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

Oxidative Cleaning in SEMS
By Ronald Vane, President XEI Scientific, Jan 15, 1999 - revised 4/20/2003

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.

TheEVACTRON SEM-CLEAN™ 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 SEM-CLEAN 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.

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™ SEM-CLEAN 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.