electron microscopy - part 2 (electron optics)

SEM operation

Magnetic Electron Lenses

• The first magnetic electron lenses were developed by M. Knoll and E. Ruska in Germany in 1932.

• Magnetic lens consist of two circularly symmetric iron pole-pieces with copper windings with a hole in centre through which beam passes.

• The two pole pieces are separated by "air-gap" where focusing actually takes place.

• Electrons have a charge and their direction of travel can be altered by an electromagnetic field.

• An electron traveling in off-axis to a uniform magnetic field follows a helical path.

• Electrons can be brought to focus by engineering the electrostatic and/or magnetic fields.

Magnetic Electron Lenses

• An off-axis electron is acted on by a magnetic force:

where, 

v = electron velocity and 

B = the magnetic field.

Magnetic Electron Lenses

• The focal length (f) of the electromagnetic lens is:

f = K (V / I²)

where:

K =   constant based on the number of turns in the magnet coil and geometry of lens

V = accelerating voltage

I =   current

SEM operation

Magnetic Electron Lenses

• Electron lenses are not as good as optical lenses in terms of defects of focus, called aberrations.

• Spherical aberrations are minimized by placing a spray aperture in front of the magnetic lens, confining electrons to the center. This results in greatly reduced beam currents.

magnetic electron lenses

• The condenser lens controls the amount of current that passes down the column.

• This is accomplished by focusing the electron beam to variable degrees onto a lower aperture.

• The sharper the focus, the less of the beam intercepted by the aperture and the higher the current.

• The ultimate electron beam spot size depends on the beam current, which is controlled by the condenser lens, and the type of filament in use.

Objective Lens

• The lens nearest to the specimen and used to focus the electron beam onto the specimen surface.

• Stigmator/scanning coils also housed in objective.

• The objective lens also influences the spot size.

Objective Lens: Scanning Coils

• Scanning coils raster the beam across the sample.

• The coils consist of four magnets.

• The beam is rastered by varying the current through these magnets.

• Deflecting the beam off-axis introduces aberrations.

Objective lens: Astigmatism

• Magnetic lenses do not have perfect symmetry and the electron beam may be elliptical.

• Astigmatism produces a reduction in resolution.

• This astigmatism can be removed by adjusting the X and Y correction controls of the stigmator coils.

The Vacuum

When a SEM is used, the column must always be at a vacuum. There are many reasons for this:

• If the sample is in a gas filled environment, an electron beam cannot be generated or maintained.

• Gases could react with the electron source, causing it to burn out, or cause ionization.

Cont’d

• The transmission of the beam through the electron optic column would also be hindered by the presence of other molecules.

• Other molecules could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image.