Electron Optics
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Electron Optics 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. Electron Optics

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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.


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Electron Optics

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


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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


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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.


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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


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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.


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Converging (positive) lens: bends rays toward the

axis. It has a positive focal length.


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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


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Electron Optics

Electrostatic lens

Must have very clean and high vacuum

environment to avoid arcing across plates


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Electron Optics

Electrostatic lens

Converging Lens

Diverging Lens


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Electromagnetic Lens

Passing a current through a single coil of

wire will produce a strong magnetic field

in the center of the coil



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Electromagnetic Lens

Pole Pieces of iron

Concentrate lines of

Magnetic force




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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.


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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.


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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


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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.


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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


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Upper Pole Piece 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

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


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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


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Lens Defects of stopping scattered electrons and thus

Since the focal lengthf 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.


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Lens Defects of stopping scattered electrons and thus

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


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Lens Defects of stopping scattered electrons and thus

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


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Lens Defects of stopping scattered electrons and thus

A few manufacturers have combined an electromagnetic (converging) lens with an electrostatic (diverging) lens to create an achromatic lens

LEO Gemini Lens


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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


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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.


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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


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Lens Defects edges of the lens so by placing an aperture immediately after the specimen chromatic aberration is reduced along with increasing contrast

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


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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


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Diffraction 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

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


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Diffraction 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

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.


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Apertures 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

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


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


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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


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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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