Plasma Cleaning Notes
WAYS OF USING THE EVACTRON DE-CONTAMINATOR
- 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 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
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
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
Details about this monitoring procedure.
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OPERATING PARAMETERS FOR THE EVACTRON DE-CONTAMINATOR
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
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
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
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
Please refer to the Documents section for the latest results for plasma cleaning with alternate gases by Vane et al..
Operating the Evactron De-Contaminator with dry Nitrogen and O2 gas mixtures
These gas mixtures will not introduce water into your system and 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 to 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 hydrogen gas mixtures. 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) for 20 W operation and 0.025 Torr (3.3 Pa) for 50 W operation.
Buyers of new CD SEMs and FIBs should specify that their tools be cleaned by the EVACTRON® system at the factory before shipping.
Energy of Radicals and Ions from the Evactron E50 Plasma Radical Source
The Evactron Plasma Radical Source (PRS) is not designed as an ion source but is an ionized neutral radical and metastable source for chemically reactive species. After leaving the plasma sheath, ions are quickly thermalized and neutralized as they enter the flowing afterglow that carries the neutral reactive species for cleaning.
Evactron® plasma cleaners are designed to use any oxygen-containing gas including air, O2, and Oxygen inert gas mixtures and can be adapted to other gases. The Evactron E50 RF power can be adjusted between 35W and 50W to change the quantity of radicals. The mean ion energy in the RF plasma varies by applied power, position, and pressure and has not been studied by us. The cleaning species used are neutral O radicals in 3Ʃu state and are carried in a flowing afterglow out of the plasma into the chamber. In air the afterglow is dominated by Nitrogen radicals. The radicals in the afterglow are thermalized and do not carry the kinetic energy of the plasma ions.
The plasma ion energies are quickly thermalized and neutralized when they pass through the plasma sheath or hit the walls as they emerge from the PRS. The plasma sheath outer edge is usually found less than 10 cm from the PRS exit. The energies of the metastable radicals are quantum states and cannot be tuned. Only the quantity of radicals is changed by adjusting RF power levels.
Information on the energy of the O radicals and metastables may be found in:
U.H. KURZWEG, A. M. BASS, AND H. P. BROIDA, Spectra of Afterglows and Discharges from Nitrogen-Oxygen Mixtures, JOURNAL OF MOLECULAR SPECTROSCOPY 1, 184-200 (1957)
It is not recommended to try to use the Evactron Plasma itself to do immersion cleaning with the plasma ions. These ions have kinetic energy from the RF fields and can do sputter etch that is more aggressive. The flowing afterglow does gentle chemical etch cleaning with neutral radicals.
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