Electromagnetic lenses, apertures and beam size
A series of electromagnetic lenses and apertures are used to reduce the diameter of the source of electrons and to place a small, focused beam of electrons (or spot) onto the specimen.
An electromagnetic lens is a coil of wire through which current flows. Because the current flow produces a magnetic field at right angles, the field pushes inwards into the hole in the centre. This acts to shape a beam of electrons travelling in their natural spiral path down the central hole.
An electromagnetic lens from an electron microscope
Focus and lenses
The purpose of a lens is to change the path of the rays in a desired direction. Glass or transparent plastic may bend light and so are used in optical lenses. However, a glass or plastic lens will stop electrons, therefore, it is not appropriate to use glass or plastic as lenses in an electron microscope. Since electrons are charged particles and they can be bent in a magnetic field, lenses for electrons are constructed with ferromagnetic materials and windings of copper wire. These produce a focal length which can be changed by varying the current through the coil. They are called electromagnetic lenses. The magnetic field bends electron paths in a similar way that solid glass lenses bend light rays. Under the influence of a magnetic field, electrons assume a helical path, spiralling down the column. This helical path can easily be demonstrated at low magnification by changing the focus up and down to cause image rotation.
There are two lens sets. The condenser lens is at the top and the objective lens at the bottom. Each does a different job. The condenser lens converges the cone of the electron beam to a spot below it, before the cone flares out again and is converged back again by the objective lens and down onto the sample. This initial convergence can be at different heights, that is, close to the lens, or further away. The closer it is to the lens, the smaller the spot diameter at the point of convergence. The further away, the larger the diameter of this point. So the condenser lens current controls this initial spot size and is referred to as the spot size control. The diameter of this initial convergence (also called a cross-over point) affects the final diameter of the spot the beam makes on the sample.
The objective lens also has some influence over the diameter of the spot size of the electron beam on the specimen surface. But its main role is in focusing the beam onto the sample. Note: a focused beam produces a smaller spot on the surface than an under or over-focused beam.
The objective aperture arm fits above the objective lens in the SEM. It is a metal rod that holds a thin plate of metal containing four holes. Over this fits a much thinner rectangle of metal with holes (apertures) of different sizes. By moving the arm in and out different sized holes can be put into the beam path.
An aperture holder: this arm holds a thin metal strip with different sized holes that line up with the larger holes. The metal strip is called an Aperture strip.
The aperture stops electrons that are off-axis or off-energy from progressing down the column. It can also narrow the beam below the aperture, depending on the size of the hole selected.
A large aperture is chosen for low magnification imaging to increase signal and for BSE and microanalysis work.
A smaller aperture is chosen for high resolution work and better depth of focus. Depth of focus is improved with the smaller angle described within the electron beam. The disadvantage is that there are fewer electrons and therefore a less bright image.
Table 4: Some examples of aperture size and purposes
Note: A numerical scale may be provided for different apertures. For example 1, 2, 3 and 4 may be used. This can run in either direction with the largest number for the largest aperture diameter or the largest number for the smallest aperture.
|Scale||Aperture diameter (microns)||Probe current||Purpose|
|4||30||Smallest||Ultrahigh resolution; Low probe current; Large depth of field|
|2||70||High resolution at high probe current; Reduced depth of field|
|1||110||Largest||Observation at high probe currents; Shallow depth of field|
During an alignment procedure to produce a good image, the aperture needs to be checked that it is centred around the beam axis. This is done by using the Wobbler control. If the image is seen to shift from side to side then the aperture needs adjusting in the X or Y direction (in and out or side to side) and is adjusted with tiny turns of the appropriate knobs until the image stops shifting.