Hummer SEM Sample Coater - Techniques to reduce charging in SEMs

Instructions for this tool: SEM Sample Coater (Hummer)

To reduce charging while SEM'ing your sample, you need to give electrons hitting your sample surface a conductive path to reach the grounded SEM stage. Thin layers <<1µm layers can still transmit some charge through then (the beam penetrates through the layer), but ≥1µm insulating layers (SiO₂, Si₃N₄, Photoresist) will prevent electrical charge from dissipating.

The most reliable method for providing this conductive path is as follows:

1. Cleave/mount your sample for imaging
2. Mount the sample onto the SEM sample holder as you would for imaging. Do this right before your scheduled SEM timeslots.
   1. If possible, place at least one conductive clip on the top surface to provide a path for charge to dissipate. This can help a lot with dissipating charge, when combined with AuPd coating.
   2. Optionally use conductive tape on the back side, but this is not completely necessary if using the clip + AuPd. It can help with mounting and keeping the sample stable while mounting the clips and during the next steps.
3. Load sample into the Hummer by putting the SEM sample holder's post into a metal tin, so the sample is oriented similarly as it will be during SEM imaging. Users often punch a small hole in the tins to enable this for SEM holders with small posts.
4. Run the Hummer coating according to the Hummer instructions, eg. 60sec is typical.
   1. If sample is still charging in the SEM after this, you can always bring the sample back to the hummer for an additional 60sec.
   2. Too thick coating and your SEM image will show a grainy AuPd coating on the sample., and lose imaging contrast between materials.
5. Keep the sample mounted in this orientation, with clip attached - you now have a thin coating on the surface that connects the sample surface → the clip, and thus → electrically-grounded stage. Don't remove the sample or the clip, or you may break/reduce the conductive coating between sample and clip/sample holder.
   1. Removing the sample can increase charging slightly, but the sample will still have a conductive AuPd coating applied either way.

The AuPd coating might remove with Gold etchant, as reported by lab users.

Technique developed by Bill Mitchell. Written by Demis D. John.