electrons, forward scattered
electrons, transmitted electrons in the scanning electron microscope, and with secondary
electrons in the focused ion beam instrument. The advantages and disadvantages of each
method are discussed in order to make it easier to choose the most appropriate technique for
grain contrast imaging.
1. Introduction
Grain contrast imaging can be performed by using a wide range of techniques and choosing the most
suitable technique is not a straightforward task. Historically, etching and light microscopy has been
used for imaging grain contrast. Instead of photons, electrons and ions can be used for imaging with a
better resolution. The most widely used imaging techniques where electrons and ions are used as
imaging sources are scanning electron microscopy (SEM) and focused ion beam microscopy (FIB),
respectively. Imaging with secondary electrons (SE), backscattered electrons (BSE) and forward
scattered electrons (FSE) are the most commonly used modes for imaging grain contrast in the SEM.
Another way to image grain contrast with high spatial resolution is by using thin foil specimens,
instead of bulk specimens, i.e. scanning transmission electron microscopy (STEM) in the SEM.
The aim of this paper is to illustrate grain contrast imaging with the techniques mentioned above
and also to discuss the strengths and weaknesses of each technique.
2. Experimental
Three different samples have been used in this work. These are: (i) a κ-Al2 O3/TiC multilayer coating
deposited on cemented carbide by using chemical vapour deposition (CVD); (ii) an oxidized pure iron
sample with a thick iron oxide scale formed after oxidation in oxygen at 600°C for 1 hour and; (iii) a
Ti(C,N) coating deposited on cemented carbide by using moderate temperature (MT) CVD.
Two different instruments have been used: (i) a Leo Ultra 55 FEG SEM equipped with an EverhartThornley (E-T) SE detector, a solid state BSE detector, an in-lens SE detector, an FSE detector
(mounted on a Nordlys electron backscattered detector (EBSD)), and a STEM detector and; (ii) an FEI
200 THP FIB with a secondary electron detector.
1
To whom any correspondence should be addressed.
c 2008 IOP Publishing Ltd
1
Electron Microscopy and Analysis Group Conference 2007 (EMAG 2007)
IOP Publishing
Journal of Physics: Conference Series 126 (2008) 012054
doi:10.1088/1742-6596/126/1/012054
3. Results and Discussion
3.1. Channeling contrast
The depth of penetration of the incoming electrons is affected by the regular arrangement of atoms in
crystalline materials. If the incoming electrons encounter a low density of atoms, there is a lower
probability that these will return to the surface as BSEs. The channeling contrast is carried by the highenergy fraction of the BSEs [1]. The channeling contrast effect is less pronounced for samples with no
or low tilt due to the higher fraction of low-energy BSEs in this case.
Figure 1 shows three images of the fine-polished Ti(C,N) sample. The image in figure 1(a) is
acquired with the E-T detector. The contrast present in this image is due to electron channeling. Even
though the positively biased E-T detector primarily collects SEs and low-energy BSEs, also some
high-energy BSEs are detected [1], which gives channeling contrast. In addition, the SE2s (SEs created
by BSEs leaving the sample) also contribute to the channeling contrast.
Figure 1(b) shows an image that is acquired with the in-lens detector, which is placed in the
electron column. The collection solid angle for high-energy BSEs is larger for the in-lens detector than
for the E-T detector, which is placed on the side. This could be one reason for the better contrast in
figure 1(b) than in figure 1(a). Another reason could be the effect of the in-lens detection system. If
more SE2 electrons are collected by the in-lens detector than the E-T detector, this will contribute to
the higher contrast.
a
b
c
400 nm
Figure 1: SEM images of the fine-polished Ti(C,N) surface acquired with (a) E-T (b) in-lens and (c)
BSED. The images are presented as acquired at the microscope. The brightness, in (a) and (b), is set to
zero in order to achieve maximum contrast. The acceleration voltage is 10 kV and the working
distance is 3 mm.
Figure 1(c) is obtained with the BSED. The high grain orientation contrast in this image is
explained by the high collection efficiency of the high-energy BSEs, which carry the channeling
contrast information.
The channeling contrast illustrated above is even more apparent when the sample is tilted 70° [1].
This effect is used for imaging with FSEs. Figure 2(a) shows an SEM image illustrating the strong
grain orientation contrast of the fine-polished Ti(C,N) sample by using FSEs. In addition, the primary
beam interacts with the lattice, creating a pattern of Kikuchi lines that can be detected with an EBSD
system. By recording these lines a band contrast map can be created. The map represents the contrast
in the Kikuchi line patterns. Thus, the patterns do not need to be indexed; they can be constructed
directly, without knowledge about the crystal structure of the material. As grain boundaries produce
less band contrast they appear dark in these maps. This is illustrated in figure 2(b), which shows the
band contrast for the fine-polished Ti(C,N) sample. The advantages with FSE imaging are the high
grain orientation contrast and the possibility to get more useful information from the EBSD analysis.
The most important drawback with this technique is that the sample surface has to be very finepolished, which is a relatively time-consuming process.
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Electron Microscopy and Analysis Group Conference 2007 (EMAG 2007)
IOP Publishing
Journal of Physics: Conference Series 126 (2008) 012054
doi:10.1088/1742-6596/126/1/012054
a
b
Figure 2: (a) an SEM
image, of the finepolished Ti(C,N) surface,
acquired with FSEs.
(b) Band contrast of an
EBSD pattern.
4 µm
5 µm
In order to investigate the resolution of grain contrast images of samples that are not tilted a series
of images were acquired. Figure 3 shows the fine-polished Ti(C,N) sample viewed at 200 kX
magnification. Figure 3(a) (in-lens, 10 kV) and 3(b) (BSED, 10 kV) both show relatively high
resolution, in the order of 10 nm. As mentioned above, the channeling contrast is carried by the highenergy BSEs. This means that only BSEs from the upper part of the pear-shaped interaction volume
are contributing to the channeling contrast. If the acceleration voltage is increased to 30 kV (from 10
kV) the resolution does not decrease significantly, as is seen in figure 3(c) (BSED, 30 kV). Although
the total interaction volume increases, the upper part of the interaction volume does not increase
considerably. This leads to only a slight loss in resolution in the grain contrast BSED image obtained
at the higher voltage. However, the contrast is lower in figure 3(c), which could depend on a lower
fraction of high-energy BSEs for higher acceleration voltage.
a
b
c
150 nm
Figure 3: SEM images of the fine-polished Ti(C,N) surface acquired with (a) in-lens (10 kV) (b)
BSED (10 kV) and (c) BSED (30 kV). The working distance is 3 mm.
The procedure for SE and BSE imaging is fast and straightforward. One disadvantage for the inlens SE images is the problem with contamination during acquisition. As the in-lens SE detector is
very surface sensitive, contamination will be very apparent in the images. This means that the samples
have to be cleaned carefully (e.g. with a plasma cleaner) and also the microscope chamber has to be
clean.
3.2. Other contrast mechanisms
Another way to image grains with high spatial resolution in the SEM is by imaging thin foil
specimens, instead of bulk specimens, with a STEM detector. This leads to a reduction of the
interaction volume without reducing the acceleration voltage. The STEM unit used in this work
consists of an electron detector with the ability of bright and dark field imaging. A STEM bright field
image of the Ti(C,N) coating (figure 4) exhibits a high grain orientation contrast. The use of high
primary beam energies causes low aberration, which results in high-resolution (< 1 nm) images [2]. A
disadvantage is that the production of a thin foil specimen is relatively time-consuming.
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Electron Microscopy and Analysis Group Conference 2007 (EMAG 2007)
IOP Publishing
Journal of Physics: Conference Series 126 (2008) 012054
doi:10.1088/1742-6596/126/1/012054
Figure 4: A STEM image, acquired at 30 kV
acceleration voltage, of the Ti(C,N) coating showing a
high grain orientation contrast.
1 µm
Due to its high spatial resolution, the FIB can be used as an imaging tool. Figure 5(a) shows a FIB
image of an ion-milled cross section of iron oxide. The grain orientation contrast in this image is
caused by the channeling of the incident ions between lattice planes of the specimen [3]. This means
that different crystal orientations affect the number of secondary electrons escaping from the
specimen, which makes the different grains appear with different contrast. However, FIB is not always
suitable for imaging grain orientation contrast. Figure 5(b) shows a FIB image of a κ-Al2 O3/TiC
multilayer coating. No grain orientation contrast is visible in the thicker, dark κ-Al2O3 layers, which
are polycrystalline, as shown in the TEM micrograph in figure 5(c). Some advantages of using the FIB
are the relatively good spatial resolution (~5 nm) and the high grain orientation contrast for many
materials. Some disadvantages are the difficulties of imaging non-conducting samples and also the
damage created by the ion beam impact on the samples.
a
c
TiC
b
κ-Al2O3
TiC
TiC/κ-Al2O3
Iron oxide
5 µm
Iron
substrate
4 µm
κ-Al2O3
1 µm
Figure 5: (a) A FIB image of an ion-milled cross section of iron oxide, (b) a FIB image of a κAl2O3/TiC multilayer coating, (c) a TEM micrograph showing two polycrystalline κ-Al2 O3 layers.
4. Concluding remarks
There are several factors influencing the choice of method for grain contrast imaging. One important
factor to be considered is the type of material to be imaged. For example, the FIB cannot be used for
imaging too non-conducting samples. Another determining factor is the availability of instruments.
Further, since some of the methods require more sample preparation than others the availability of the
sample preparation equipment and time has also to be taken into account. The spatial resolution
necessary is also important to consider before choosing method.
Acknowledgments
This work was financed by the Swedish Foundation for Strategic Research program CROX, the
National Graduate School in Materials Research and Knut and Alice Wallenberg foundation.
References
[1] J. Goldstein et al., Plenum Press, New York, 2003.
[2] J. P. Vermeulen, A Novel STEM Detector System, 2005 Imaging & Microscopy 1 22–23
[3] M. W. Phaneuf, Micron 30 (1999), 277-288.
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