Any variation of emission current with time is unwanted; however, all electron sources suffer from short- and/or long-term
instability in different degree. Short-term instability should be especially minimized so that image flicker in STEM mode and instability of EELS and EDS mapping can be avoided. In EMs (especially in TEMs), the temporal coherency effects mainly comes from the small instabilities in the accelerating voltage and electron gun emission over time, which will give the illumination a small energy spread, and from variations in the lens currents, which induces focus variation with time. Moreover, the steady emission current of the electron gun may change to a different steady current if the filament is switched off and on. Therefore, the filament should always be left on even when the TEM specimens or recording film is changed. This requirement needs pre-pumped airlocks between the electron gun and EM column and between the EM column and film chamber which are common in modern electron beam equipments.
As listed in a table in page1409, the tungsten (W) hairpin filaments are usually stable extremely (with a current drift of only ±0.05% per hour). Their instabilities last for only a few minutes with new filaments and for an even shorter time if old filaments are switched on. LaB6 filaments are less stable than the W hairpin filaments. However, similar to W filaments, if they are not overheated, the instability of their emission currents will be ±0.1% per hour after their initial instabilities. Field emission sources are relatively unstable both over short times and over long times, and thus special maintenance is needed to compensate for the instabilities with time.
In general, the instability of the source emission can be due to many causes, for instance:
i) Mechanical drift/vibration of the electron gun;
ii) Ion bombardment of the filament by gas ions;
iii) Adsorption of residual gases presenting in the microscope column;
iv) Whisker formation on the filament.
The W filament is less sensitive to the vacuum level than the LaB6 and field emission sources. The W filaments are normally stable at vacuums between 10-6 and ~10-3 Torr, while LaB6 filaments are stable only when the vacuum is better than 10-6 Torr. The thermal field emission sources require a vacuum of the order of 10-7 Torr for stable operation and the cold field emission sources are ~10-10 Torr. In general, the better the vacuum, the greater the stability.