Hi Nicolas,
I use time to time such currents on FE (Cameca SXFiveFE). Never had used any tool to find out what the spot size in reality is as it was clearly comparable with high resolution SEM, however not in standard user settable conditions. It is quite known that in normal conditions (using as intended by manufacturer) the resolution is worse than on Jeol SuperProbe...
But... I am not casual user which are happy being holded by hand. And I believe at max current it beats Jeol; especially at low voltages as be it 5kV or 30kV it is possible to stream all those electrons to the sample. Hearing from Karsten that there is such a strong handicap on Jeol in this regard, it just makes me more happy we have Cameca.
Anyway the trick and requirements is very simple.
1) Well maintained FE gun that includes: no ring-collapses, no arching, no poisoning.
2) decently physically centered column
3) ability to fiddle around manually with all lenses
If nr. 1 is met, then tip itself emits supper sharply round beam with no eye visible astigmatism. Moreover if tip is well maintained it can give about 0.1% beam stability/24h.
The idea is borrowed from Gemini columns (used by CZ FESEM's), where beam crosses only once - at surface of the sample. Crossing beam brings in astigmatism due to electron-electron interactions.
So the trick is to put zero strength on C2 lense (and probably C1 too if on Jeol, but I don't know if this trick is at all achievable there), and then rely on the integrated EL lense of the FE tip (again that needs user access, and other probes than Cameca deny it to user for that parameter) focusing the beam (with EL) up to moment of bypassing all apertures which is in axis of the column. The software and firmware will go nuts with C2 (and C1?) as zero and will give no magnification information as C2 and C3 values are used to calculate that. However the image will be as sharp as we see on our high resolution ZEISS FESEM's, thus closer to tens of nm, rather than hundreds. Again as there is no valid scale, I never tried to test it with any software, also seen no point in that.
Downsides of such trick:
* only the max current emitted from emitter (800-1.2µA, depends from setup); it is not possible to granularly tune that to particular value (actually it is but that will destabilize the emitter).
* WDS counting electronics will be enormously over-saturated for major elements. Even new dead time correction equations wont help.
* C3 will be working in different conditions than designed, long term stability of keeping such beam focused is unknown.
P.S. You wont drill the hole with such currents, if coating the sample with good coater in multi-pulse mode and mineral is resilient against electron beam. I.e. when applying 1µA beam on the rutile, I had never seen any disintegration spot mark. There are also some other minerals which can withstand insane amount of current.