Transmission and electron microscope Tem ppt

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1. Transmission Electron
Microscope (TEM)

2. TheTEMsystemandcomponents:
•Vacuumsystem
•ElectronGun
•ElectronLenses
•SampleStage
•MoreElectronLenses
•ViewingScreen
•CameraChamber

4. TEM Illumination control
•Filament saturation
•Filament centering
•Spot size (Condenser
Lens Current)
•Condenser aperture

5. How to focus the TEM
•Control objective lens current
•Adjust astigmatism correction coils
•Use large screen at low magnifications
•Use small screen at high magnifications
•Beware of lingering on an area too long

6. Focusing on a hole in a thin carbon film
(a)underfocusedobjective lens (bright
fringe);
(b) at focus (no fringe);
(c) overfocusedobjective lens (dark
fringe).
Note also the change in appearance of the
carbon fine grain.
Magnification ~750,000X.

7. Objective aperture located between
upper and lower parts of polepiece,
just under the specimen. The
major function of the aperture
is to help remove peripherally
deflected electrons to enhance
image contrast.
In addition to the specimen and
objective aperture, a chilled
anticontaminator blade (see Figure
6.34) may also be inserted just
above the specimen (or sometimes
above and below the specimen) to
prevent contaminants from
condensing on specimen.

8. 8
Astigmatism Correction
•Older stigmators were composed of pairs of magnetic slugs
that could be mechanically rotated into position to
compensate for astigmatism.
•Newer microscopes use primarily electromagnetic stigmators
since they are less expensive to build, easier to use, and
somewhat more precise in their correction.
•Electromagnetic stigmators may consist of eight tiny
electromagnets encircling the lens field.
•By varying the strength and polarity of various sets of
magnets, one can control both amplitude and azimuth in
order to generate a symmetrical magnetic field(Figure 6.35).
•When stigmators become dirty, they will no longer effectively
compensate for astigmatism and must be withdrawn from the
microscope and cleaned.

9. 9
(A) Conceptual drawing of
electromagnetic stigmator
showing orientation of eight
electromagnets around the
lens axis. Strength and direction are
controlled by
adjusting appropriate combinations of
magnets to generate a symmetrical field.
The stigmator is located
under the condenser and the objective
lens polepieces.

10. Astigmatism Correction
•Must correct condenser lens, objective lens and projector lens
separately
•Use both objective stigmator selectors
–Use the first stigmator
–Use the second one
–Refocus
–Repeat

11. Contrast Considerations
•Resolving power is good , but…
To see image features contrast is needed
•How to increase contrast…?
•(SEM contrast is derived from local topography
and/or differential interactions of beam with sample)

12. Contrast Considerations
•Mass-thickness Contrast (absorption)
–Density x thickness
–More dense or thicker areas look darker due to
absorption of beam electrons.
–Thickness fringes due to destructive interference
as beam traverses the sample
–Use stains to highlight specific areas
•Uranium, manganese, osmium
•Coat and/or shadow areas to generate contrast

13. Contrast Considerations
•Other “deficiency” contrast mechanism
–Electron scattering
•Random
–Amorphous materials change electron trajectories
•Regular
–Crystalline materials change trajectories uniformly

14. Contrast Considerations
•Most of the beam
–Goes right through the sample unperturbed
–Other elastic interactions change the direction of
electrons which can be selected for or eliminated
from the image forming beam.

15. Brightness Considerations
•Type of source (W, LaB6, FE)
•Higher magnification means a strong Int-Proj
lens, which means lower intensity
•Hard to see on fluorescent screen
•Ways to mitigate
–use lower mags
–converge beam with condenser lens
–align beam as needed

16. Beam Energy Considerations
•Higher voltages produces
–shorter wavelength
–better resolution
–greater depth penetration of sample

17. Beam Energy Considerations
•Lower voltages produces
–greater contrast due to larger scattering angles for
slow(er) moving electrons
–less depth penetration
–larger proportion of electrons involved in inelastic
collision events

18. Electron Beam-Sample Interaction

19. MagnificationResolution at Object (nm)
2,000 10.0
20,000 1.0
50,000 0.4
100,000 0.2 Most photographic emulsions used in electron microscopy
can resolve image details of ~20µm, thus the resolution
of object details will depend on the image magnification
as shown in the table (resolution = 20µm/magnification):

20. 20
How to Obtain High Resolution
•2. Adjustments to the gun, such as the use of higher
accelerating voltages,will result in higher resolution for the
reasons already mentioned in the discussion on high contrast.
•Chromatic aberration may be further lessened by using field
emission guns since the energy spread of electrons generated
from such guns is considerably narrower. (The energy spread
for tungsten = 2 eV while field emission = 0.2–0.5 eV.)
•In an electron microscope equipped with a conventional gun,
a pointed tungsten filament will generate a more coherent,
point source of electrons with better resolution capabilities.

21. 21
How to Obtain High Resolution
•3. Use apertures of appropriate size.
–For most specimens, larger objective lens aperturesshould be used to
minimize diffraction effects.
–If contrast is too low due to the larger objective aperture, smaller
apertures may be usedbut resolution will be diminished.
–In addition, they must be kept clean since dirtwill have a more
pronounced effect on astigmatism.
–Small condenser lens apertures will diminish spherical aberration, but
this will be at the expense of overall illumination.
–The illumination levels may be improved by altering the bias to effect
greater gun emissions; however, this may thermally damage the
specimen.

22. 22
How to Obtain High Resolution
•It may take nearly an hour for the eyes to totally adapt to the
low light levels, and this adaption will be lost if one must leave
the microscope room.
•Alignment must be well done and stigmation must be checked
periodicallyduring the viewing session.
•The circuitry of the microscope should be stabilized by
allowing the lens currents and high voltage to warm up for 1
to 2 hours before use.
•Bent specimen grids should be avoidedsince they may place
the specimen in an improper focal plane for optimum
resolution.
•In addition, they prevent accurate magnification
determination and are more prone to drift since the support
films are often detached.

23. 23
Magnification
•Besides forming images with high resolution, the
lenses of the electron microscope are able to further
magnify these images.
•Magnification refers to the degree of enlargementof
the diameter of a final image compared to the
original.
•In practice, magnification equals a distance
measured between two pointson an image divided
by the distance measured between these same two
points on the original object, or

24. 24
Magnification
•Consequently, if the image distance between
two pointsmeasures 25.5 mm while the
distance between these same two points on
the objectmeasures 5 mm, then the
magnification is

25. 25
Magnification
•As will be discussed later, there are at least
three magnifying lenses in an electron
microscope: the objective, intermediate,and
projector lenses.
•The final magnification is calculated as the
product of the individual magnifying powers
of all of the lenses in the system as shown in
Equation 6.6.

26. Exposure Considerations
•Low intensity situations lead to longer
exposure times
•Vibration will make edges blurry
•High intensity situations lead to short
exposure times (and concomitant error)

27. Sample Stability Considerations
•High intensity or long exposure situations may
cause sample to degrade (bonds break,
polymer chain-scission, etc.)
•Remember the contamination square in the
SEM??? Same thing happens in the TEM-you
will grow a nice carbon bump on samples as
you look at them.

28. TEM Sample Prep for Materials

29. Imaging Modes in the TEM
Bright Field Mode
Dark Field Mode
Diffraction Mode

30. Bright Field Imaging
•Main portion of the transmitted beam is
used to form the image
–zero-order beam

31. Dark Field Imaging
•If the transmitted beam
is excluded from the
image formation process
–off-axis imaging
–tilted beam imaging

32. TEM Imaging:
Ray Paths

33. 33
(top) Darkfield image obtained by tilting illumination system.
(bottom) Same specimen viewed in standard bright field
mode. Specimen consists of inorganic salt crystals.

34. Electron Diffraction
•Elastic Scattering Events
–Bragg diffraction
•nl=2d sinq

35. Electron Diffraction
•Four conditions in Back Focal Plane (BFP) of the
objective lens:
–No sample No reflections (only transmitted beam)
–Amorphous Transmitted beam + random scattering
–PolycrystalTransmitted beam + rings
–Single crystalTransmitted beam + spots

36. Electron Diffraction
Angle of incidence ~1/2
0
to even come close to
satisfying the Bragg condition.
Therefore only the lattice planes close to parallel to the
beam are involved in diffraction.

37. Electron Diffraction
•Think of TEM as a
diffraction camera
Transmitted Beam
Diffracted Beam
R
L
Rd=lL
R is measured
d is the unknown
lis the electron wavelength
L is the camera length
(lLis the camera constant)

38. Electron Diffraction
•Au (111) ring [2.35 Å d-spacing]
With 200KV and L=65cm the (111)
ring should be at about 7.5mm from
the transmitted beam
Rd=lL
R=0.027A*650mm/2.35A

39. TEM Images
Metal particles Polymer mix Electron Diffraction