The Evactron® Decontaminator is used by both scanning electron microscope manufacturers and users to ensure 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, such as carbon buildup on EUV windows.
Hydrocarbons diffusing into the area impinged by the electron beam will react with the beam. This reaction will cause the hydrocarbon to polymerize into higher molecular weight compounds. In an SEM the carbon will build up exactly where you are trying to image, and the results are black squares. In EUV systems, the carbon will build up on the surface of the mirror and reduce its reflectivity. Even a small amount of contamination in the chamber can cause problems.
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® Decontaminator lets a small stream of gas, such as room air, oxygen or hydrogen, into the vacuum chamber. The stream of gas passes through a radio frequency (RF) generated, low power plasma. The plasma creates radicals (atoms), which then flow through the chamber and ash the hydrocarbons. If room air or oxygen is used, the products of the ashing process are CO, CO2, and H2O which can be pumped out of the chamber.
There are two parts to the Evactron® Decontaminator. For almost all models, the compact Plasma Radical Source is ~22 x 13 x 13 cm and can easily be attached to a SEM chamber. The Controller, for model 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. Other methods for monitoring contamination and Evactron® cleaning effectiveness include using a Residual Gas Analyzer.
In addition, carbon count rates, using the EDS in the spot mode, can be used to monitor carbon contamination, as well as XPS and Auger measurements of carbon.
The best place to install an Evactron® Decontaminator 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® Decontaminator as close to the top of the chamber as possible.
The Evactron® Decontaminator is very easy to install. There is a KF40 fitting on the Plasma Radical Source (PRS). Electron Microscopy Sciences sells adapter flanges for almost all ports on an SEM. Please contact us if there is a need for a custom adaptor flange. To install, one vents the SEM or vacuum chamber to atmosphere, removes the blank flange to the port on which the Evactron® Decontaminator will be installed, and attaches the adaptor flange and the PRS. Finally, one finds a level and convenient location for the Evactron® Controller, plugs the Controller into a wall socket (the Evactron® Decontaminator can be run with almost any standard wall socket, provided the correct power cable is used), and connects the cable between the Controller and the PRS. The Evactron® Decontaminator is now installed and the vacuum chamber is ready to be pumped down.
We recommend room air because it is free, safe and effective for cleaning mildly contaminated vacuum chambers. 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 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 which also chemically etch and remove hydrocarbons.
Hydrogen is also an effective gas for removing hydrocarbons, if a non-oxidizing environment is necessary.
The Evactron® Decontaminator 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. Once the chamber is pumped down below 2.0 Torr, the instrument will automatically start cleaning. When the chamber pressure is stabilized, the RF power is turned on, and the plasma is activated, the Evactron® Decontaminator will begin cleaning. After the cleaning is stopped, the chamber will pump down to high vacuum.
The Evactron® Decontaminator 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® Decontaminator. This way, unexpected venting of the chamber by the Evactron® Decontaminator will not occur. The venting step can be removed by the user if needed, but the user will need to make sure that the Evactron® Decontaminator will not harm any sensitive instruments in the vacuum chamber. For details on removing the venting step, please contact us.
The Evactron® Decontaminator runs between 5 and 20 Watts forward RF power. The use of higher power on the Evactron® Decontaminator would overheat the matching network. 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® Decontaminator 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.
For long time runs, we urge you to work with your SEM service engineer to configure the vacuum system on your SEM to allow for an extended cleaning by the Evactron® Decontaminator.
How often you run the Evactron® Decontaminator depends on how often you see the contamination problem reoccur. Contamination in hard-to-reach areas may move into the accessible areas of your chamber and eventually cause problems. If this occurs, you might need to use the Evactron® Decontaminator more frequently. As the chamber becomes cleaner, you can scale back on how often you turn on the Evactron® Decontaminator.