XEI Scientific, Inc.
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems

The EVACTRON® Anti-Contaminator and De-Contaminator
Stops Artifacts and Removes Hydrocarbons and Organics.

Updated August 2007

How Evactron® RF Plasma Cleans
SEMS, FIBS and fine film substrates:

Evactron® RF plasma creates oxygen radicals that sweep hydrocarbons, organics, and surface carbon from Electron Microscopes and fine vacuum coating systems. Atmospheric contaminants get swept through the roughing pump. After cleaning, turn off the Evactron® RF plasma and you get great SEM photos and cleaner substrates for fine vacuum coatings.

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Oxidative Cleaning in SEMS
By Ronald Vane, President XEI Scientific

Oxidative EVACTRON® cleaning of the SEM specimen chamber and in-situ cleaning of the specimen gives the microscopist a new tool for combating the persistent problem of contamination. Hydrocarbons and other organics are oxidized by oxygen radicals (oxygen atoms) to form H2O, CO, and CO2 gases that can be easily pumped from the SEM.

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. Oxygen plasmas are more reactive, and Oxygen/Argon mixtures have proved to be very effective for cleaning. But inside the SEM Argon and pure Oxygen plasmas should be avoided because possible damage to the instrument.

 The EVACTRON Decontaminator (D-C) system provides a safe method for plasma cleaning inside the SEM. Air as a diluted oxygen gas mixture is chosen as the plasma reactant gas. Air is passed through a low-power RF glow-discharge to create oxygen radicals inside a generator mounted on a specimen chamber port. The generator chamber is subject to the same vacuum as the specimen chamber, and the vacuum pressure and gas flow are controlled by the EVACTRON SD-C controller. The radicals are carried out of the plasma into the whole of the specimen chamber by convection. In the chamber they react with all exposed surfaces including the specimen if present. The plasma itself is confined to the generator chamber, which prevents ion bombardment damage to the instrument or specimen.

Chemistry and Thermodynamics

The Evactron process is successful because the HC 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 breakdown the chain.
The most thermodynamic favorable reactions are C double bond oxidation, followed by hydride extraction, followed by C-C single bond oxidation. C-C single bond oxidation is only slightly exothermic which accounts for the very slow degradation of polymers compared to single chain HC compounds. The C-C single bond is usually broken and oxidized after an adjacent bond is oxidized to create a reactive site. In fluorocarbons C-F bond oxidation is very endothermic and these compounds are non-reactive.
The effect of Evactron cleaning on other materials can be predicted by looking at the oxidation chemistry of the target material. If a stable oxide layer is formed as on most metals, Evactron oxidation will stop.

EVACTRON cleaning controls the temperature of the plasma as an important part of the method for generation the oxygen radicals from air. When oxygen is ionized a series of reactions lead to the formation of oxygen radicals:

     O2 + O+ --> O2+  +  O

      O2+ + e- --> O + O

Compared to the ions these radicals are long-lived species and may leave the plasma region.

The ionization potential of oxygen is 12.1 eV and nitrogen is 15.6 eV. Thus oxygen ionization takes place in a lower temperature or lower energy plasma than nitrogen. By lowering the average temperature of the electron-energy- distribution oxygen ionization is favored. When nitrogen ions are produced in an air plasma they react with O radicals by the following fast reactions:

        N2+  +  O --> NO+  +  N

        N + O --> NO+  +  e-

Thus two oxygen radicals are destroyed by every nitrogen ion produced. Because nitrogen is the major constituent of air, this destruction takes place quickly once nitrogen ionization begins. In addition, the reaction product NO+ is a stable ion that is unable to react with the neutral diatomic gases in air and reacts with hydrocarbons to form nitrogen oxide polymers that are resistant to further oxidation and removal. In EVACTRON cleaning an operating pressure and plasma temperature are adjusted such that the oxygen radical flux to the surfaces is maximized.

Using EVACTRON Cleaning

EVACTRON cleaning is a fast process, and cleaning in most cases can be done in less than 5 minutes. After the EVACTRON process is completed a nitrogen purge can be done for short time to flush the reactant product gases out of the chamber before full evacuation. There are four ways to use the EVACTRON process to clean SEMs and specimens:

    · SEM chamber cleaning: EVACTRON cleaning is used with a nitrogen purge to clean a dirty chamber before specimens are introduced.

    · Pre-analysis cleaning. A specimen is EVACTRON cleaned inside the SEM chamber before the electron beam is turned on to remove residual hydrocarbons that may have been carried in on the specimen surface.

    · Mid-analysis cleaning. A specimen exposed to the electron beam shows sign of contamination on the area of interest. The specimen is left in place, the electron beam is turned off, and an EVACTRON cleaning cycle performed to remove the contamination.

    · Post-analysis cleaning. After electron beam analysis the specimen is EVACTRON cleaned to remove any contamination residues so that the specimen can undergo subsequent analysis or use where cleanliness is required.

The EVACTRON system offers the microscopist a new tool for keeping his SEM and specimens clean for better analysis, identification, and metrology. By cleaning in-situ within the SEM it offers time saving convenience in the pursuit of better results.

  • Quickly remove hydrocarbon deposits from chamber walls and samples
  • Clean samples inside the SEM
  • Fast - cleans in minutes, not months
  • Activated Oxygen cleaning action. Low power RF plasma radical source
  • Compact - mounts externally on chamber
  • Uses air as oxygen source
  • Optional N2 purging capability
  • Safe - No sputter etching
  • Ideal for Materials Science and Metrology
  • Patented US 6,105,589, US 6,452,315, and US 6,610,257

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The EVACTRON®  system quickly cleans the interiors of electron microscopes including the stages, and specimens. It is an Anti-Contamination or Decontaminator accessory for electron microscopes and is used for hydrocarbon and organic contamination control in vacuum systems. It is used for black square and raster burn control, stopping oil condensation and drip from X-ray windows, plasma cleaning of high vacuum systems. It can do remote glow discharge cleaning, and hydrocarbon removal in vacuum chambers. The Evactron Decontaminator is a SEM Anti-contaminator or SEM Anticontaminator for RF Plasma cleaning and RF Plasma Ashing of vacuum chambers, AMC (Atmospheric Molecular Contamination) removal, or organic contamination control and removal in any high vacuum system. It removes Organics as an decontaminator.

Evactron® anticontamination system is for SEM and FIBs, EDX and EDS window cleaning. An RF Plasma Asher in Electron Microscopes that uses a low-power, air-plasma inside the specimen chamber to create oxygen radicals, all surfaces are reactively cleaned of hydrocarbons and residual oils in-situ. This prevents the coating of scanned areas with hydrocarbon polymer by the electron beam during SEM focusing or analytical work (no black squares or raster lines). It is designed for use microscope technicians with only simple training. Specimen cleaning by Evactron plasma oxidation may be done inside the specimen chamber either before or after examination by the electron beam. In addition to plasma oxidation, the system allows for SEM-CLEAN nitrogen purge cleaning of the SEM chamber and vacuum system when the plasma is not on.


EVACTRON System on SEM

This cleaning process takes place while the SEM chamber is at 0.6 Torr in rough pumping mode and is of a short duration. (2 - 10 minutes). The plasma cleaning is activated by operator command when needed. When activated the SEM is vented first to partial vacuum and then evacuation recommenced. At the correct pressure air is bled into the plasma source and then the RF plasma is ignited to create activated oxygen radicals for cleaning. After short time the plasma and air leak are turned off and the system is pumped down to remove the residual oxidation products. The SEM is soon ready for operation.

The feed gas to the plasma is air that is fed into the chamber through the exclusive XEI designed, low-power, RF plasma generator which maximizes oxygen radical output from air. Air is used because it is always available, and will not create the explosive conditions in oil sealed vacuum pumps that pure oxygen could. Argon is not used because it may accumulate and flood the ion pumps used in many SEMs on their electron guns.

EVACTRON Oxygen Radical Source on SEM port

A Nitrogen purge may be used to complete the cleaning and removes water vapor from the chamber. The Nitrogen purge function may be operated separately from the plasma function to suppress backstreaming from the roughing pumps.

Oxygen radicals are highly reactive to surface hydrocarbon scum or AMC (Atmospheric Molecular Contamination). The volatile reaction products of H2O, CO2, and CO are easily pumped away by the vacuum system. The plasma is confined to the generator region to minimize the bombardment of surfaces by free electrons or energetic ions that could cause sputtering or polymerization of the surface hydrocarbons or damage to sensitive SEM parts. The low power of the plasma and its confinement prevents ion etching and damage to detector windows and other sensitive surfaces within the SEM chamber. The system consists of a non-sputtering, glow discharge generator with a gas and power feed-through on a microscope port, RF Generator and matching network, vacuum gauging and control electronics.