The document provides information about electron microscopes. It discusses how electron microscopes work by using a beam of electrons instead of light to illuminate samples. This allows for higher magnifications and resolutions compared to light microscopes. It describes the key components of transmission electron microscopes and scanning electron microscopes, including the electron gun, electromagnetic lenses, sample stage, detectors, and vacuum system. It also explains how electron microscopes can be used to study thin sections and surface topography of samples at nanometer or micrometer scales.

1. HAFIZ
MUHAMMAD
WASEEM
BA BA G
LAHORE
Micrometry

2. ELECTRON MICROSCOPY
UNIVESITY OF EDUCATION
LAHORE PAKISTAN

3. INTRODUCTION
source of illumination:
beam of accelerated electrons.
this microscope has a high
resolution of images,
able to magnify objects of very
small size,
captured on a
phosphorescent screen.
Ernst Ruska:
in 1931built the First Electron
Microscope
,

5. © 2013 FEI
Comparing Microscopes
LIGHT MICROSCOPE ELECTRON MICROSCOPE
Use of vacuum No vacuum
Entire electron path from
gun to camera must be
under vacuum
The source of
illumination
The ambient light source is
light for the microscope
Electrons are used to “see” –
light is replaced by an electron
gun built into the column
The lens type Glass lenses Electromagnetic lenses
Magnification
method
Magnification is changed by
moving the lens
Focal length is charged by
changing the current through
the lens coil
Viewing the
sample
Eyepiece (ocular)
Fluorescent screen or
digital camera

6. The electron gun generates electrons.
Two sets of condenser lenses focus the electron beam on
the specimen and then into a thin tight beam.
Working Principle of Electron Microscope

7. Continued……
• To move electrons down the column, an accelerating voltage (100
kV-1000 kV) is applied between tungsten filament and anode.
• Ultra-thin sections of specimen (20-100 nm ) are used.
• at least 200 times thinner than those used in the optical
microscope.

8. Continued……
• The electronic beam passes through the specimen and
• electrons are scattered depending upon the thickness or refractive
index of different parts of the specimen.

9. Continued……
• The denser regions in the specimen scatter more electrons
and therefore appear darker in the image since fewer
electrons strike that area of the screen. In contrast,
transparent regions are brighter.

10. Continued……
• The electron beam coming out of the specimen passes to the
objective lens, which has high power and forms the intermediate
magnified image.
• The ocular lenses then produce the final further magnified image

11. DIFFERENT TYPES OF ELECTRON
MICROSCOPES
1. Transmission Electron Microscope (TEM)
2. Scanning Electron Microscope (SEM)
3. Scanning Transmission Electron Microscope (STEM)

12. 1.Transmission electron microscope
(TEM)
• In it a beam of highly focused electrons is directed toward a
thinned sample (<200 nm).
• The highly energetic incident electrons interact with the atoms in
the sample
• This produces characteristic radiation and particles providing
information for materials characterization.

13. Continued……
• Information is obtained from
• both deflected and non-deflected transmitted electrons,
• Back scattered and secondary electrons, and
• emitted photons.

14. Transmission electron microscope (TEM)

15. Transmission electron microscope (TEM)
objective aperture
selected area aperture
condenser aperture
electron source
first condenser lens
second condenser lens
fluorescent screen
Micro condenser lens
specimen (thin)
objective imaging lens
objective condenser lens
diffraction lens
intermediate lens
first projector lens
second projector lens
projector chamber
(Illustration of a TEM shown)

16. Instrument components
• Electron gun
• Generates electrons
• Condenser system
• lenses & apertures for controlling illumination on specimen.

17. Continued……
• Specimen chamber assembly:
• Objective lens system
• A. image - forming lens - limits resolution;
• B. Aperture - controls imaging conditions.
• Projector lens system:
• magnifies image or diffraction pattern onto final screen.

18. Working Principles
1. A thin specimen is irradiated with an electron beam of uniform current
density.
2. Electrons emitted from electron gun, that illuminate the specimen through a
two or three stage condenser lens system.
3. Objective lens provides formation of either image or diffraction pattern of the
specimen.
4. The electron intensity distribution behind the specimen is magnified with a
three or four stage lens system and viewed on a fluorescent screen.

19. SCANNING ELECTRON MICROSCOPE (SEM)
It provides a valuable combination of
• high resolution imaging,
• elemental analysis, and recently,
• crystallographic analysis.
It is used for inspecting topographies of specimens at very
high magnifications.
SEM magnifications can go to more than 300,000 X.

20. SCANNING ELECTRON MICROSCOPE (SEM)

21. COMPONENTS OF A TYPICAL SEM
• The main components of a typical SEM are
• electron column,
• scanning system,
• detector(s),
• display,
• vacuum system and
• electronics controls.

22. • Electron Column:
• electron gun and
• two or more Electromagnetic lenses operating in vacuum.
• Electron Gun:
• It is located at the top of the column.
• Generates free electrons &
• accelerates these electrons to energies in the range 1-40 keV in the SEM.
•

23. • Electromagnetic Lenses (Condenser Lenses):
• Create a small, focused electron probe (beam) on the Specimen.
• After the beam passes the anode it is influenced by two electromagnetic
lenses that cause the beam to converge and pass through a focal point.
• Apertures:
• one or more apertures may be found.
• They reduce and exclude extraneous electrons in the lenses.

24. Components of SEM
• Scanning System:
• The electron beam is focused to a small diameter, and scanned across the
surface of a specimen by electromagnetic deflection coils inside the
objective lens.
• Specimen Chamber:
• The lower portion of the column is called the specimen chamber.

25. • Secondary Electron Detector:
• it is located above the sample stage inside the specimen chamber.
• Stage Controls:
• Specimens are mounted and secured onto the stage which is controlled by a
goniometer.
• The manual stage controls are found on specimen chamber and allow for x-
y-z movement, 360 rotation and 90 tilt