NPGS SEM Optimization

NPGS: SEM Optimization

In general, the optimization of the SEM will involve the following steps. It may help to visualize the process as working from the top of the column down to the sample.

Properly Saturate the Filament. This should be done at the kV to be used for lithography. Note the for lithography, you should not simply use the previous setting for the filament saturation, even though that is often the procedure for SEM microscopy. Setting the filament at too low of a current will make it sensitive to small changes, while too high of a current will significantly reduce the filament life. For field emission microscopes, the filament saturation may be fully automatic. Some W and LaB6 SEMs have an automated saturation, however, manual saturation may still be better.

Set the Gun (filament) Shift/Tilt Controls. The tilt setting will normally be adjusted to maximize the current on the sample. Note that the shift setting may be adjusted during a physical column alignment procedure or by a procedure that changes the beam current over a wide range of values. Some people will simply adjust it to maximize the sample current, just like the tilt. Please consult your SEM manual or SEM service representative for the recommended procedure (although don't be surprised if you get different recommendations). Note that on lower cost models, the SEM may not have electronic gun shift/tilt controls and may only have knobs at the top of the column to physically shift the gun, which will be used to maximize the beam current. (In this discussion, "beam current" means the current actually hitting the sample, while some SEM brands may use the term with a different meaning.) For field emission microscopes, the gun coils may not be adjustable by the user.

Set the SEM Beam Current: The final steps should be done at the beam current (and kV) which will be used for the subsequent lithography. Also, if the microscope offers an option to "clear" the hysterisis in the objective and/or condenser coils, it should be used after larger changes in the focus and/or beam current settings.

Center the Aperture. Most SEMs will have a "wobble" mode that automatically changes the focus back and forth (this mode may also be called "Lens Modulation" or something similar). While this mode is active, the image will swing back and forth on the screen if the aperture is poorly centered. I like to find a small, bright, nominally round, speck of something which fills about 1/3 of the imaging screen when the image field is about 1 to 3 microns in size. This speck should be in reasonably good focus before centering the aperture. With the "wobble" mode active, the X and Y positions for the aperture should be moved to make the image motion change from "bad" to "good" to "bad" and back again. This ensures that you can fully observe the middle "good" position, which produces the minimum motion on the imaging screen. Independently set the X and Y positions for minimal oscillation of the image while in the "wobble" mode of the SEM. Note that if there is significant astigmatism, it may be difficult to set the aperture centering correctly. In that case, you should jump ahead to the astigmatism correction step, come back to center the aperture, and then repeat the astigmatism step.

Astigmatism/Focus Correction. The astigmatism correction should be done on a SEM gold calibration standard as seen above. Initially, quickly move the stage to the smallest speck of anything you can see on the sample. Then, adjust the focus and astigmatism. Repeat this process of moving to the smallest speck of dust or whatever, then adjusting the focus and astigmatism, until you are at an image field size of 1 micron or smaller. Note that sometimes you may want to go to a lower magnification and search around to find a better small speck to use for the next step. Also, adjusting the astigmatism may be skipped at the lower magnifications. Once you are at an image field size of 1 micron or smaller, the imaging field may need to be moved frequently, since contamination will often quickly degrade the image over such a small field.

In general, if any of the steps have produced large changes from the previous settings, you should back up a few steps, or even to the first step, and start over. When the SEM is well optimized, you should be able to go through all steps and quickly confirm that the parameters (knobs or digital settings) are at their optimal values. It should also be obvious that the last two steps are typically mixed together, however, the last adjustment will always be minimizing the astigmatism while at the best possible focus.

More detailed information on SEM optimization can be found on the JEOL page "A Guide to Scanning Microscope Observation".