Beam chopper for subnanosecond pulses in scanning electron microscopy

Abstract

A beam deflection system for an AUTOSCAN scanning electron microscope is described. The primary electrons (PE) are deflected in the electrostatic field of a symmetrical deflection system consisting of two trough-type travelling-wave structures with a bandwidth of 8 GHz. For routine measurements the system is driven with pulse generators limiting the PE pulse width to 350 ps; it is optimised like the electron gun to an acceleration voltage of 2.5 kV and can be externally adjusted. The degradation of local resolution due to defocusing and the virtual shift of the crossover is largely avoided by applying deflection voltages of opposite polarity to the two structures and by limiting the beam path within the deflection system.