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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.