Gun Scans

[Joe Boesenberg]

Hi All,

Both the Cameca and JEOL microprobes have a feature called "gun scan" (Cameca has both "gun scan high" and "gun scan low"), which allows you to see and image the electron distribution (I think) at a certain point in the column. For the Cameca you can image near the filament (for the high) and then much lower in the column after the beam has been focused. The JEOL appears to only have one gun scan, and I think it is only the one near the filament. Can anybody explain to me how this is done and what exactly I am looking at? I have asked both Cameca and JEOL engineers and no one seems to be able to answer this question. I know it requires the secondary electron detector, but I am not sure how.   Thanks Joe

[sem-geologist]

I don't know much about Jeol, albeit we have about 20 year old Jeol SEM...

I know it requires the secondary electron detector, but I am not sure how.

How do you know? I am sure that on Cameca EPMA beam (HIGH/LOW) image is made with in-axis placed in Faraday cup (SX100) or with Faraday plate (SXFive (+FE), beam is deflected of-axis). There is no SE detector needed for that. The SE detector on our SX100 is even not powered currently with cable - thus 100% it is not needed for emission scanning.

There is indeed possiblity to get beam emission image with secondary electron detector - that kind of thing is used on some of SEM when there is no in-column built in Faraday cup. I.e. Zeiss FE SEMs use in-lense detector for that purpoise and is able to give image of the beam. And it is rather not possible to make these kind of images by chamber mounted SE...

Getting back to Cameca and Jeol... Cathode or emitted electron stream can be physically off-axis/misaligned. To get it into central electron optical axis which (theoretically) run through center of C1 and C2 condensers, splash aperture, objective aperture (which normally is adjustable) and center of probe forming C3 condenser - there is two sets of X, Y deflectors at most upper part of electron column just below the anode aperture. Cameca call those sets High and Low where High is just below the anode, and "Low" is adjacently further below it (not at bottom of column). Jeol (at least on their SEM) call it Shift and Angle (or something otherwise bizarre). Simple SEM can't make image of emission not only that it lacks of in-column Faraday cup, but also it lacks scanning hardware for these deflectors (albeit somehow it has some automatic means to align the gun...). Cameca hardware has scanning ability for both high and low alignment deflectors (of course that has some additional cost). And now you can already guess why Jeol has only beam scan...

I am convinced that having both "beam high" and "beam low" scanning ability it is possible to do incredible good gun alignment across different currents and acceleration voltages. I.e. Field emission version of SXFive while making alignment automatically jumps between different C2 values to make sure alignment is perfect for different currents.

Now what does these images represents?:
I believe Gun scan High represents beam scanned onto faraday cup with low deflectors fixed and high delflectors scanning, and "beam scan low" with high deflectors fixed (to set value) and Low deflectors scanning. There is small difference as between these two modes – I believe both "Beam scan" have C2 condensor switched off and C1 less or more focusing with no beam crossings down down to the Faraday cup. Cameca FE EPMA has no C1 and thus do different beam scan with C2 engaged. On SX100 beam scan high in some rare circumstances can project the kind of representative image of emission from the hairpin. "Beam Scan Low" more represent beam after going thought apertures and condensers, it is wishful to have it as round as possible as that beam is then going to C3 and being focused to point - anything going out of round shape will cause image aberrations. The Unix version of Peaksight GUI had nice educative real-time representation of how beam is formed in the column... unfortunately disk died and we had to dispose the unix machine and I was stupid enough not making some recording of that splendid feature.

[Joe Boesenberg]

Many thanks for the explanation. On the JEOL, you cannot generate the gun scan with the BSE detector working, only with SE detector selected, thus I assumed the image was generated using the secondary electron detector.

Joe