The Evactron® RF plasma creates oxygen radicals that chemically etch and remove hydrocarbons, organics, and surface carbon from Electron Microscopes and other vacuum systems. Contaminants are ashed into volatile products which are removed through the roughing pump.

Plasma ashing and glow discharge cleaning of samples have long been a cleaning method available for sample preparation for SEMs and TEMs, but requires expensive auxiliary equipment. Argon and oxygen plasmas are normally used. Argon cleans by a sputter etching mechanism. However inside an SEM (or other vacuum chamber) plasma that sputter etch should be avoided because of possible damage to components within the chamber.

The EVACTRON Decontaminator (D-C) system provides a safe method for plasma cleaning inside the SEM. Air, as a diluted oxygen gas mixture, can be chosen as the plasma reactant gas. Air is passed through a low-power RF glow-discharge to create oxygen radicals inside a small source chamber, called the Oxygen Radical Source (ORS) mounted on a larger vacuum chamber (see figure above). The source chamber is subject to the same vacuum as the main vacuum chamber, and the vacuum pressure and gas flow are controlled by the EVACTRON’s D-C controller. As seen in the figure above, the radicals are carried out of the plasma into the whole of the main chamber by convection. In the chamber they react with all exposed surfaces including the specimen if present. The plasma itself is confined to the ORS, which prevents ion bombardment damage to the instrument or specimen.

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The use of low-power RF plasma in the EVACTRON D-C is an important aspect in the generation the oxygen radicals from air.  In low-power plasma a large amount of the electron energy is distributed to vibrational excitation of oxygen and nitrogen molecules (see Figure 2-32 on p. 66 in Plasma Chemistry by A. Fridman).  Oxygen has a lower dissociation energy (5.1 eV) than nitrogen (9.7 eV), so these vibrational excitations are more likely to lead to oxygen molecule dissociation than nitrogen molecule dissociation.  A small fraction of oxygen molecules will be dissociated by electron bombardment to form oxygen radicals, and these radicals will leave the ORS and go into the main vacuum chamber.  Greater RF power will lead to a greater number of nitrogen and oxygen ions, which in turn will lead to the formation of secondary products such as NO. 

  
The Evactron process is successful because the hydrocarbon oxidation products are volatile in vacuum. Oxygen radicals oxidize hydrocarbons and form volatile oxides. The oxidation generally begins with hydride extraction (hydrogen atom removal) that creates more reactive sites on the hydrocarbon chain. These sites, when ashed by subsequent O radicals, further break down the chain into volatiles. For fluorocarbons the C-F bond oxidation reaction is very endothermic and these compounds are non-reactive. The effect of Evactron cleaning on metals and other materials can be predicted by looking at the oxidation chemistry of the target material. On most metals a stable oxide layer has already formed on the metal and Evactron oxidation will not penetrate this oxide layer.

EVACTRON System on SEM