NetNotes


When was the last time you cleaned or replaced the apertures in


the column? One can replace them or clean them in a plasma asher. Usually, it is cheaper to replace them. John Minter jrminter@gmail.com


Steve Chapman of Protrain in the UK has responded in the past to


similar pulsing issues. In those instances, Steve’s vast experience pointed to a dirty column. When the column gets dirty, little arcs between the main beam and the ground side of the column are common. Maybe, aſter you exhaust all other possibilities, you should open the column and clean it. In SA mode only, when the objective lens is on, pulsing suggests the issue could be dirt and or fibers around the objective lens area. Pete Eschbach peter.eschbach@oregonstate.edu


Tank you for your help. Sorry I did not mention that by SA


mode, I did not mean selected-area electron diffraction. On FEI instruments, the SA imaging mode is when the magnification is higher than 5600X or 6200X. We have a beam pulsing issue in both imaging and diffraction modes. Te FEI technician changed the filament, checked out the HT circuit, and replaced the 30 V double-deflection coil. I checked it out today and there is beam pulsing at 60 kV, 80 kV, and 100 kV at magnifications higher than 2650X. I can’t see pulsing at low magnifications. Tere was irregular beam pulsing last month, but now it is regular pulsing. Te condenser apertures, objective apertures, and SA aperture were replaced a month ago. Tank you. Sara Rizzo sararizzo125@gmail.com


Can you qualify your description of pulsing a bit better? Does


the entire image appear to be pulsing or only the beam? Use a holey carbon film and alternately spread the beam over a larger area covering multiple “holes” versus focused within a single hole. Is the effect and magnitude the same? Nestor Zaluzec zaluzec@anl.gov


As people have already mentioned, there can be a number of causes


for pulsing. If the beam intensity is flickering but the image is stable, there is likely charging/discharging on the Wehnelt cap. Basically, it is changing the “emission” setting of the gun by affecting the bias voltage. If you see the image size pulsing, it suggests that there is HT instability. When there is an arc in the gun, the HT will momentarily drop. Te lower-energy electrons will be deflected more by the lenses, and you will see the image rapidly expand then shrink back to its original size. If you see a stable image but the beam diameter fluctuates, there is current instability in the C2 lens. If the image magnification fluctuates, the instability is most likely in one of the imaging lenses. An instability in the diffraction lens will cause the objective aperture to change focus in diffraction mode and the SA aperture to change focus in image mode. Henk Colijn colijn.1@osu.edu


If low magnification is stable and the problem only appears in


SA mode, then the problem is not likely to be gun or HT hardware/ circuitry related. It is not likely caused by contamination or charging either. It is likely to be an electronics issue. Henk explained general symptoms completely. Troubleshooting is required. By “30 V double- deflection coil” - do you mean a 30 V doubler unit on the leſt side of the power supply cabinet? Perhaps a silly question - is the wobbler ON? Is any alignment mode ON? (Tere are several such modes). Vitaly Feingold vitaly@sia-cam.com


Vitaly, sorry for my mistake, I meant a 30 V doubler unit. Neither


wobbler nor alignments are ON. Tank you, everyone. I will show your comments to our technician, hopefully, he can fix it the next time. I will keep you updated. I have uploaded a video of beam pulsing here: https://www.mediafire.com/file/9nfnix4ck23jozv/beam+pulsing.mp4/ file Tank you again. Sara Rizzo sararizzo125@gmail.com


2021 September • www.microscopy-today.com


In the video I saw the magnification changing by a few % - is


this the symptom? (Your camera was moving so it was hard to tell). If yes, then either the HT isn’t stable, or the lens(es) current(s) isn’t stable. Tere was one big jump in diffraction mode—was this also a symptom? On a general note - when recording a video, please fix the camera (smartphone or whatever) in space so the camera itself doesn’t move. Tankfully it was possible to reference image to circles on the phosphor screen, but next time rest your smartphone against a wall or TEM, so the field-of-view won’t move. Are the symptoms what I described above? Vitaly Feingold vitaly@sia-cam.com


Sorry about that, I recorded new videos with a fixed camera recorded


at mag 5600 X, spot size 3 and beam pulsing with no sample: https:// www.mediafire.com/file/61o25rfa319ywjp/beam+pusling+no+sample. mp4/file Beam pulsing with sample: https://www.mediafire.com/file/ bzvizngisfevl4q/beam+pulsing+with+sample.mp4/file Te beam is pulsing in and out. Tank you. Sara Rizzo sararizzo125@gmail.com


Looks like a discharge in the gun. Possibly the Wehnelt. A look


at the caustic spot might glean some insight. Regards, Jaap Brink jbrink@jeol.com


Arcing in the gun HT to ground would cause a way more dramatic


effect. Your videos don’t show that. Extra conductivity in the gun or HT cable (to ground) once present usually goes only one way, not back and forth as in your videos. Arcing or extra conductivity in the gun cathode-to-Wehnelt could be somewhat similar to your videos except it would cause obvious brightness/emission changes. Your videos don’t show that either. Same as before - lens(es) current(s) or HT isn’t stable. Vitaly Feingold vitaly@sia-cam.com


Wonderful videos, and really very useful in diagnosing the issue.


Ok, so here is my best guess first thing to look at, the lens reference resistors. Both the C2 and the OBJ lens ones. Te CM12 is based on the EM420 and aſter several years the lens reference resistors that compare the current in the resistor to the lens go bad. It is like a Wheatstone bridge circuit and the lens voltage/current is compared to the balanced power resistor for stability, when they go bad. Tey act as a LR circuit and pulse slowly as they fail. I would have your tech spray with a cryogen and check. If you freeze them they stabilize, and then fail when they warm up. Let us all know how it goes. It is now like a game to the community. David Bell dcb@seas.harvard.edu


I hadn’t thought of the lens measuring resistors (aka “foot” resistors).


However, now that you mention it, that sounds like a likely source of the instability. I seem to remember that they were on a water cooled heatsink. I’ll put my vote in for your suggestion! Henk Colijn colijn.1@osu.edu


Te thermometer connected to the scope shows 68–70o F. Tere


are other scopes in the room, all of them connected to one chiller, none of them have this issue. Sara Rizzo sararizzo125@gmail.com


What do you think about a leakage current from the lens coil to


the lens housing? Tis may not occur very oſten but is easy to measure. You only need an insulation tester: Unplug the lens coil and connect the tester at one of the lens coil contacts and at a cooling tube or water connector of the lens. (Do not disconnect water!) Bad insulation from the coil to housing will cause a leakage current which will deteriorate the stability of the lens current. If the insulation is bad, you need a new coil. Winfried P. Send winfried.send@gmail.com


Generally correct. Devil is in details. Firstly, it is not necessarily a lens coil. Diagnosing this mode of failure is a highly test-condition-dependent.


73


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