Inscription of sem

1. Scanning Electron
Microscopy-SEM
Submitted to :Jawaria Jahngir
Submitted by-
1- Muhammad Shoaib
2- Atta-Mohy-Ud-Din
3- Atia Rani
BS Induction.-2nd Semester
Department of Physics
University of Gujrat

2. 2
 Light Sources
 Introduction
 Sources of visible light
 Lamp as a Source of light
 LASER as a Source of light
 IR radiations
 Fourier transform
 Used in instruments
 Summary
 References
Table of Contents

3. Electron Microscope
★ The electron microscope is a type of microscope that uses a
beam of electrons to create an image of the specimen.
★ Have a higher resolving power than light microscope and can
reveal the structure of smaller objects.
★ Electrons are very sensitive to magnetic fields and can
therefore be controlled by changing the current through the
lenses.
★ Used to investigate the ultrastructure of a wide range of
biological and inorganic specimens including cells,
Microorganisms, biopsy samples, metals, and crystals.

4. Scanning Electron Microscope - SEM
❏ Von Ardenne first constructed STEM in 1938
by rastering the electron beam in a TEM
followed by first commercial SEM in 1965.
❏ The typical scanning electron microscope
laboratory contains a machine with 2
components:
❏ 1. the microscope column, including the electron gun
at the top, the column, down which the electron beam
travels, and the sample chamber at the base.
❏ 2. the computer that drives the microscope, with the
additional bench controls

5. Principle:
When the accelerated primary
electrons strikes the sample , it produces
secondary electrons . these secondary
electrons are collected by a positive
charged electron detector which in turn
gives a 3- dimensional image of the
sample.

6. COMPONENTS OF SEM
➢ A source (electron gun) of the electron beam which
is accelerated down the column.
➢ A series of lenses which act to control the diameter
of the beam as well as to focus the beam on the
specimen.
➢ A series of apertures which the beam passes
through and which affect properties of that beam;
➢ An area of beam/specimen interaction that
generates several types of signals that can be
detected and processed to produce an image or
spectra.

7. Scanning process and image formation
★ STEP-1
❏ A beam of electrons is produced at the top of the microscope by
heating of a metallic filament.
★ STEP-2
❏ The electron beam follows a vertical path through the column of the
microscope.
❏ It makes its way through electromagnetic lenses which focus and
direct the beam down towards the sample.
❏ In the final lens, which deflect the beam horizontally and vertically
so that it scans in a raster fashion over a rectangular area of the
loop.

8. Scanning process and image formation
❖ STEP-3
❏ Once it hits the sample, other electrons are ejected from the
sample.
❏ Detectors collect the secondary or backscattered electrons, and
convert them to a signal that is sent to a viewing screen similar to
the one in an ordinary television, producing an image.
❏ When the accelerated beam of electrons strike a specimen they
penetrate inside it to depths of about 1 μm and interact both
elastically and inelastically with the solid, from which various types
of radiation emerges -

9. Magnification
➔ Magnification in a SEM can be controlled over a range of about 5 orders of
magnitude from x25 or less to x 250,000 or more.
➔ Unlike optical and transmission electron microscopes, image magnification in the
SEM is not a function of the power of the objective lens
➔ Magnification in the SEM depends only on the excitation of the
scan coils which determines the focus of the beam.

10. Sample Preparation
❖ Starters-
samples need to be coated to make them conductive. Most often, a thin layer of gold
works.
❖ Sample Cleaning-
A clean sample is essential for image clarity.
For biological samples, use appropriate buffers or distilled water for cleaning the
samples. Use a surfactant if the sample requires more vigorous cleaning like in
metals.
❖ Sample Fixation and Dehydration
Use a fixative like glutaraldehyde or osmium vapor to maintain the structural details
of the sample.

11. Interference and Troubleshooting

12. Charging
Interference- Charging is produced by buildup of electrons in the sample and
their uncontrolled discharge, and can produce unwanted artefacts, particularly in
secondary electron images.
❖ When the number of incident electrons is greater than the number of electrons
escaping from the specimen, then a negative charge builds up at the point where the
beam hits the sample.
Troubleshooting -The level of charge will relate to the energy and the number
of electrons. The energy of the electrons is related to the kV so reducing kV can
reduce charging. So reducing the number of electrons by adjusting these parameters
can also reduce charging.

13. Edge effects
Interference-
❖ Edge effects are due to the enhanced emission of electrons from edges and peaks
within the specimen.
❖ They are caused by the effects of topography on the generation of secondary
electrons.
❖ Poor signal intensity occurs in those regions shielded from the detector, such as
depressions.
❖ Troubleshooting--Topographic contrast is also enhanced by Backscattered
electrons emitted from regions of the sample facing towards the detector.
❖ Lowering the beam kV can reduce edge effect.

14. Lack of detail of surface structures
Interference-
❖ At high kV the beam penetration and diffusion become larger and result in signal
(electrons coming out of the sample) being generated from deeper within the
specimen. This can obscure fine surface structures.
❖ It will also increase BSE and so the image will start to show changes in contrast based
on composition.
Troubleshooting-
The solution therefore, for obtaining fine surface structure is to exclude these
backscattered electrons by using lower kVs such as 3-10kV. Hence lower energy provides
better detail of surface structure.

15. Specimen damage
Interference-
❖ Irradiating a specimen with an electron beam results in a loss of the beam energy to
the sample in the form of heat.
❖ A higher kV results in a higher temperature at the irradiated point and this can
damage (e.g. melt) fragile specimens, such as polymers or proteins, and volatilise
waxes or other sample components.
❖ This can ruin a sample (as well as contaminate the SEM chamber).
Troubleshooting-
❖ The solution is to lower the beam energy, sometimes down to a few kV.

16. Low contrast of cellular structures
Interference-
❏ Insufficient stain
Troubleshooting
❏ Tannic acid is a mordant and can improve the staining.

19. References
1. Michler, Goerg H. "Scanning electron microscopy (SEM)." (2008): 87-120.
2. Reed, Stephen Jervis Brent, and Stephen Jarvis Brent Reed. Electron microprobe analysis.
Vol. 2. Cambridge: Cambridge University Press, 1975.
3. Lee, Robert Edward. Scanning electron microscopy and X-ray microanalysis. PTR
Prentice Hall, 1993.
4. Reimer, Ludwig. "Scanning electron microscopy: physics of image formation and
microanalysis." (2000): 1826.
5. Google photos.

20. THANK YOU !