Electron microscopist in here?

[Sukhoi]

SEM is an FEI XL30 with a FEG. We're doing higher resolution work than this scope has been used for much previously, and am having big problems with massive periodic image shifts. Yesterday I was imaging at 50000 X, and was getting 50-100 shifts every ~50 seconds for example.

The period seems to vary with microscope parameters, so I don't think it's an environmental issue. What would be likely causes? Crap in the column charging up and then releasing? Bad lens?

 

[franksta]

The image is drifting, jumping back and forth, or something else? Is your gun current stable?


I've got a LEO 1550 and it would be fine for a while and then the image would jump. It ended up being a leak in the vibration isolation system (like a floating optics table). It was easy enough to fix but kind of a pain to track down.

 

[silverpig]

Sounds like something charging up to me. Is your sample conductive? Is it grounded?

I tried doing some e-beam lithography using an SEM on a glass substrate once. The charge kept getting built up and then discharging causing all sorts of image jumping issues.

 

[Sukhoi]

Samples vary but are generally aluminum, so charging of the sample is minimal. That micrograph is Al. The image slowly drifts, but then has the big jumps. It does appear that the periodicity of the big jumps varies with either kV and/or mag. I have some time booked later today so I'll try to figure out which one it is.

Interesting point on the vibration isolation system. Yesterday we noticed there is a leak at the compressed air dryer, so maybe it's not getting the required PSI (though leak is small). If I can change the jump period with the kV / mag I can rule this out.

I did read a suggestion that insulated wires in the chamber can cause significant charging/drift issues. I know there are two wires for some stage attachment or detector that usually sit unused...wonder if they could be the issue.

 

[franksta]

Sounds like something charging up to me. Is your sample conductive? Is it grounded?

I tried doing some e-beam lithography using an SEM on a glass substrate once. The charge kept getting built up and then discharging causing all sorts of image jumping issues.

I would generally run an ultra thin layer of chrome under the resist for e-beam lithography. If the piece can tolerate having it there of course. I also have a sample holder with four really small copper clamps that helps dissipate charge.

 

[Sukhoi]

Samples vary but are generally aluminum, so charging of the sample is minimal. That micrograph is Al. The image slowly drifts, but then has the big jumps. It does appear that the periodicity of the big jumps varies with either kV and/or mag. I have some time booked later today so I'll try to figure out which one it is.

Interesting point on the vibration isolation system. Yesterday we noticed there is a leak at the compressed air dryer, so maybe it's not getting the required PSI (though leak is small). If I can change the jump period with the kV / mag I can rule this out.

I did read a suggestion that insulated wires in the chamber can cause significant charging/drift issues. I know there are two wires for some stage attachment or detector that usually sit unused...wonder if they could be the issue.

Didn't have time to do the kV / mag variation today. Won't be on the microscope until next Thu unfortunately, so I'll update this at that point in time.

 

[silverpig]

I would generally run an ultra thin layer of chrome under the resist for e-beam lithography. If the piece can tolerate having it there of course. I also have a sample holder with four really small copper clamps that helps dissipate charge.

Yeah most of our stuff was highly doped silicon. I just stuck it to the holder with carbon tape and never had issues with that.

The glass was just a test to see if I could get it to work without too much trouble, but it turned out to be more trouble than it was worth.

 

[Sukhoi]

Didn't have time to do the kV / mag variation today. Won't be on the microscope until next Thu unfortunately, so I'll update this at that point in time.

So I got on the SEM yesterday after all. I varied voltage from 5-15 kV, spot size from 3-5, and mag from 35000-150000. The period of the shift never changed from 41 seconds. So what's that mean?

 

[ScottMac]

Does the beam current jump?
It could be a vacuum leak, such that you pump down to some level, some O ring, bushing, or seal fails, pressure goes up (tweaks the filiment) and causes the jump or displacement.
I used old TEM/STEM stuff and very little SEM (Joel, Zeiss, Phillips....), but I worked at Zeiss as a field tech on their EMs for four years.

It could be a power supply issue too.

What kind of HV pumps do you use? Ion getter, turbomole ular? Or just diff pumps?

 

[Sukhoi]

So the lab manager finally got FEI out. They think there is an issue with the vibration isolation table and ordered a couple new parts to install next week. FEI could not think of any electrical problems that could cause the symptoms. I'll update everyone if it turns out to not be the table. Thanks for the suggestions!