m109 sem(f)
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Table of Contents
1. Objective .................................................................... 2
2. Introduction ............................................................... 2
3. Experimental Work .................................................... 3
3.1 Equiment and Auxillaries: .................................. 3
3.2 Experimental conditions: ................................... 3
3.3 Set up and Experimental Procedures: ................ 34. Results ........................................................................ 4
5. Discussion .................................................................. 9
6. Questions ................................................................. 10
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1. Objective
The main purpose of this experiment is to obtain the basic knowledge of SEM
analysis. Characterize different samples and learn the impact on resolution,
depth of field and different type of contrast when different electronaccelerating voltages and working distances are applied.
2. Introduction
Since light-optical microscopy is limited by visible light (wavelength 400nm
700nm), the maximum resolution is about 200nm, the electron microscopy
took its advantages in both high magnification and depth of focus. Scanning
electron microscopy (SEM) is well performance on the material surface
analysis.
SEM device is briefly shown in the figure 1. It mainly consists of an electron
emitting system, lens system, specimen chamber, and operating screen. The
certain vacuum is needed in the whole device. An additional sample changing
chamber is separated from the main chamber, providing a saving of time
during changing samples.
Figure 1 construction of SEM
In SEM, electrons are accelerated towards the surface of specimen. The
interaction takes place between electrons and specimen, secondary electrons
(SE) and back scattered electrons (BSE) are two important signal escaped
from the surface, containing an energy ESE 50eV and EBSE 50eV. They are
caught by detectors and then converted to image.
In an electron gun, electrons are produced by thermal emission. The filament
is heated up to operate the emission of free electrons. The accelerating
voltage accelerates the electrons towards the anode. The impact ofaccelerating voltage is respectively important to the resolution of surface
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image, surface structure, edge effects and sample charge up.
Working distance is the distance between sample and the final lens. It
impacts the resolution of image and depth of field.
3. Experimental Work
3.1 Equiment and Auxillaries:
Smart SEM (Scanning Electron Microscope) (Brand: ZEISS Ultra Series
Detection Plurality with Gemini Column.)
Three samples for characterizing the structure and learning the
influence\effect of variables like accelerating voltage, magnification and
working distance etc. on the result and qualities of the micrograph.
Sample 1: Gold coating on Si substrate
Sample 2: ZnO (Zinc-oxide) coating on Si Substrate
Sample 3: Sn (Tin) on carbon coated, coated with gold to increase
conductivity and reduce the charge effect.
3.2 Experimental conditions:
Vacuum in the SEM column: 4.8x10-7
bar
Vacuum in the filament\electron gun: 5.8x10-10
bar
Others variables like magnification, working distance and acceleration voltage
is mention below the micrographs.
3.3 Set up and Experimental Procedures:
3.3.1 Set up
The SEM consists of a filament chamber where the electron source is
produced. At least two pumps are needed in order to create a required vacuum
inside it.
The sample is placed into the specimen chamber through an exchangechamber without venting the whole system, as getting the vacuum is a long
time procedure.
In the control panel of the SEM the focus, magnification, contrast, brightness
and stigmator correction buttons are found.
The access to the digital imaging scanning system and the digital imaging
processing system is made through the monitor of the computer placed aside
the control panel. These programs are used to manipulate the image that one
wants to get from the SEM.
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3.3.2 Experimental procedure
The first thing to do is to install the sample in the chamber of the SEM; gloves
must be worn so there is no contamination in the vacuum. For sample to be
firm at their position, the sample must be screwed in a sample holder.
Once the sample is ready, it is introduced in the exchange chamber, prior vent
of it. After that, vacuum is applied in the exchange chamber so that the valve
between the exchange and specimen chambers can be opened and the sample
is transferred to the latter.
After completing this procedure, the monitors are turned on. The high voltage
is turned on, so it goes up until 12 kV. When the set-up of the SEM is done,
Live SE is turned on in the Digital Imaging Scanning System to find the
sample and focus it. Once the sample is found and focused, the magnification
is increased in order to see the surface of the sample much clearer. With theDigital Imaging Processing System the photographs of the samples surface
are saved and measures are taken, if necessary. The whole procedure is
repeated for the remaining 2 samples.
After the analysis of the samples, accelerating voltage put off. Finally, the
sample is taken out and the exchange chamber has to be vented.
4. Results
Figure 1: Shows the surface micrograph of sample 1 at 917X Mag, 12KV Acc.
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volts and 2.6 WD.
Figure 2: Shows the micrograph of sample 1 (gold on Si substrate) at 2002X
Mag, at same Acc. volts and WD as that of figure 1.
Figure 3: Shows the micrograph of sample 1 (gold on Si substrate) at much
higher Mag 33470X.
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Figure 4: Shows the micrograph of sample 1 with astigmatism at 9320X.
Figure 5: Shows the micrograph of sample 1 without stigmatism at 9320X.
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Figure 6: Shows the micrograph of sample-substrate-sample holder interface
of sample 2 at 309X Mag for studying the density contrast effect in SEM
micrographs.
Figure 7: Shows the microstructure of zinc oxide film (sample 2) at 3170X
Mag and 12 KV Acc. Volts.
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Figure 8: Shows the microstructure of zinc oxide film on Si substrate (sample
2) at 3170X Mag and 1 KV Acc. Volts.
Figure 9: Shows the microstructure of sample 3 (tin on carbon) at 11130X
Mag, 12 KV Acc. Volts and 2mm WD.
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Figure 10: Shows the microstructure of sample 3 at 11150X Mag, 12 KV Acc.
Volts and 9.2mm WD.
5. Discussion
Figure 1 to 5 correspond to the structure of thin film of gold (sample 1) at
different magnification, the purpose behind that was to characterize the thin
film of gold. Figure 1 reveals the network of gold (thin film) deposited on the
silicon substrate, gold thin film has better contrast than silicon substrate which
was looking as black in the background. At higher magnification Figure 2
shows that gold thin film (sample 1) composed of spherical, trigonal and
hexagonal particles. At more higher magnification then 2000X it can be seen
from figure 3 that the small spherical particles joined with the trigonal and
hexagonal particles, aslo from figure 3 one can find the size of spherical
particles which was approximately about 600 nm to 800 nm.
Figure 4 show the presence of astigmatism which is basically the type of
aberration in the objective lens, caused image stretching in one direction. It is
very difficult to get information from such images because image was too blur,
distorted and give false information as was stretched in one direction. Figure 5
has the same magnification as that of figure 4 but it without astigmatism. It
is observed that the effect of astigmatism was more prominent at higher
magnification.
An image is said astigmatism free if it has no unidirectional defocussing when
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the objective lens was changed to under or over focus at a little high
magnification.
Figure 6 is the sample- substrate-sample holder interface, one can see the
better contrast of ZnO than Si substrate and Aluminium sample holder,furthermore it was noted that Si substrate has better contrast the Al sample
holder which was looking dark in the image. This effect is called Z contrast in
SEM in which High Z (atomic number) material show better contrast than
lower Z materials.
The effect of accelerating potential can be observed by comparing figure 7
and 8 while keeping the magnification and working kept constant. Figure 7
contain high resolution, more edge effect and unclear surface structure where
figure 8 contain less edge effects, low resolution but it contain clear surface
structures. Moreover, it was concluded from both the micrograph that thinfilm of zinc oxide (sample 2) has a star like structure.
The effect of working distance on the image can be seen clearly by comparing
figure 9 and 10 while keeping magnification and acceleration potential keep
constant. Higher working distance increased the resolution while the depth
of field decreases, this can be seen by viewing figure 10 that contain high
resolution while high depth of field obtain in figure 9. Moreover, figure 9 and
10 correspond to the sample which tin (Sn) coated on carbon, it was clear
from the figure that the round particles were tin which were spread along the
carbon matrix. Also high contrast of tin spherical particle then carbon matrix
is because of Z contrast.
6. Questions
1. How can the range of usage of a light-optical microscope be extended?
Resolution can be defined as the smallest distinct difference between two
distinct points. Any resolution more this would be proved fruitless.
The dependence of resolution can be found by Rayleigh rule;
r = (0,61) / (sin)Higher resolution can be achieved by lowering the wavelength and decreasing
the distance between the sample and the lens, as well as higher index of
refraction.
Changing of wavelength is near impossible in light microscope whereas
increasing the lens magnification is also quite difficult. Hence only the
distance between the objective lens and the sample can be decreased.
2. Which are the advantages/disadvantages of transmission electron
microscopy (TEM) in comparison to scanning electron microscopy (SEM)?
The advantages of TEM in comparison with SEM are; it has a higher
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resolution (as small as 0.2 nm) and that it shows a direct imaging of
crystalline lattice and delineates defects (like dislocation, twin boundaries and
tilt boundaries) inside the sample, while SEM shows only the surface of the
sample.
The disadvantage of TEM in comparison with SEM is that anelectron-transparent sample is required and its preparation is difficult.
3. Which are the scattering types that can occur at atoms in case of accelerated
electrons? Give some examples.
Elastic or inelastic scattering can occur.
In the case of the elastic scattering there is no significant energy loss,
therefore the electron can leave the solid. One example would be the
backscattered electrons, which are created by the interactions between
electrons from the probe and atoms or between electrons from the probe and
the crystal lattice.The inelastic scattering occurs when there are energy losses and the electron
remains in the solid. Some examples are: Secondary electrons, X-rays, Auger
electrons, formation of electron-hole pairs resulting in cathode-luminescence.
These are created due to the transfer of energy from the primary electrons to
the solid.
4. Field emission SEM: Describe the principle. Which are the advantages of
this method?
The principle of a Field Emission SEM (FESEM) is as follows. Electrons
generated by a Field Emission Source (cold cathode field emitter) are
accelerated in a field gradient under vacuum. The beam passes through
electromagnetic lenses, focusing onto the specimen, so that different types of
electrons are emitted from the sample. A detector gets the secondary electrons
and an image of the sample surface is constructed by a comparison of the
intensity of the secondary electrons to the scanning primary electron beam.
The image is displayed in a monitor.
The advantages of FESEM are:
- It produces a clearer and less electrostatically distorted image with a spatial
resolution lower than 1.5 nm (3-6 times better than regular SEM).- High quality, low voltage images are obtained with negligible electrical
charging of samples.
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