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Electron Optics

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Not an optical instrument (no image forming lens) but uses electron optics. ... electrons can be eliminated by placing an aperture beneath the specimen. ... – PowerPoint PPT presentation

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Title: Electron Optics


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.
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 -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. 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
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