Micro-Optics Ltd

Fieldemissions

We have successfully become involved with this demanding area of scanning electron microscopy over the last few years. In this device class the challenges have always related to the emitter and the ultra-high vacuum (UHV).

JEOL exclusively uses so-called cold field emission cathodes in its older FSEMs. We can also exchange this emitter and carry out all the required ancillary work – properly and expertly and at lower cost of course!

We work with well-known companies to service and repair the required UHV system. These companies can provide repair services and supply new parts for the various FSEM vacuum components. 

Complete view - please click on picture

Principles of cold field emissions

The cold field emission cathode generally consists of a tungsten single crystal with a fine tip which is mounted on a bent tungsten wire. The curvature radius of the tip is approx. 100 nm. A < 310 >orientation of the tungsten crystal is used to keep the discharge area for the electrons small.

With an extraction voltage of 3 – 8 KV this achieves a field strength of 3-5x 10-7 V / cm on the surface of the tip at which adequate beam currents are generated. A high beam current density (105 A/cm2) and a high directional beam value are achieved as a result of the very small discharge area.

The low energy fluctuation range of just 0.2 eV suppresses the level of chromatic error even at low acceleration voltages. As good a vacuum as possible (better than 10-7 Pa) is required to keep the damage to the tungsten single crystal tip caused by ion bombardment low whilst also reducing the occupation of the tip by foreign atoms and molecules.

comparison of the individual beam sources