Novel approach for surface scanning

1. Scanning Electron
Microscope
by : MAHESH BIRADAR
Dept. of genetics and plant breeding
UAS DHARWAD

2. History
• The first scanning electron microscope
with higher magnification were developed
by - Manfred vonArdenne 1937.
• First SEM developed for bulk samples -
Zworykin et al. in 1942
• First commercial SEM developed -
Cambridge Scientific Instrument
Company as “Stereoscan” in 1965.
Manfred vonArdenne

3. Scanning Electron Microscope (SEM)
• Produces images of a sample by scanning it with a focused
beam of electrons in a raster scan pattern.
• Electrons interact with atoms and produces various signals that
contain information about the sample's surface topography and
composition.
• Resolution
• Depends on the size of the electron spot, which in turn depends on
the magnetic electron-optical system which produces the scanning
beam.
• Is not high enough to image individual atoms, as is possible in
the TEM … so that, it is 1-20 nm

4. M. von Ardenne's first SEM Schematic representation of SEM

5. PRINCIPLE
Basic principle : A beam of eˉ is generated by a suitable
source ( tungsten filament or a field emission gun).
Electron beam is then accelerated through a high
voltage (e.g. 20 kV).
Passed through a system of apertures and
electromagnetic lenses to produce a thin beam of eˉ.

6. Then the beam scans the surface of the specimen.
Electrons are emitted from the specimen by the
action of scanning beam.
Collected by a suitable positioned detector.

7. +ve
-ve

8. COMPONENTS
Tungstenfilament
2400oC

9. Electron Gun
W hairpin
LaB6 crystal
FEG

10. 2.LENSES
Condenser lens –determines the number ofelectrons
in the beam which hit the sample by reducing the
diameter of the electron beam.
Objective lenses -changes the position of thepoint
at which the electron are focused on thesample.

11. 3. SCANNING COILS
 Are used to raster/scan the e-beam across the sample
surface
Thee-beam can be scanned in a rectangular raster across the
surface of the sample bymeans of a series of “scan
coils” situated above the objective lens.
4. SAMPLE CHAMBER
Aplace where the sample was mountedon.

12. X-ray
photon

13. SCATTEREDELECTRONS

14. 5. DETECTORS
 detect the secondary and backscattered electrons.
Have + charges

15. Electron Detectors
Objective
lens
Sample stage

17. 6. VACUUM CHAMBER
Used to protect the electronic beam from
interference with air.
control the number of electrons which reach
the sample.
control the final convergence angle of the
electron beam onto the sample
7.Aperture

18. HOW THE IMAGE WILLCREATED?

19. Beam spot image at different stage of heating

20. Topographical Contrast
Topographic contrast arises because SE generation depend on the
angle of incidence between the beam and sample.
Bright
Dark

21. SAMPLE PREPARATION
• Appropriate sized samples will be prepared to fit in to
SEM
• Mounted rigidly on a specimen holder - specimen
stub
• For imaging in the SEM, specimens must be
1. Electrically conductive
2. Electrically grounded
1. Cleaning the surface of the specimen
2. Stabilizing the specimen
3. Rinsing the specimen
4. Dehydrating the specimen
5. Drying the specimen
6. Mounting the specimen
7. Coating the specimen

22. 1.Cleaning the surface of the specimen
• Surface contains many unwanted deposits, such as dust, mud, soil etc
2. Stabilizing the specimen
Hard, dry materials such as wood, bone, feathers, dried insects, or shells can be
examined with little further treatment, but living cells and tissues and whole, soft-
bodied organisms usually require chemical fixation to preserve and stabilize their
structure.
Stabilization is typically done with fixatives.
3. Rinsing the specimen
Sample must be rinsed -- remove excessive fixatives.
Fixation -- performed by incubation in a solution of a buffered chemical fixative, such
as glutaraldehyde, sometimes in combination with formaldehyde and other
fixatives.
Fixatives that can be used are:-
Aldehydes, Osmium tetroxide, Tanic acid, Thiocarbohydrazides

23. 4. Dehydrating the specimen
Water must be removed
Air-drying causes collapse and shrinkage, this is commonly achieved by replacement of water in
the cells with organic solvents such as ethanol or acetone.
Dehydration -- performed with a graded series of ethanol or acetone.
5. Drying the specime
 Specimen should be completely dry
 Otherwise the sample will be destroyed
6. Mounting the specimen
Specimen has to be mounted on the holder
Mounted rigidly on a specimen holder called aspecimen stub
Dry specimen -- mounted on a specimen stub using an adhesive such as epoxy resin or electrically
conductive double-sided adhesive tape.

24. Charge-up

26. Sample coating is
intended to prevent
charge-up phenomenon
by allowing the charge
on the specimen surface
go to ground through
the coated conductive
film.
This charge up phenomenon can be prevented by the coating
the non-conductor sample with metal (conductor).

27. • Coating the specimen
• Toincreasetheconductivityofspecimenandtopreventthehighvoltage charge on the
specimen
Coated with thin layer i.e., 20nm-30nm of conductive metal.
All metals are conductive and require no preparation before being used.
Coating the specimen
Non-metals need to be made conductive, which is done by usinga device
called a "sputter coater.”
Conductive materials
 Gold, Gold-palladium Alloy, Platinum,
Osmium, Iridium, Tungsten, Chromium
and Graphite

28. A spider coated with gold

29. ADVANTAGES
 Wide array of applications.
 3D & topographical imaging.
 Versatile information gathered
from different detectors.
 Works faster ( completing SEI,
BSE & EDS).
 Most SEM samples require
minimal preparation action.
DISADVANTAGES
 Expensive, large & must be housed
in an area free of electric, magnetic
and vibration interference.
• Maintenance involves keeping a
steady voltage, currents to
electromagnetic coils and circulation
of cool water.
• Special training is required to
operate an SEM as well as prepare
samples.
• SEMs are limited to solid, inorganic
samples small enough to fit inside
the vacuum chamber that can handle
moderate vacuum pressure.

31. SE Images - Topographic Contrast
1m
Defect in a semiconductor device
The debris shown here is an oxide fiber
got stuck at a semiconductor device
detected by SEM
Molybdenum
trioxide crystals

32. SCANNING ELECTRON
MICROSCOPIC IMAGE
OF THE TONGUE

33. Scanning electron micrographs of the early human embryo

34. Pollen and Stamens Wool fibers

36. THANK YOU…