Electron Gun Using a Field Emission Source

Abstract

A new electron gun has been built which features mechanical and optical simplicity. Theoretically, it can produce a focused spot having a radius smaller than 50 Å and provide 1000 times more intensity than a hot filament system having a similar final spot size. The increase in intensity is made possible by using a field emission electron source operating at a pressure of 10⁻⁹ Torr, which is provided (without baking) using commercially available pumps. The small spot is produced by using two properly shaped electrodes which accelerate and focus the electrons from the tip. It would take a hot filament gun and at least two additional lenses to replace this field emission gun when a spot radius less than 100 Å is required. Even then the brightness of the conventional source would be too low to make use of the small spot size obtained. The optical properties for the new gun were predicted on a computer and experimentally confirmed in a new scanning electron microscope. The aperture aberration coefficient was measured to be no more than a factor of two greater than the theoretical value of 1.5 cm. A spot radius of 250 Å has been measured, and this value is to be compared with the theoretical value of 150 Å. Although it was convenient to measure the spot directly only at a relatively large image distance (11.3 cm), calculations imply that the gun can provide a spot radius less than 25 Å when very small image distances are used. The gun can be used in pulsed operation because all optical properties are constant for a given voltage ratio so that application of the electrode voltages by means of a voltage divider provides automatic focusing for arbitrary changes in the applied voltage. The methods used to make and operate reliable high field emission tips are reviewed, and a technique is described for changing the required tip voltage to obtain a given emission current.