The Evactron E50/E50 E-TC/E16, ES and U50 models are new vacuum accessories for removing trace hydrocarbons from SEMs, FIBs and HV and UHV vacuum chambers. XEI’s plasma R & D team has designed these systems with sixteen to fifty watt cleaning power, providing fast, reliable high-performance cleaning. The critical RF generator and impedance matches are upgraded versions of units used in XEI’s fully featured Evactron De-Contaminators. Flowing afterglows give high oxygen radical concentrations for fast cleaning due to longer mean free paths in the 2-20 milliTorr region. A fixed input gas flow rate removes the need for pressure gauges and variable gas flow valves. They are affordable, easy to use, and require no adjustment to clean in any vacuum system equipped with a turbomolecular pump. No special gases are needed for operation as they use room air (alternate gas models are available). Simply install the Evactron on a vacuum chamber, apply vacuum and switch on. “They just work.”

The new Evactron E50/E50E-TC/E16 and U50 models use up to fifty watts to generate flowing afterglow cleaning with air, removing carbon compounds from vacuum chambers operating with turbomolecular pumps. The unique new system has instant ignition from any vacuum level. Compact and powerful, it uses a 50-watt external RF hollow cathode excitation to produce plasma for downstream cleaning. Most commercially available remote plasma sources have been designed for semiconductor equipment and thus are usually derated for the plasma cleaning of other vacuum systems. Evactron systems are specifically designed to clean electron microscopes safely. This high-rate process cleans in minutes, removing carbon acquired during exposure to ambient air, system assembly, poor vacuum practice, etc. The removal of carbon speeds pump down time, improves instrument performance and stops carbon deposition by charged particle beams on optics and other sensitive surfaces.

The Evactron ES was designed to be integrated with new Tescan FIB/SEMs that are commanded by the SEM computer so that it can only be operated when the high voltage is off to the electron gun and detectors, and the gun valve is closed. Oxygen cleaning plasma can destroy the electron gun by poisoning the emitter. Evactron models have always provided protection of electron guns through the SEM interlock system.

XEI Scientific research shows that high cleaning rates are achieved by the Evactron E50/E50E-TC/E16/U50 and ES models by a combination of factors. These are:

• Remote hollow cathode plasma source
• Low air flow between 15 and 25 sccm into the plasma
• Pumping with a turbo molecular pump to pressures below 30 mTorr (4 Pa)
• UV light from flowing afterglow
• RF power level
• Distance from the plasma source

The Evactron remote hollow cathode plasma source gives the plasma electrons a more mono-energetic energy profile than that of ICP-type sources where the electron energy spread is high because of multiple collisions in the plasma. This is important because ICP plasma uses most of the power applied to create heat, thus a remote hollow cathode will be more efficient in producing oxygen radicals than ICP plasma. Keeping the power under fifty watts with a hollow cathode also avoids the ionization of nitrogen. Nitrogen ions are very reactive with oxygen radicals and destroy them in two-body collisions, lowering the cleaning rate.

The flow rate of air into the plasma is important both to maintain the quantity of Oxygen molecules to be excited and to lower the pressure in the chamber into the molecular flow range allowing the operation of turbomolecular pumps at full speed while plasma cleaning. XEI research has shown that the ideal flow rate is 20sccm +/- 5 sccm. This flow rate maintains plasma ignition in all Evactron PRS (plasma radical sources). Depending on turbomolecular pump speed and size, the pressure in the PRS will be between about 15 mTorr and 30mTorr (2Pa -4 Pa) and in the chamber near the turbo pump 1 mTorr to 22 mTorr. These pressures and these flow rates are tolerated by most turbomolecular pumps with active cooling (air or water) without overheating.

Vacuum pressure: For high cleaning rates it is also important to achieve the longer free paths found in molecular flow conditions. Two body collisions with diatomic species do not destroy the oxygen radicals used in the cleaning process since the excess energy cannot be lost by allowed quantum transitions. Three body collisions allow excess energy to be lost as kinetic energy and thus remove oxygen excited species from the vacuum. The cleaning rate quickly drops with distance from the plasma source as the pressure rises. XEI experiments show that in chambers less than 10 liters in volume that cleaning reaction rates rise rapidly until the pressure is below 30 mTorr (4Pa). Below this pressure the cleaning rates vary more by chamber shape, location and distance from the plasma source than by pressure. There tends to be a plateau between 10 and 30 mTorr where the cleaning rate is more a function of input gas flow than pressure. Pressure does affect the distribution of the active cleaning species coming from the plasma source. The cleaning species move by flow into the chamber. At higher pressures there is more scatter and cleaning is better on surfaces that are not in the line of sight of the plasma. At lower pressures there are longer free paths and cleaning is concentrated on surfaces in the line of sight. Pressures below 1 mTorr cause spot cleaning in the chamber.

Figure 1. Simple 2D Monte Carlo Model showing mean free path λ (red) and 10λ (black)

Distance from Plasma: Cleaning rates strongly decrease with distance from the plasma and increase with declining pressure. Location of measurement and the geometry of the chamber also have strong influence on the result. XEI research is doing direct measurements and theoretical model calculations to fully understand these functions in the cleaning space.

Figure 2. Etch rates versus distance and pressures measured with ES PRS at 20 watts

Pop ignition is a proprietary method (patent pending) that allows the RF power and air flow to be turned on in such a way that the plasma ignites every time. Pop ignition works even when the main chamber is initially at very high vacuum or below and very large turbo molecular pumps are used. It does require a vacuum under 1 Torr or 133 Pa to ignite.

Low pressure mode cleaning should be limited to under 5 minutes and never more than 10 minutes to prevent oxidation of lubricants and other materials. XEI tests have shown that at 20 cm from the plasma sheath cleaning rates are 10-60 times faster than Classic Evactron cleaning at low vacuum (200-600 mTorr).

With an oil filled roughing pump the flow rate of 20 sccm of the EP and ES will perform classic mode Evactron cleaning at pressures between 200 and 600 mTorr.  While slower than “flowing afterglow cleaning” it is adequate for small chambers such as the Softclean™ chamber or others of less than 4 liters of volume.

The EP uses a KF 40 (NW40) clamp type fitting as standard. A CF 2.75 copper gasket fitting is an option. The ES is fitted with an SEM to Evactron adapter flange to be mounted on an SEM port as chosen by the SEM manufacturer.

Only the Evactron U50 is bakeable to 150°. All other models are not bakeable.

The new Evactron E50 E-TC and U50 configured for alternate gases generate oxidative or reductive plasma for hydrocarbon decontamination and modification of surface functional groups. The external hollow cathode plasma radical source (PRS) provides fast and reliable plasma and UV cleaning without sputter etch damage or debris. The compact PRS makes the Evactron E50 E-TC and U50 versatile solutions for FIB/SEM chambers, load locks, or sample prep chambers. Create optimal bonding conditions and hydrophilic sample surfaces, regenerate tungsten probe tips, and optimize imaging and analysis with hydrocarbon-free specimens.

The Evactron U50 has no o-ring seals in the plasma radical source.  All seals are metal-to-metal and we use NASA-rated epoxy methods, allowing no outgassing and maintaining UHV-rated vacuum levels. The plasma radical source is bakeable to 150°C and a range of alternate gases may be used to generate oxidative or reductive plasmas.