SEM beam alignment: The Happy Place
Aligning Milly’s electron beam is tricky business. It’s a process involving hours (or even days!) of pain and uncertainty that might, just possibly, result in a better microscope imaging experience.
The procedure below is what I’ve been able to cobble together from a variety of sources. I am not a professional microscope technician and by no means an expert. I accept no responsibility if you are foolhardy enough to attempt to use the information below. It might cost you time, and possibly even damage your microscope. You have been warned!
Then again, how much worse can it be compared to the procedure that shipped with the scope? The official documentation tends to be scant at best and wildly inaccurate (or at least wildly misleading) at worst.
I will say this: Using this procedure I was able to take photos I’m actually happy with. I’m also sure that if I spend several more days on alignment I can improve the quality even more, maybe even by a half a percent or so. Since Milly is taking beautiful photos at the moment I’m not sure I will actually attempt further alignment any time soon, but it is tempting every time I sit down at the console.
It’s the curse of the hobby microscopist: If you know that the scope could resolve 2nm features or less back when it was brand new in 1980, isn’t it your duty to make sure that it still can? Even if you never actually need to image at 650,000x?
Yep. We’re cursed.
You will want to use jeol buddy for this job.
Now for the procedure!
After a significant change to the scope (such as installing a new electron emitter, wantonly turning the OL or NC knobs, or moving the scope), you will need to align the beam. This process may take several hours to a few days depending on your needs and level of dedication.
It is tempting to simply find the beam and make the nicest picture you can by turning knobs at random. This is a valid approach, but you will be optimizing the beam for a specific acceleration voltage at a specific beam current at a specific OL aperture position at a specific NC aperture position at a specific working distance for an arbitrary location in the column at an arbitrary CL astig adjustment at an arbitrary OL astig centering at an arbitrary OL astig adjustment.
Many of these parameters are interdependent, and while you may have luck getting an image at random, you won’t get the best possible performance. Even worse, changing any single parameter will mean significant time adjusting the rest of them to compensate for your poor beam alignment.
An ideal microscope will have its beam aligned perfectly to the physical center of the column. The gun chamber will be perfectly clean and the vacuum will be stable at 1e⁻⁸ Pa or better. The beam will be perfectly round and will remain centered at every possible acceleration voltage and probe current size. The beam current will remain at 8µA or 12µA and will not fluctuate. The beam will pass through the center of the NC and OL apertures with every adjustment coil energized to the center of its possible range. The beam will encounter no stray electric or magnetic fields on its path, and the table will have no vibration. The stage will be sufficiently cooled to minimize thermal noise. Finally, the beam will illuminate your sample with maximum brightness but without adding undue charge, the secondary electrons will reflect into the dead center of a pristine scintillator, the digitizer will record a perfect signal with no 60Hz or other noise, and the microscope will take amazing photographs.
You will never have an ideal microscope. Instead, we will shoot for the Happy Place.
In the Happy Place, you can adjust most scope parameters without needing to make much in the way of corrections. You can turn the probe current knob to adjust the beam current to fully illuminate whatever you are shooting (bigger for more brightness, smaller for more detail) without significant shift in the image. Any adjustment needed should only involve a couple of clicks on a couple of knobs. The stored microscope parameters will handle making whatever adjustments are needed to keep the beam centered and round.
Note that Milly will only store some required parameters in memory and occasionally loses all of them, so use the jeol_buddy.py script to help fill in the gaps.
The only way to find the Happy Place is to first align the beam to the condenser lens (CL), then to the objective lens (OL).
These are notoriously difficult to follow. Contradictory. Crazy. Full of pseudo-Japanese. Possibly for a completely different scope. Use whatever documents you have as a source of inspiration, but they will not actually work even if followed to the letter.
WARNING: The following process is (as yet) incomplete, but it gets results.
- Warm up the scope. Briefly press the flash button. Wait two hours.
If you don't flash at the beginning of a session, you'll get an erratic beam.
Why not strong flash?
Early in the emitter's life you should strong flash about once per week, and normal flash once at the beginning of a session (and possibly once again after 6 to 8 hours of use). As the emitter ages, it degrades and requires more frequent flashing.
Do I really have to wait two hours?
If you don't wait two hours, the beam will start strong but will not be stable throughout the entire procedure.
Mood lighting? Really?
Yes, really. You will need to pick out subtle variations in a slow scan pattern, let your eyes adjust to low light now.
On the JEOL 6320F, 8mm is in the "snorkel zone" where the near field has a big influence on the image quality. The top of a the holder (or a blank stub) should be 8mm from the place where the beam enters the specimen chamber.
If you usually image at a different height, or if you encounter image artifacts during the alignment procedure, you can shift the stage up or down as needed. But try to stay at whatever height you intend to do most of your imaging, and on the 6320F that should be 8mm.
The collector is used when imaging at 8mm, but it will deflect the beam during alignment.
Why constant current mode?
If you forget to set constant current mode, the beam will start quite bright and taper off over the course of several minutes.
Why not a higher acceleration voltage?
If you never use very low acceleration voltages, you might want to start this procedure at 5kV or so instead. It is impossible to align the beam perfectly at every acceleration voltage, so to save time try to concentrate on the ranges where you typically work. But keep in mind that if you skip the very low voltages, you won't be able to use them effectively without a full realignment.
I still can't see anything!
- Did you turn brightness and contrast up on the beam and on the CRTs?
- Are you in standard SEM (HR) imaging mode with COMP, TOPO, and AUX disabled?
- Are you sure you've got an 8uA beam in CNST mode?
- Are you sure the OL and NC apertures are retracted?
- Wiggle the probe current knob and turn it back to 1.
- If you still can't see anything despite moving the gun alignment screws SLOWLY through most of the search space, you have a serious problem. Try the lower SEI detector, double check your wiring (if the scope was recently assembled), etc.
Why flat? Can't I use the alignment target or whatever happens to be under the scope?
If you don't find a flat surface, the alignment pattern can be affected by whatever the beam happens to be hitting.
You will also be hitting the stage at high power for an extended time, so having nothing on the stage will minimize contamination caused by offgassing.
Why does this matter? Didn't we just make a picture?
The picture was intended to get a rough beam and find the focal distance to the flat surface. But if you don't start the alignment process at 0,0 you may run up to the end of the gun adjustment scale.
Using SEM1 instead of SEM (HR) disables the position shift adjustments. You want to align to dead center, so you can either set position shift x/y to 0 or (safer) disable it altogether.
The blob is too big or too small!
Try adjusting contrast. If that doesn't help, you can turn the probe current knob. When in CL alignment mode, the probe current knob makes a fine adjustment. Remember to turn it all the way back up (counter-clockwise) when you're done.
I still can't get a picture!
It doesn't have to be good, but you should be able to produce something. If not, go back to CL alignment mode (step 13) and try again.
- Each time it should be easier to make a round-ish blob with a dot in the middle when in CL alignment mode, and your pictures should look better in OL alignment mode.
- After a pass or two, try the ASTIG adjust in OL mode to make the best possible picture.
- Always work on improving the image using this loop: wobble on, center the physical OL, wobble off, focus, astig, mag until it's fuzzy again, repeat.
This guide is for TEM but it has some excellent information and illustrations relevant to SEM.