XEI Scientific, Inc.
The EVACTRON® Anti-Contaminator and De-Contaminator
Frequently Asked Questions (FAQ)
Who uses the Evactron Decontaminator (D-C)?
What causes black squares and other kinds of carbon contamination build-up in my electron microscope images?
Why do I see contamination using a low kV electron beam versus using a high kV electron beam?
How does the Evactron D-C clean my electron microscope/vacuum chamber?
Will the Evactron process harm my sensitive detection equipment such as EDS detector, EBSD, or WDS?
What size is the Evactron D-C?
How can I determine how contaminated my chamber is? How can I gauge the effectiveness of Evactron cleaning?
Carbon count rates using the EDS in the spot mode can also monitor carbon contamination. XPS and Auger measurements of carbon can also be used.
Where do I install the Evactron D-C on my electron microscope/vacuum chamber? How easy is it to install?
You recommend using room air as the oxygen containing gas. Can I use other oxygen containing gases?
How do I run the Evactron process on my instrument?
For the Evactron C, the controller does not set the pressure in the chamber automatically. A needle valve on the ORS may need to be manually adjusted to obtain the desired pressure setting in the chamber.
What pressure does the Evactron D-C work? Why do I need to vent my chamber?
What RF power range does the Evactron D-C work?
How much time should I run the Evactron D-C? How often?
How often you run the Evactron D-C depends on how often you see the contamination problem reoccur. Contamination in hard-to-reach contamination may move into the accessible areas of your chamber and eventually cause problems. If this occurs, you might need to use the Evactron D-C frequently. As the chamber becomes cleaner, you can scale back how often you turn on the Evactron D-C.
RF Plasma Cleaning Systems for Electron Microscopes
and High Vacuum Systems
Stops Artifacts and Removes Hydrocarbons and Organics.
The Evactron D-C is used by both SEM manufacturers and users to insure pristine conditions for nanometer scale high resolution images. It has also been used with great success in CD-SEMS, nanoprobers and FIBs. It can be used to rid any vacuum chamber of unwanted hydrocarbon contamination.
Positively charged hydrocarbons ionized by the electron beam are attracted to the surface being imaged, negatively charged by the electron beam. Carbon will pile up exactly where you are trying to image, and the results are black squares or carbon build-up. Very little contamination in the chamber will cause this problem. Also, residual carbon adsorbed on surfaces being imaged will be polymerized by the electron beam.
High kV electron beams have a lower reactive cross-section with carbon containing molecules than low kV electron beams. They will penetrate through carbon adsorbed on surfaces.
The Evactron D-C lets in a small stream of oxygen containing gas, such as room air, into the vacuum chamber. The stream of gas passes through a radio frequency (RF) generated, low power plasma. The plasma creates oxygen radicals (oxygen atoms), which then flow through the chamber and ash the hydrocarbons. The products of the ashing process are CO, CO2 and H2O, which can be pumped out of the chamber.
Several studies have shown that the Evactron D-C is almost harmless to EDS detectors, EBSD, and WDS. Only neutral radicals leave the low power RF plasma, not sputtering ions.
There are two parts to the Evactron D-C. For almost all models, the compact Oxygen Radical Source is ~22 x 13 x 13 cm and can easily be attached to an SEM chamber. The Controller, for models C, 10, and 25, is 14 x 23 x 18 cm and can fit on any tabletop. Models 40 and 45 are rack-mounted (9 x 49.5 x 18 cm).
Contamination can be monitored by observing the rate of formation of black squares. Alternatively, the build-up of contamination piles on a surface when in the spot mode can also be used to monitor carbon contamination. A monitoring procedure for contamination in SEM chambers was recently presented and can be found here. Other methods for monitoring contamination and Evactron cleaning effectiveness include using a Residual Gas Analyzer.
The best place to install an Evactron D-C is on a port away from the vacuum pump port. For example, if your vacuum pump port is on the bottom of your chamber, put the Evactron D-C as close to the top of the chamber as possible.
Pure oxygen has been shown to be more effective than room air at cleaning hydrocarbon contamination. However, dry pumping systems (i.e. no pump oil) need to be used in order to minimize fire risk. Dry air or dry oxygen/nitrogen mixtures can also be used, but they are less effective at removing hydrocarbons than room air because of the lack of water vapor to make hydroxyl (OH) radicals.
The Evactron D-C first needs to be enabled for cleaning. The enabling procedure is either pressing a button or turning on a couple of switches, depending on the model. Then, the vacuum chamber needs to be vented briefly so that the Evactron pressure gauge reads over 2.0 Torr. For all models but the Evactron C, once the chamber is pumped down below 2.0 Torr, the instrument will automatically start a cleaning. When the chamber pressure is stabilized, the RF power is turned on, and the plasma is lit, the Evactron will begin cleaning. After the cleaning is stopped, the chamber will pump down to high vacuum.
The Evactron D-C typically runs at pressure in the 0.2 to 1.0 Torr range. The models 25, 40 and 45 can run as low as 50 mTorr. The venting step is an interlock to make sure that the pressure in the vacuum chamber is being raised by the instrument's evacuation control system and not the Evactron D-C. This way, unexpected venting of the chamber by the Evactron D-C will not occur.
The Evactron D-C runs between 5 and 20 W forward RF power. The use of higher power on the Evactron D-C would overheat the matching network, and studies have shown that the rate of cleaning levels off as the forward RF power is increased, due to a greater production of nitrogen ions. These ions remove oxygen radicals from the plasma.
This depends on how contaminated the chamber is. You will need to run the Evactron D-C for a long time if you are working with an extremely contaminated chamber. Sometimes overnight runs are necessary. For less contaminated chambers, shorter run times are needed.