1. history about sem...sem stands for ‘scanning electron microscopy’. the earliest known work...
TRANSCRIPT
1
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
1. HISTORY ABOUT SEM
SEM stands for ‘Scanning Electron Microscopy’. The earliest known work describing
the concept of a Scanning Electron Microscope was by M. Knoll (1935) who, along with other
pioneers in the field of electron optics, was working in Germany. Subsequently M. von Ardenne
(1938) followed by Zworykin et al. (1942), working in the RCA Laboratories in the United
States also tried for construction of SEM but unfortunately they could not get success, it did
suffer from the slight problem. Finally Dennis McMullan and Oatley in 1948 built their first
SEM. By 1952 this instrument had achieved a resolution of 50 nm.[1]
2. I�TRODUCTIO�
The simplest of all microscopes is the hand lens or magnifying glass which is a single
biconvex lens of glass or plastic. But as all we know “�ecessity is the Mother of Invention”
means that without ever having to need anything, nothing would have been invented. The need
of certain things, you can help improve your life and the lives of others. The day by day
requiring of things need to know about the things, we have to correlate our necessity with
existing materials, we have to tailor the things in order to fulfill our requirements. For a
particular application the compatibility of any material can be decided by their structure, their
properties etc. So how can we determine the structure of any material? This need invented many
microscopes, the SEM is one of them.[2, 3, 4]
As the name ‘Scanning Electron Microscope’ sounds that the electrons are used for
imaging the material. The next question arises in mind, what happened when electrons incident
on the specimen surface. Figure 1 shows the interaction of electron beam with matter. We can
2
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
realize that how can this electron beam helps us to know about specimen or any matter. I will
discuss this in detail one by one in my next section.
3. I�TERACTIO� OF ELECTRO� BEAM WITH MATTER
When the primary electron beam strikes the specimen in the scanning microscope a number
of different emission and absorption processes occur (figure 1).[2,4]
(1) Secondary Electrons: These are low energy (tens of eV) electrons originate from
specimen atoms by collision with high energy electrons.
(2) Backscattered Electrons: These backscattered electrons are generated from primary
beam which have interacted with atoms in the specimen and have been turned back out of
the specimen again. They may have energies ranging from the full primary beam energy
down to the level of secondary electrons.
Figure 1: (a) Interaction of Electron Beam with Matter, (b) Range of Energy of Emitted
Electrons
3
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
(3) Auger Electrons: As higher energy electrons interact with specimen they will knock out
inner shell electron. In order to fill this inner shell electron vacancy outer most electron
jump from higher energy level to lower one with the emission of X-rays. If these X-rays
have sufficient energy then they are able to knock out electron from any shell depending
on their energy, these electrons are known as Auger Electrons and has the energies up to
1 to 2 keV.
(4) Transmitted Electrons: If specimen the specimen is thin enough then some of the
incident electrons penetrate through it. These transmitted electrons may have been
deflected from the line of the primary beam and may have lost energy by collisions.
(5) Light: Many specimens emit light under electron bombardment, by the process of
Cathodoluminescence.
(6) Charge on Surface: The net current remaining in the specimen after all the above
processes have added to or diminished the primary beam current is conducted to the
earth.
4. CO�STRUCTIO�
The essential parts of SEM column are shown schematically in figure, numbered as follows:
(1) Electron Gun: This is a conventional triode gun, usually with a tungsten filament. It will
deliver a total electron current of up to 250 µA at energies adjustable between 1 and 30
keV.
4
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
Figure 2: Simplified Cross Section of the Column of a Scanning Electron Microscope
(2) Double Condenser Lens: These lenses are magnetic electron lenses, consists by a coil of
copper wire carrying a direct current, surrounded by an iron shroud, project a
demagnified image of the source onto the surface of the specimen. Interaction with this
field causes electrons to be deflected towards the axis, giving properties analogous to
those of convex glass lenses used for focusing light. The strength of the lens can be
controlled by varying the current in the coil.[3,4]
(3) Objective lens (Final condenser lens): This lens projects the diminished image of the
electron crossover as a spot focused on the surface of the specimen.
(4) Specimen Chamber: The chamber accommodates the specimen holder and mechanisms
for manipulating it, as well as detectors for the various emissions (electrons, light, X-
radiation) which will form the scanning microscope image. The specimen stage is able to
5
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
move in three mutually perpendicular directions X, Y, Z, the last being parallel to the axis
of the column. The specimen can be tilted and rotated so that every point on its surface
can be brought under the electron beam and examined.
(5) Vacuum pumps: Electron beam instruments must be evacuated sufficiently well to avoid
damage to the electron source and high-voltage breakdown in the gun, as well as
allowing electrons to reach the specimen without being scattered. Taking these
considerations into account, it is desirable for the operating pressure to be below 10-5
mbar.[2]
5. THE I�TERPRETATIO� OF SEM MICROGRAPHS
(a) Effects of Tilt: In figure 3 we can easily visualize the combined effects of changes in
secondary emission and perspective when a specimen is tilted.[5]
(a)
6
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
Figure 3: SEM micrograph (Scale bar 30µm) of a deposited egg of Acrosternum
marginatum with, (a) Tilt angle 0o, (b) 30
o, (c) 60
o
Reference: Klaus W. Wolf et al, Micron, 34, 57, (2003)
In figure 3(a, b, c) the same sample is seen at 0o, tilted 30o and 60o from the respectively.
In figure 3b micrograph at 30o the region around pores are brighter than micrograph at 0o
because high secondary electron emission is combined with a high efficiency of
collection. Further increment in the tilt angle (at 60o) the secondary emission would be
expected to be high because of the surface inclination to the scanning beam, but the final
effect is of low brightness, because the emitted and backscattered electrons will be
immediately reabsorbed by the surrounding surfaces.
(b) Effects of Magnification: In SEM, magnification is increased by the reduction in
scanning area. From figure 4, it is clear that at lower magnification the product is bundle-
like structure with lengths of ~500 nm but as the magnification is increased from 20K X
to 120K X the micrograph becomes clearer and we conclude that these structures are
composed from small rods of 500 nm length and ~50 nm in diameter. [6,7]
(b) (c)
7
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
Figure 4: (a) Low- and (b) high-magnification SEM images of the as-synthesized
bundle-like Te nanostructures
Reference: Jun Li et al, Solid State Sciences, 10, 1549, (2008)
(c) Effects of bias voltage: SEM image intensity is a result of back scattered electrons or
secondary electrons, or both, we applied a bias (50 eV) to the sample by modifying a
Faraday-cup sample stage figure 5c. Secondary electrons (defined as <50 eV energy) are
considered to be produced mainly through interactions between energetic beam electrons
and weakly bound conduction-band electrons in metals, or outer-shell valence electrons
in semiconductors and insulators. Most secondary electrons are emitted with less than 10
eV energy; thus, a positive bias on the specimen will suppress the emission to be
collected by the detector. In contrast, the BSEs are those incident electrons that
underwent elastic scattering from the sample and changed their direction while losing
little energy; therefore, the bias will not affect the image intensity.[8]
8
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
Figure 5: SEM images of a thick holey carbon film (~20-nm-thick) on top of a Ti grid and
covered with a thin (~2-nm-thick) carbon film with, (a) 0 eV, (b) +12.7 eV bias, (c) effect of
accelerating voltage (Eo) and atomic number (Z) on the droplet-shaped volume inside the
specimen
Reference: Y. Zhu et al, #ature Materials, 8, 808, (2009)
When a small bias is applied, the contrast of the thin carbon film generated by secondary
electrons disappears almost completely, whereas the contrast of the thick grid stays the same.
6. OPERATI�G MODES OF THE SEM
6.1 Bright Field and Dark Field Image
The bright field (BF) image is formed from transmitted electrons (Appendix 1) and dark
field (DF) image is formed from scattered electrons (Appendix 2). Micrographs are shown in
(b) (c) (a)
9
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
figure 6b it is clear that the BF image has more information about crystallographic orientations
of molecules than Secondary electron image (figure 6c). In multiphase materials with varying
electron transmission the BF images can be difficult to acquire with good contrast in all phases
then the DF mode is often more useful.[9]
Figure 6: Images of the molybdenum disilicide composite thin foil sample. (a) SE – SEM
image with labels (b) BF – SEM image (c) DF – SEM image
Reference: M. Halvarsson et al, Journal of Physics, Conference Series, 126, 012075, (2008)
Al2O3
Mo5(Al, Si)3
Mo(Al,Si)
Pt (a) (b)
(c)
10
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
6.2 Transmission Electron Mode
Thick substrates commonly used in BSE detection will generate BSE signals that exceed
the BSE signals from nanoparticles many times resulting in a poor image contrast. The unwanted
signal contributions of the substrate can, in principle, be reduced by using thin film substrates;
however, this does not help to enhance the small portion of electrons that are scattered back from
the nanoparticles. Since more electrons are transmitted and scattered in the forward direction, a
measurement of transmitted electrons results in higher signals and thus in better signal-to-noise
ratios.[10]
Figure 7: SEM images of silica particles deposited on holey carbon film, (a) Dark field image
(b) Bright field image
Reference: E Buhr et al, Meas. Sci. Technol, 20, 084025, (2009)
11
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
7. DISCUSSIO�
As I have discussed above, in SEM a beam of electrons is generated in the electron gun,
this beam is attracted through the anode, condensed by a condenser lens, and focused as a very
fine point on the sample by the objective lens. The scan coils are energized (by varying the
voltage produced by the scan generator) and create a magnetic field which deflects the beam
back and forth in a controlled pattern. The varying voltage is also applied to the coils around the
neck of the Cathode-ray tube (CRT) which produces a pattern of light deflected back and forth
on the surface of the CRT. The pattern of deflection of the electron beam is the same as the
pattern of deflection of the spot of light on the CRT.
Actually what happened, when an electron beam incident on the specimen, it will interact
with electron cloud of atoms as well as positively charged nucleus. Because of the positive
charge of nucleus they will attract towards nucleus whereas because of negatively charged
electron cloud they will repel from these electrons. Depending on certain phenomenon these
electron beam will form the image of specimen.
Electrons impinging on solid materials are slowed down principally through ‘inelastic’
interactions with outer atomic electrons, while ‘elastic’ deflections by atomic nuclei determine
their spatial distribution. Some leave the target again, having been deflected through an angle.
Both these ‘backscattered’ electrons and ‘secondary’ electrons dislodged from the surface of the
sample are used for image formation. In addition, interactions between bombarding electrons and
atomic nuclei give rise to the emission of X-ray photons with any energy up to 1-10 µ, the
energy of the incident electrons, resulting in a ‘continuous X-ray spectrum’ (or ‘continuum’).
‘Characteristic’ X-rays (used for chemical analysis) are produced by electron transitions between
12
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
inner atomic energy levels, following the creation of a vacancy by the ejection of an inner-shell
electron.
If we use thin specimen then transmitted electrons can form image. Because for thin
sample the number of effective scattering centers is lesser since the number of atomic layers is
less.
8. CO�CLUSIO�
The wave nature of moving electrons is the basis of the electron microscope. The
resolving power (appendix 3) of any optical instrument is proportional to the wavelength of
whatever is used to illuminate the specimen. In the case of a good microscope that uses visible
light, the maximum useful magnification is about 500X; higher magnifications give larger
images but do not reveal any more detail. Fast electrons, however, have wavelengths very much
shorter than those of visible light and are easily controlled by electric and magnetic fields
because of their charge.
In a scanning electron microscope (SEM), current-carrying coils produce magnetic fields
that act as lenses to focus an electron beam on a specimen and then produce an enlarged image
on a fluorescent screen or photographic plate. To prevent the beam from being scattered and
thereby blurring the image, a thin specimen is used and the entire system is evacuated.
13
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
APPE<DIX
1. Bright Field Image: The image formed by using the transmitted beam only referred to as
the bright image. To get this the objective aperture is inserted in the back focal plane of
the objective lens around the transmitted beam thus blocking all diffracted beams.
2. Dark Field Image: Dark field image is formed the diffracted beams only and is obtained
by inserting the objective aperture around the particular beam.
Figure: showing dark field patch
3. Resolving Power: The resolving power of a microscope is given by
0.61λ / µ sin α
Where µ is the refractive index of the medium,
µ sin α, is the numerical aperture.
14
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
The resolving power of electron microscope ~2 Å as compared to ~1700 Å for optical
microscopy.
4. Depth of Field: Depth of field denotes the vertical distance in the object space that can be
focused on the plane of the final image without causing any aberration in the image. This
can be derived to give a value of 2d/α where d is the resolving power ~2Å and α is the
semi-apex angle of the cone of rays (0.01 – 0.001). The depth of field is therefore about 2
µm while the specimen thickness is less than 1500Å.
5. Depth of Focus: Depth of focus determines the height in the image space over which the
image can continue in sharp focus. This value comes to 2dM2/α, which will be about 2
metres.
15
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
References:
1. The scanning electron microscope and its fields of application.
Smith KCA and Oatley CW, Br. J. Appl. Phys. 6, 391, (1955)
2. The principles and practice of electron microscopy.
Ian M. Watt
3. Electron Microscope Analysis and Scanning Electron Microscopy in Geology
S. J. B. Reed, Cambridge
4. Electron Microscopy, Principle and Fundamentals
S. Amelinckx, D. van Dyck, J. van Landuyt, G. van Tendeloo
5. Optical illusions in scanning electron micrographs: the case of the eggshell of
Acrosternum (Chinavia ) marginatum (Hemiptera: Pentatomidae).
Klaus W. Wolf, Walton Reid et al, Micron, 34, 57, (2003)
6. Self-assembly of inorganic magnetic nanocrystals: a new physics emerges
M.P. Pileni, J. Phys. D: Appl. Phys. 41, 134002, (2008)
7. Surfactant-assisted synthesis of bundle-like nanostructures with well-aligned Te nanorods
Jun Li et al, Solid State Sciences, 10, 1549, (2008)
8. Imaging single atoms using secondary electrons with an aberration-corrected electron
microscope
Y. Zhu et al, #ature Materials, 8, 808, (2009)
9. Thin foil analysis in the SEM.
M. Halvarsson et al, Journal of Physics, Conference Series, 126, 012075, (2008)
10. Characterization of nanoparticles by scanning electron microscopy in transmission mode
16
A. Raja Annamalai, Y7106063, MME, IIT Kanpur
E Buhr et al, Meas. Sci. Technol, 20, 084025, (2009)
11. www.vcbio.science.ru.nl/images/TEM-SEM
12. www.materialscience.uoregon.edu/ttsem/sem_mos.gif
13. http://mse.iastate.edu/microscopy/home.html
14. www4.nau.edu/microanalysis/Microprobe-SEM/