Plasma Cleaning Notes
WAYS OF USING THE EVACTRON DE-CONTAMINATOR
There are five ways to use the Evactron process to clean SEMs and specimens:
- Specimen cleaning: When not being used to clean the SEM chamber, the Evactron Decontaminator can be mounted on a separate vacuum chamber system. Samples such as those mounted on a TEM sample holder can be cleaned by the Evactron Decontaminator.
- 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 into the chamber on the specimen surface.
- Mid-analysis cleaning: A specimen exposed to the electron beam shows signs 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 be used where cleanliness is required.
HOW THE EVACTRON DE-CONTAMINATOR WORKS
The Evactron Decontaminator consists of three parts. The Plasma Radical Source (PRS) mounts onto the vacuum chamber, and the Controller either fits on a tabletop or is rack mounted. A cable bundle connects the PRS and the Controller.
The Evactron Decontaminator PRS consists of a patented RF plasma electrode attached to a matching network inside a small vacuum chamber attachment. A metering valve assembly and a pressure gauge are also added to this attachment. The plasma produces radicals – atoms – as the active cleaning species. If room air is used, a small number of hydroxyl radicals are produced by the Evactron Decontaminator from the water vapor in the air. These radicals react with carbon containing contamination to make gas phase products such as H2O, CO and CO2 if room air is used. The EVACTRON system provides a N2 purge function for removing these gases after plasma cleaning.
INSTALLING THE EVACTRON DE-CONTAMINATOR
As a general rule, the Evactron PRS port should be mounted as far away as possible from the vacuum pump port so that the maximum amount of surface area and chamber volume will be cleaned. The Evactron system cleans in a line-of-flow action, so surfaces located between the Evactron PRS port and the vacuum pump port are cleaned the best.
For example, if the Evactron PRS is placed on upper ports of a Hitachi FE SEM chamber, then good cleaning action on the specimens on the stage is observed. However, less cleaning action is observed if the Evactron is placed on lower ports of the same SEM chamber.
XEI Scientific, Inc. offers adapter flanges for almost all SEM chambers which can be used to mount the Evactron PRS to almost any port. Custom adapter flanges are also available; please contact us.
STARTING THE EVACTRON DE-CONTAMINATOR
The Evactron De-Contaminator operates at chamber pressures between 50 mTorr (0.20 Torr for the Evactron C) and 1.0 Torr (2.0 Torr for the Evactron C).
Once the Evactron PRS is installed and connected to the controller, it is a good idea to pump down the system in order to make sure that Evactron installation has not introduced leaks into the chamber.
The Evactron De-Contaminator can be started by enabling the controller (separate instructions for each model) and then venting the chamber briefly so that the Evactron pressure gauge reads over 2.0 Torr. The Evactron Decontaminator will then automatically start cleaning. Once the chamber starts pumping down again, the pressure stabilized, the RF power turned on, and the plasma activated, the Evactron will begin cleaning.
NB: For Evactron C users, the needle valve on the PRS may need to be manually adjusted so that an operating pressure between 0.2 and 2.0 Torr is obtained.
DETERMINING THE CLEANLINESS OF THE CHAMBER
For SEM chambers, the cleanliness of the chamber is assessed by continuously imaging a clean Si surface at high resolution for 10 minutes, then lowering the magnification to see if a black square or carbon build-up has appeared. The appearance of black squares/carbon build-up indicates contamination in the chamber. This procedure can be repeated after separate Evactron runs in order to determine how much cleaning is needed.
Details about this monitoring procedure.
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OPERATING PARAMETERS FOR THE EVACTRON DE-CONTAMINATOR
Recommended operating parameters for room air and long cleaning times: Pressure = 0.4 Torr, Forward RF Power = 14 Watts.
Studies of cleaning rates have shown that setting the Evactron Decontaminator to these parameters will be effective at removing carbon containing contamination. The pressure level is recommended in order to accommodate a variety of chamber sizes and pumping speeds. The power level is recommended in order to maximize the lifetime of the impedance match on the unit.
If the Evactron Decontaminator can stabilize a pressure lower than 0.4 Torr, a higher cleaning rate will occur. A stable pressure will result in a steady pressure reading (within ±0.02 Torr) for Evactron Decontaminator units with pressure gauge feedback (models 10, 25, 40 and 45). For the Evactron model C, users can determine if a steady stream of air is entering the vacuum chamber by gently placing their finger on the inlet and feeling for steady suction.
LENGTH AND FREQUENCY OF EVACTRON CLEANING
How often and for how long the Evactron De-Contaminator is used depends on how contaminated the chamber is. Extremely contaminated chambers will need long cleaning times. Once contamination has been mostly removed, shorter cleaning times can be used. It is recommended that low RF power (14 W) be used during very long cleaning times. Higher RF power (17-20 W) can be used for runs no longer than two days.
The Evactron D-C will clean contamination from accessible surfaces between the Evactron and vacuum ports. Contamination in crevices or other difficult to reach areas inside the vacuum chamber will not be easily cleaned by the Evactron De-Contaminator. After Evactron cleaning, this hard-to-reach contamination may migrate into the accessible areas of your chamber and eventually cause problems. The frequency of Evactron use will depend upon how long it takes for this migration to affect your results. As the chamber becomes cleaner, these migration effects will become less, and the frequency of Evactron use can be decreased.
TYPES OF CONTAMINATION
The Evactron De-Contaminator cleans carbon and hydrocarbon contamination. Sources of this contamination include dirty specimens, pump oil backstreaming, oil built-in during manufacturing, lubrication, fingerprints, and the atmosphere.
Radicals react fastest with short chain and unsaturated hydrocarbons. We have had good results cleaning pump oils, skin oils and greases.
The Evactron De-Contaminator will not remove fluorocarbons from the vacuum chamber. Also, a preliminary study has shown that carbon nanotubes are fairly inert to the Evactron cleaning process. The reaction with polymers such as photoresist is slow.
CHAMBER SIZE AND PUMPING SPEED
The Evactron De-Contaminator has been proven to remove hydrocarbon contamination from almost all SEM chambers.
Large Chamber Cleaning
The Evactron De-Contaminator has been proven to be able to clean the large chamber of CD SEMs. Longer cleaning times and higher pump speeds are recommended for the best results.
Long pump down times allow recontamination
Spending hours after Evactron cleaning to reach a higher vacuum is counter-productive. The Evactron removes contaminants in the critical areas of the SEM. They are then no longer in equilibrium with all the surfaces in the chamber. By a slow desorption process these unreacted contaminants will redistribute themselves in the chamber and contamination will return. Within 3 hours, migration of contaminants around a large vacuum chamber has been observed. It is well known that a single finger print can contaminate a whole vacuum system in 10 minutes under ultra high vacuum. Contaminant migration is also a big problem on other large chambers. Long pump down times are typically for water vapor removal, and water vapor does not degrade EM images.
Faster Pumps = Faster Cleaning
The Evactron maintains a constant cleaning pressure by varying the input leak rate to match the pump speed. If more radicals are needed for faster cleaning, use a roughing pump with a greater pumping speed. This will increase the gas flow rate into the plasma, which increases oxygen radical production, thus increasing the cleaning speed.
GASES WHICH CAN BE USED WITH THE EVACTRON DE-CONTAMINATOR
Operating the Evactron De-Contaminator with dry Nitrogen and O2 gas mixtures
This gas mixture will not introduce water into your system. It can be used as a substitute for air. However, it will also result in slower cleaning rates, since there will be no hydroxyl radicals produced by the Evactron De-Contaminator.
Operating the Evactron De-Contaminator with Argon and O2 gas mixtures has been shown to be effective.
Operating the Evactron De-Contaminator with pure oxygen has been shown to greatly increase the cleaning efficiency. It has also been shown reduce the rate of polymerization of hydrocarbons, which in turn reduces the time needed to clean the system. As with air cleaning, the best plasma pressure and power set-points for cleaning are dependent upon the size of the vacuum chamber and the speed of the pump used. Warning: Pure oxygen is highly oxidizing. Fire and explosion hazards exist if pure O2 is used with oil filled vacuum pumps.
For users who need a non-oxidizing environment, the Evactron De-Contaminator can be used with pure hydrogen gas. Preliminary experiments at XEI have shown that users will achieve the best cleaning rate if the chamber pressure is set to 0.1 Torr (13 Pa) and 20 W.
Buyers of new CD SEMs and FIBs should specify that their tools be cleaned by the EVACTRON® system at the factory before shipping.
Still have questions that we did not answer above?
Contact XEI Scientific, Inc. with any further inquiries. We will be glad to help you.
Phone: 1 (650) 369-0133 / Fax: 1 (650) 363-1659