Frequently asked questions about field emission sources

Q. Are FE sources less reliable than thermionic sources?
A. No - they can be very reliable, working over many months continuously.

Q. What are the advantages of Schottky field emission compared to cold field emitters e.g. W(310) or W(111)?
A. The major advantages are better beam current stability, less stringent vacuum requirements and the fact that there is no need for periodic emitter flashing to restore the emission current.

Q. What are the vacuum requirements for operation of a Schottky field emission source?
A. Normally 10^-9 mbar or better in the source region.

Q. What are the brightnesses of Schottky sources as compared to thermionic sources?
A. Schottky emitters have brightnesses of about 10⁸ Acm^-2sr^-1 whereas thermionic emitters have about 10⁵ Acm^-2sr^-1.

Q. Do Schottky emitters need replacing very frequently?
A. The lifetime of a Schottky emitter is determined by the life of the Zr reservoir in the tip structure. This is normally about 8000 hours.

Q. Are there special steps to be taken in turning on a Schottky source?
A. Care is needed to avoid thermal shock to the emitter and to avoid electrical breakdown transients as the extraction voltage is applied. Further, the operating temperature should not exceed 1850 K.

Q. Can any thermionic source be replaced by a Schottky field emitter?
A. In principle yes. There may be a need for differential pumping of the source in order to maintain adequately low pressure.

Q. What performance can be obtained if an LaB₆ source is replaced by a Schottky field emission source on a PHI 600 series probe or its equivalent?
A. Upgraded PHI 660 systems with the YEO3/P660 product has improved the spatial resolution to close to that of the PHI 680.