Technology Notes

The Magnetic Lens - Basic Principles

The unique magnetic lens of AXIS electron spectrometers has two functions. It provides high collection efficiency and high spatial resolution. All electron optical lenses have aberrations which limit the spatial resolution (the definition or "fuzziness") of a image in the image plane. The effects of these lens aberrations can be minimised by limiting the collection angle of the lens (increasing the "f- number"). Reducing the collection angle, however, reduces the sensitivity of the overall electron optical system, decreasing the signal to noise ratio and thus placing a practical limit on ultimate spatial resolution for the system. In a practical XPS instrument the imaging field must be relatively remote from the sample surface, to allow proper access by the X-ray source and other devices. At long distances lens aberrations are large, and to obtain a reasonable degree of image resolution the input lens has to be apertured down, resulting in poor sensitivity. In order to minimise the aberrations of the first lens, it must be as close as possible to the specimen. Ultimately the smallest lens aberrations are achieved when the specimen is immersed in the imaging field. With electrostatic lens systems, a specimen would then have to become an electrode of the lens, which clearly raises problems for surface analysis applications

The alternative to these restrictions is to use a magnetic lens of unconventional design. The magnetic snorkel lens, unlike conventional magnetic lenses, produces a field which is not confined within the mechanical lens structure.This enables a sample to be supported some distance away from the lens and provides a clear line of sight for excitation sources.

The magnetic lens applied to electron spectrometry is a Kratos invention and is the outcome of years of ground breaking research and development in this field. It is protected under several world patents.

References
A.R.Walker, "Charged Particle Energy Analyser", European Patent Specification 0 243 060 B1; United States Patent No. 4810879
T.Mulvey, "Magnetic electron lenses" Ed. P.W. Hawkes, Springer-Verlag, New York, 1982. ISBN 3-540-10296-5

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