Technology Notes

The Magnetic Lens -
Further Detail and the Charge Balance System

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A distinctive property of a magnetic lens is the rotation of the image with respect to the object. This image rotation is caused by the helical path that an electron is forced to follow in a magnetic field. For a projector magnetic lens this image rotation can be expressed as

For any given working distance the term is made constant. As the energy spectrum is scanned (i.e. V varied) the current, I, is adjusted. By this means focal length, lens aberrations and image rotation are fixed. Software "geometry corrections" (more flexible than hardware alternatives), then provide cosmetic offset for this fixed value, just as in standard electron microscope practice.

While high energy electrons are transported along the field axis in the manner desribed above (note that the magnetic field provides a pure transport mechanism: the energy of the electrons is not altered at all by the field), low energy electrons are strongly held in the imaging field and begin to perform cyclotron motion, with orbits touching the diverging field lines. The charge balance plate at the circumference of the lens provides sufficient repulsive energy to direct these trapped thermal electrons back to their point of origin, exactly retracing their path to the sample surface to meet it at the point at which they left.

When "extra" electrons are needed, for example when using a monochromator (which provides considerably fewer low energy photoelectrons, due, amongst other reasons, to its small spot size and exclusion of bremsstrahlung), then a hidden filament near the charge balance plate injects electrons into (from this view point) the converging magnetic field. This takes them down to the surface until charge balance (an equipotential surface) is attained.

Thus a very exact "charge neutralisation" regime is provided with very low energy electrons, which results in high energy resolution, mimum sample damage and the exclusion of differential charging artifacts.

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