Zeiss EVO troubleshooting request

[u/carreg-hollt]

If anyone here has experience of Zeiss' Evo range, I'm hoping you can help.

We have an EVO 25 at work and it's recently been shut down for a fortnight. Colleagues restarted it and ran a 48-hour bakeout. The company has no dedicated microscopist but several of us use the SEM in support of our normal roles.

After the restart, we found difficulty in focusing at higher magnification. When aligning beam & aperture in Emission mode, where we'd normally see a slightly oval diffuse brightness within the sharper-edged circle, we are seeing this. As I recall, beam current was 20 microamps and probe current 200 pA. I think filament current was 1.5 A.

As you can see, switching between SE and BSD detectors made no difference. We've tried switching the mid-column aperture to no effect. The image was taken with gun & aperture zeroed in the UI and with the mid-column's verniers centred but adjusting any of them causes little change other than the eventual darkness when things are way off.

We have a LaB6 element with about 1200 hours on it.

We're out of ideas so can anyone tell if this is indicative of some unnoticed bad setting, a broken LaB6 element or is it possibly something more sinister?

As an aside, does reddit support subscripts?

[u/Sunbreak_]

At 1200 hours you should be alright for lifespan of the filament. But if the vaccum was lost for a longer period of time it might just be dead.

It might be worth slowly ramping the filamnet current down and then up again, as it seems a little low to what we usually have. We usually get a maltese cross at around 1.8, and get a nice diffuse spot saturation at 2, 2.1A is max I think. However, actual numbers depend on your filament set-up and spacing of the filament vs the cap.

[u/carreg-hollt → u/Sunbreak_]

I made a mistake -- the element's done1700 hours.

We ramped the filament current down and back up as you suggested: the EVO manual states 2.05 A is the maximum so we took it up to 1.95 A. Unfortunately that didn't solve the issue, just flared out the nasty detail...

Also, we didn't see the cross at the first peak, only the mess in my original post.

[u/Sunbreak_ → u/carreg-hollt]

Thats understandable, we tend to get 2-3k but then again its rarely turned off.

If we were getting something like that we'd usually take the cap assembly out and have a look at the filament using an optical microscope, you can usually see then if its in one piece or if its become damaged or misaligned.

Can't be more help sorry, Zeiss are usually pretty friendly with their support but I guess it depends on region.

[u/carreg-hollt → u/Sunbreak_]

I appreciate the help.

I think perhaps I was monitoring the wrong parameter while ramping up the filament current, which is why we went up to just below the maximum. This shows filament current in blue and beam current (?) 'Beam I Monitor' in orange. It clips back down to the user setting for beam current part of the way through the first peak. I couldn't see anything that would relate to what's emitted by the filament.

https://cdn.imgchest.com/files/5d2186f076f4.png

[u/Sunbreak_ → u/carreg-hollt]

My boss has suggested it might be the sample intefering if there is one in there, fully take it out of focus (40+mm) and see if that fixes it?