Hummer SEM Sample Coater - Techniques to reduce charging in SEMs
To reduce charging while SEM'ing your sample, you need to give electrons impinging from the top SEM cone a conductive path to reach the grounded SEM stage. Thin layers <<1µm layers can still transmit some charge, 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:
- Cleave/mount your sample for imaging
- Mount the sample onto the SEM sample holder as you would for imaging. Do this right before your scheduled SEM timeslots.
- 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.
- 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 during the next steps.
- 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.
- Run the Hummer coating according to the Hummer instructions, eg. 60sec.
- If sample is still charging in the SEM after this, you can always bring the sample back to the hummer for an additional 60sec.
- Too thick coating and your SEM image will show a grainy AuPd coating on the sample., and lose contrast between materials.
- Keep the sample mounted in this orientation, with clip attached - you now have a thin coating on the surface that connects the surface to the clip, and thus to the sample 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.
- Removing the sample can increase charging slightly, but the sample will still have a conductive AuPd coating applied either way.
Technique developed by Bill Mitchell. Written by Demis D. John.