SEM condenser lens pole piece cleaning and objective aperture replacement.


My old uncle SEM didn’t have much maintenance in the past, so I had to start some ordinary cleaning of its parts. There were some astigmatism problems, which at some magnification I couldn’t fix with the stigmator. The stigmator is a circuit which controls 8 electromagnetic coils which bend the electron beam so that its section shape is as close to a circle as possible. Any deviation from a circle shape will introduce some spherical aberration to images which leads image to look out of focus and deformed.

The next two images show a spherical aberration correction through the stigmator. First image shows blurr due to spherical aberration, which is corrected in the second one.


Inside the colum of the SEM, there are some tiny apertures inside the pole piece assembly, the diameter of which may vary from a few tens of microns up to about 250 in more modern ones. In my old SEM there are two 500 micron condenser apertures on 0.02 mm thick x 2 mm diameter molybdenum foils, which clean and shape the beam. The smaller the aperture, the thinner the beam gets, the higher the possible resolution at signal cost, since current on speciment will be lower.
If contamination deposits on their edge, this may lead to spherical aberrations in the image.

To check the condition of the apertures and eventually remove and clean them, the pole piece assembly must be removed from the column by lifting it with a dedicated tool as shown in the below picture.


The last picture shows the parts of the pole piece assembly and the position of apertures.

Once checked the apertures, I couldn’t see any deposit of contamination on them, so I decided to leave them there without cleaning them. For a correct cleaning I would need to build a vacuum evaporator in the future, but probably replacing them with platinum ones will be cheaper. One aperture costs around 70$.

The cleaning of the metal parts, copper ones excluded, took place by rubbing them with cotton swabs and ammonia and then a final sonic bath in acetone. Tungsten is soluble in ammonia, so deposits of it can be removed with some patience by rubbing the surfaces for a good amount of time. The same procedure applies to anode and wehnelt cap as tungsten from the filament deposits on them as it evaporates. Wehnelt needs to be cleaned each time the filament is replaced, while anode is suggested to be cleaned every 24 months, which in my case means that I wont probably need to do it ever again, considering the few times I will be using the machine.


I had to use a wire to lift the pole piece because someone who previously attempted this operation forgot to screw the anode on the assembly, making it impossible to lift it with the tool, as the tool itself needs to be inserted around the anode. I wonder how it was possible for the SEM to actually work with the polepiece probably not being in the correct position! This is why some people should be kept FAR away from this kind of equipment… Maintenance of these machines requires precision, extreme care and a lot of time. Everything should always be kept clean, like all the tools used, everything has to be kept clean from dust or lint. It’s a good practice to cover ALL the parts with aluminium foil immediatly after removing them from the column and immediatly after taking them from the last sonic bath in acetone and let them air dry while covered by foil. Acetone evaporates very fast.

After putting all the parts together and pushing the pole piece back inside the column, I took out the objective aperture holder from inside the chamber in order to replace the aperture with a new one. The old aperture had a 250 micron hole and was made of molybdenum. The new one is platinum and 240 micron hole.

The old objective aperture was checked under a stereoscope and it was super dirty!
Once replaced, I could correct spherical aberrations a lot better than before!


The tiny copper part in the above picture is the objective aperture holder. It’s kept in position just by pressure by a copper ring around it. That’s not a good mechanical solution for this critical task indeed.



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