1. Electron Optics

2. Transmission Electron Microscope
Optical instrument in that it uses a lens to form an image
Scanning Electron Microscope
Not an optical instrument (no image forming lens) but uses electron optics. Probe forming-Signal detecting device.

3. Electron Optics
Refraction, or bending of a beam of illumination is caused when the wavelength enters a medium of a different optical density.

4. Electron Optics In light optics this is accomplished when a
wavelength of light moves from air into glass
In EM there is only a vacuum with an optical
density of 1.0 whereas glass is much higher

5. Electron Optics In electron optics the beam cannot enter a
conventional lens of a different optical density.
Instead a “force” must be applied that has the
same effect of causing the beam of illumination
to bend.

6. Electron Optics In electron optics the beam cannot enter a
conventional lens of a different optical density.
Instead a “force” must be applied that has the
same effect of causing the beam of illumination
to bend.
Electromagnetic Force or Electrostatic Force

7. Classical optics: The refractive index changes
abruptly at a surface and is constant between the
surfaces. The refraction of light at surfaces separating
media of different refractive indices makes it possible
to construct imaging lenses. Glass surfaces can be
shaped.
2) Electron optics: Here, changes in the refractive
index are gradual so rays are continuous curves rather
than broken straight lines. Refraction of electrons
must be accomplished by fields in space around
charged electrodes or solenoids, and these fields can
assume only certain distributions consistent with
field theory.

8. Converging (positive) lens: bends rays toward the
axis. It has a positive focal length. Forms a real
inverted image of an object placed to the left of the
first focal point and an erect virtual image of an
object placed between the first focal point and the
lens.

9. Diverging (negative) lens: bends the light rays
away from the axis. It has a negative focal length.
An object placed anywhere to the left of a diverging
lens results in an erect virtual image. It is not
possible to construct a negative magnetic lens
although negative electrostatic lenses can be made

10. Electron Optics Electrostatic lens
Must have very clean and high vacuum
environment to avoid arcing across plates

11. Electron Optics
Electrostatic lens
Converging Lens
Diverging Lens

12. Electromagnetic Lens Passing a current through a single coil of
wire will produce a strong magnetic field
in the center of the coil

13. Electromagnetic Lens

14. Electromagnetic Lens Pole Pieces of iron Concentrate lines of
Magnetic force

15. Electromagnetic Lens

16. Electromagnetic Lens

17. The two force
vectors, one in the
direction of the
electron trajectory
and the other
perpendicular to
it, causes the
electrons to move
through the magnetic
field in a helical
manner.

18. The strength of the magnetic field is determined by the number of wraps of the wire and the amount of current passing through the wire. A value of zero current (weak lens) would have an infinitely long focal length while a large amount of current (strong lens) would have a short focal length.

19. A TEM image is made up of nonscattered electrons (which strike the screen) and scattered electrons which do not and therefore appear as a dark area on the screen

20. Some of the scattered electrons will only be partially scattered and thus will reach the screen in an inappropriate position giving a false signal and thus contributing to a degradation of the image. These forward scattered electrons can be eliminated by placing an aperture beneath the specimen.

21. The design of an electromagnetic lens results in a very strong lens with a very short focal length thus requiring that the specimen lie within the lens itself along with an aperture to stop the highly scattered electrons

22. Upper Pole Piece
Specimen
Aperture
Lower Pole Piece
Both the specimen rod and the aperture rod assembly have to be inserted into the lens. They are made of nonmagnetic metals such as copper, brass, and platinum

23. While a small opening objective aperture has the advantage of stopping scattered electrons and thus increasing image contrast it also dramatically reduces the half angle of illumination for the projection lenses and thus decreases image resolution

24. Lens Defects Since the focal length f of a lens is dependent
on the strength of the lens, if follows that different wavelengths will be focused to different positions. Chromatic aberration of a lens is seen as fringes around the image due to a “zone” of focus.

25. Lens Defects
In light optics wavelengths of higher energy (blue) are bent more strongly and have a shorter focal length
In the electron microscope the exact opposite is true in that higher energy
wavelengths are less effected and have a
longer focal length

26. Lens Defects
In light optics chromatic aberration can be corrected by combining a converging lens with a diverging lens. This is known as a “doublet” lens

27. Lens Defects
A few manufacturers have combined an electromagnetic (converging) lens with an electrostatic (diverging) lens to create an achromatic lens
LEO Gemini Lens

28. The simplest way to correct for chromatic aberration is to use illumination of a single wavelength! This is accomplished in an EM by having a very stable acceleration voltage. If the e velocity is stable the illumination source is monochromatic

29. The problem arises when electrons are differentially scattered within the specimen slowing some more than others and thus producing poly-chromatic illumination from a monochromatic beam.

30. The effects of chromatic aberration are most profound at the edges of the lens so by placing an aperture immediately after the specimen chromatic aberration is reduced along with increasing contrast

31. Lens Defects
The fact that wavelengths enter and leave the lens field at different angles results in a defect known as spherical aberration. The result is similar to that of chromatic aberration in that wavelengths are brought to different focal points

32. Spherical aberrations are worst at the periphery of a lens so again a small opening aperture that cuts off the most offensive part of the lens is the best way to reduce the effects of spherical aberration

33. Diffraction
Diffraction occurs when a wavefront encounters an edge of an object. This results in the establishment of new wavefronts

34. Diffraction
When this occurs at the edges of an aperture the diffracted waves tend to spread out the focus rather
than concentrate them. This results in a decrease in resolution, the effect becoming more pronounced with ever smaller apertures.

35. Apertures Disadvantages Advantages
Decrease resolution due to effects of diffraction
Decrease resolution by reducing half angle of illumination
Decrease illumination by blocking scattered electrons
Advantages
Increase contrast by blocking scattered electrons
Decrease effects of chromatic and spherical aberration by cutting off edges of a lens

36. If a lens is not completely symmetrical objects will be focussed to different focal planes resulting in an astigmatic image

37. The result is a distorted image
The result is a distorted image. This can best be prevented by having as near to perfect a lens as possible but other defects such as dirt
on an aperture etc. can cause an astigmatism

38. Astigmatism in light optics is corrected by making a lens with a corresponding defect to correct for the defect in another lens
In EM it is corrected using a stigmator
Which is a ring of electromagnets positioned around the beam to “push” and “pull” the beam to make it more perfectly circular