Invited Paper VT-WeA1
Plasma Cleaning of SEMs and Large Vacuum Systems

Wednesday, October 21, 2015, 2:20 pm, Room 230B

Vacuum-based processes can suffer harmful effects from the presence of adventitious hydrocarbons that result from various sources such as oils and solvents as well as work-pieces. The problem of carbon and hydrocarbon contamination in vacuum chambers of scanning electron microscopes (EM) and other ion beam instruments is well known. An effective tool at removing hydrocarbon contamination from electron microscope chambers is remote, or downstream, plasma cleaning. Electrically neutral radicals flow from the plasma source into the chamber so that carbon compounds are removed by chemical reactions.

Plasma cleaning of hydrocarbons for electron microscopes and vacuum chambers is a simple version of the more complex plasma etch and ashing technologies used in semiconductor production and other plasma processing. Its premise is straight forward: remove carbon compounds and do no damage to the instrument. Doing this requires a small plasma source that can be mounted on an instrument port. Desirable properties are 1) Use air as an oxygen source for oxygen chemical etch, 2) Avoid ion sputtering . 3) operate at low power to avoid heat and high sheath energy potential 4) Produce a narrow electron energy distribution. 5) operate over wide pressure range.

The Evactron® De-Contaminater from XEI Scientific is a hollow cathode RF plasma device that meets these criteria. First developed to operate at low vacuums produced by roughing pumps, it has been now been modified to work with high vacuum produced by turbo molecular pumps (TMP). Now Evactron cleaning can be initiated with a TMP at full speed and vacuum of < 10-7 Torr. The flow of gas through plasma raises the chamber pressure during cleaning but the TMP retains full speed. If the chamber pressure drops below 15mTorr (2 Pa ) the mean free path becomes long enough that a pink flowing afterglow fills the chamber. The afterglow is the result of reduced recombination rates of radicals and metastables at the lower pressures. The flowing afterglow is a marker for the presence of the oxygen radicals that do the plasma cleaning. The cleaning volume and rates are greatly increased with flow afterglow cleaning.

The pink flowing afterglow from air plasma is caused by nitrogen metastables and contains many UV emission lines that can desorb water vapor and hydrocarbon vapors from surfaces in the chamber which speeds pump down after plasma cleaning. RGA mass spectrometry results have shown remarkable decreases of the partial pressures of all gasses in UHV chambers if flowing afterglow cleaning is done during pump down. If this effect can be used to avoid bake out of UHV systems, considerable time savings may be achieved.