We report on three compound magnetic and electrostatic lenses for low-voltage applications: an immersion condenser lens, a compound spectrometer objective lens for electron beam testing, and an objective lens for a low-voltage scanning electron microscope (SEM). A common characteristic of all lenses is the electrostatic retarding field in front of an intermediate, or the final crossover image. This keeps electron energy high as long as possible and results in a reduced Coulomb interaction and low aberration constants. In the case of the objective lens for low-voltage microscopy, we have calculated 3.7 mm for the spherical and 1.8 mm for the chromatic aberration constant at a beam energy of 500 eV and a working distance of 4 mm. With such a lens, which allows tilting even of large specimens, a resolution of 11 nm should be obtained.

Frosien, J., Plies, E., & Anger, K. (1989). Compound magnetic and electrostatic lenses for low-voltage applications. Journal of Vacuum Science & Technology B: Microelectronics Processing and Phenomena, 7(6), 1874–1877.