A field-emission illumination system using a new optical configuration

Abstract

A field‐emission electron‐beam illumination system employing an ``oxygen processed'' field emitter and a specially adapted optical configuration is described and experimentally evaluated. A preaccelerator lens, acting directly upon the beam emerging from the field‐emission diode, allows the accelerator section to be used in a field‐lens mode where the gun crossover remains stationary within the accelerating lens for a wide range of operating conditions. The gun is particularly insensitive to accelerator aberrations and to changes in the high voltage, tip voltage, and beam current, making it particularly suitable for high‐voltage microscopy. Operating at 50 kV under conditions favorable for conventional transmission electron microscopy, measurements indicate a brightness of over 10⁸ A/cm²/sr at a beam current of 10⁻⁸ A with 10 μA total emission current in satisfactory agreement with theoretical predictions. The discussion includes some general design considerations and performance predictions necessary for optimizing the field‐emission system for application in conventional transmission electron microscopy.