Backstreaming of pump oil vapors
Systems evacuated with oil-based pumps may be victims of backstreaming of pump oil vapors into the fore- line, vacuum chamber, and upstream throughout the system. The back- streaming referred to here is the act of pump oil vapors moving against the flow of molecules traveling from the vacuum chamber to the pump(s). In this case, we refer to these oil vapors as moving upstream, and eventually into the vacuum chamber. At high pressures, where the number of molecules from the chamber outnumber pump oil vapor molecules emanating from the pumping system, flow upstream is greatly mitigated. Oil vapor back- streaming is significantly more pronounced at lower operating pressures.
This backstreaming, or backmigration, is distinct from any pump oil vapors that may be pulled into the chamber during an abnormal event, such as a power failure. Improper start-up and shut down procedures can also allow oil and vapor to move toward the vacuum chamber. Issues with the exhaust of an oil-based vacuum pump, where the pressure in the exhaust is higher than recommended by the manufacturer will also compromise the flow of vapors out of the system and may promote them to move upstream.
Backstreaming can occur with any oil-based pump, including diffusion and rotary vane pumps. The type of oil used in these pumps differs in terms of function but in both cases, there is a strong possibility that oil from a rotary vane pump, when backstreamed into a diffusion pump, will be heated and reach a vapor pressure high enough to backstream unless mitigation efforts are employed. This inadvertent mixture of oil types will also cause the rate of diffusion pump oil backstreaming into the vacuum system to increase. Depending on materials used in vacuum systems, gaseous oil vapors may have sticking coefficients approaching  and as such they will condense on cooler solid surfaces and will not be returned to the pumping system. Backstreaming is a well-known phenomena and pumps are typically equipped with accessory devices which help mitigate upstream migration. Diffusion pumps can be augmented with baffles and water or cryogenically cooled traps at the entrance of the pump.
The baffle assembly is designed to provide a cooled, high surface area, structure which serves as a location for oil vapors to re-condense and return to the oil reservoir of the diffusion pump. There is also the possibility that diffusion pump oil may break down if temperature limits are exceeded and reduce to more volatile components. There is also the possibility that oil vapors from a mechanical pump backing a diffusion pump will move through the diffusion pump and into the vacuum chamber (see below). In the cooled absorptive traps described above, should cooling fail and those surfaces get hot, any trapped oil vapors will be re-released into the system .
For mechanical, oil-sealed, rotary vane vacuum pumps, backstreaming becomes critical around the cross over pressure from rough to high vacuum. Studies done on backstreaming at various pressures indicate that at high gas flow rates, from the chamber to the pump, the weight gain measured on a sample, i.e. a pipe of 30 cm in length and 2.5 cm in diameter, was small, at about 10 μg/h . But at lower gas flow rates, below 10.5 sccm, see fig. 3, the gases rushing from the chamber are not sufficient to mitigate the backstreaming of pump oil vapors. The resulting rate of weight gain from pump oil vapors moving upstream was 100 times greater or 100 μg/h.
For systems using an oil-sealed mechanical pump to back a turbomo- lecular pump the are two opportunities for backstreaming. As shown above, there is a significant opportunity for backstreaming during rough pumping. While it may be counter intuitive, pump oil vapors can also work their way through a turbomolecular pump spin- ning at 60,000 rounds per minute. In molecular flow there is no directionality to the flow of molecules. To the extent that molecules of pump oil vapors can find a clear pathway upstream through a turbo pump they can make their way into the vacuum chamber. Fig. 4 details the backstreaming of mechanical pump oil vapors through diffusion and turbomolecular pumps and shows that use of a trap (b) is nearly equivalent to the removal of oil based pumps (d).
Backstreaming can minimized by keeping the inlet pressure of the pump, or the cross-over, in viscous flow, above 0.133 hPa or 100 microns, thereby staying above the transitional flow region, as shown in Fig. 5, where 1 sccm of N2 effectively stops pump oil backstreaming. This may require some pressure controls when trying to stay in viscous flow when the ultimate pressure of the pump may be 1.33 x 10-2 hPa or less . A schematic of a vacuum system equipped with an active pressure control system is detailed in the Fig. 6.
If pressure cannot be controlled with an integrated closed loop downstream system, an upstream control system which utilizes a purge gas can also be employed if the process can tolerate a high operating pressure. The purge gas flow can be managed with a mass flow controller or manually with an appropriate valve capable of very fine resolution. The purge gas can keep the pump inlet pressure high enough to allow high numbers of purge gas/vapor collisions in order to prevent upstream oil migration.
Vacuum pump oil is also likely to 'creep' along surfaces of interconnecting pipes such that it can migrate into a vacuum chamber. This is a serious possibility in the roughing line between a chamber and a mechanical pump and is only somewhat mitigated in the high vacuum stream where a turbomolecular pump may interrupt the direct connect of a mechanical pump to a vacuum chamber. Surface migration of vacuum pump oil, which is also stimulated by vibration, can be interrupted by the insertion of a material which does not wet to vacuum pump oil, like Teflon.
Most modern rotary vane pumps are equipped with a fail-safe device called an anti-suck-back valve. This device is designed to close immediately in the event of an interruption in the normal operation of the pump, sealing off the inlet of the pump so that oil and oil vapors are not allowed to move upstream through the plumbing and into the chamber. Older, belt-driven, rotary vane pumps do not have this feature. Those vintage designs require immediate venting upon shut down in order to prevent the backflow that will occur because of the differential pressure between the pump oil case, or reservoir (nominally 1013 hPa) and the foreline.
When clean, pristine, and unencumbered by particulates from the vacuum system or solidified oil deposits, the anti-suck-back valve can be an effective protection mechanism against up reverse flows of liquid and vaporous oils. But the valve needs to be properly monitored and maintained so that it is fully functional in the event of a power failure. The valve typically closes within 0.5 seconds of a power-off event. In the event the pump is not properly vented after shutdown the rubber seal of the anti-suck-back valve will eventually degrade. It should not be used as a valve but as an emergency device. For example, a pump that is not in service should be vented to atmosphere.
An additional preventative measure to fight backstreaming is the installation of a gate valve between the pumping system and the vacuum chamber. A gate valve can effectively isolate the vacuum chamber from the pumping system so that the chamber can be vented while the pumping system remains in vacuum. Further, when venting the vacuum system, with the gate valve isolating the vacuum chamber, it is best to introduce the vent gas close to the gate valve so that the direction of flow is toward the pumps and away from the chamber. Injecting vent gas near the pumps encourages flow in the direction of the vacuum chamber - promoting backstreaming.
Tests for backstreaming
There are several methods to test a system for backstreaming. The techniques vary in terms of complexity, equipment required, and the level of detail on amounts and rates of backstreaming. A few methods are detailed in the following paragraphs.
One highly quantitative method is called the coupon method. In this method, clean coupons of optically flat materials, like polished silicon wafers, are placed around a vacuum system where backstreamed oil, liquid or vapors, are likely to condense. A series of system evacuation cycles, using the oil-based pumps, is performed at intervals of 1, 2, 3 or more hours. Coupons can be harvested and analyzed at each interval. Analysis options include the use of an ellipsometer which is sensitive to the growth of thin films, using the refractive optical index of the pump oil. A scanning electron microscope is also a useful tool for imaging the condensation of pump oil vapors. These techniques have been used successfully for vary large vacuum systems including the Space Power Facility, near Sandusky, Ohio, operated by NASA.
Using these methods and an evaluation model adapted to consider the formation of islands versus continuous films, as shown in Fig. 8, researchers were able to quantify the height of the islands of pump oil at approximately 220 angstroms after 72 hours. The coverage area is on the order of 70%. This implies a backstreaming rate (average of several runs) of 0.37 x 10-6 mg/(cm2 min). Shorter runs gave a rate of 0.1 x 10-6 mg/(cm2 min). which implies that the rate diffusion pump oil vapor escaping the pumping system increases over time. Coupons placed near to the inlet of the diffusion pump showed higher accumulation than those placed at a distance from the pump, for example deposits of 150 angstroms near pump inlets versus 34 angstroms at locations distant from pumps. With this knowledge an effective mitigation system can be developed.
The mass gain, or weight gain, method, can also be used to measure pump oil backstreaming. This technique can be used to look at weight change for an entire chamber or the change in mass of a coupon. As mentioned above, pump oil backstreaming has been reported at rates on the ordering 10 cm x 10 cm x 0.3 cm, has a surface area 212 cm2 and a volume of 30 cm3 and a weight of 236.1 grams. After 100 hours of pumping, the sample will have gained: 212 cm2 x 6,000 minutes x (0.37 x 10-6 mg/ (cm2 min) or 0.000471 grams.
That weight gain is on the order of 0.0002 % of the sample weight. A highly accurate weight scale will be required to make this measurement, and even with the best available, this approach may be prohibitive.
Perhaps a more accessible approach is to look at how water behaves with the surface of a sample. The wettability method looks at the behavior of water on the surface of metal to determine if the sample is cleaned or contaminated. While this will not provide a quantitative measure, like the two approaches discussed previously, it does serve as a go/no-go test for backstreaming of vacuum pump oils.
When totally clean, metal surfaces, including steel, aluminum and brass, are hydrophilic, meaning 'water loving' such that water will spread out across the surface. Water will sheet off the clean steel, as shown in the image on the left of fig. 9. Water will tend to bead up on a contaminated, hydrophobic, or 'water fearing' sample. Fig. 10, below, provides detail of the interaction of a water droplet with contaminated and clean surfaces. For our purposes, hydrophilic surfaces are considered clean, and hydrophobic surfaces are considered contaminated.
Mitigation of backstreaming can be achieved by:
- Inserting low conductance pathways to provide resistance to the counterflow of pump oil vapors, using long, narrow (small inside diameter) forelines (plumbing between turbomolecular pumps and oil sealed backing pumps). The insertion of a planned low conductance section will provide a tortuous pathway to the back-flow of gases and vapors. A low conductance section creates other issues - so it must be balanced with pumping speed so that the performance metrics such as ultimate pressure and time-to-pressure are maintained.
- The use of traps between oil sealed pumps and vacuum systems, either adsorbent, like the diffusion pump baffle featured in Fig. 2, or absorptive, room temperature or cryogenic, using physiosorptive or chemiosorbtive mechanisms to stop the upstream flow of oil vapors.
- Use inserts of materials, like Teflon, that do not wet to pump oil and will interrupt pathways for oil creep or migration.
- The use of dry pumps between chambers and wet pumps, such as Roots blowers.
- Use of specialized pump oils like Fomblin have demonstrated a 30 % reduction in backstreaming (Fomblin tests to reduce backstreaming by about 30%, 15 micrograms per hour to 10 micrograms per hour .
- Use dry pumps in place of wet pumps, like dry scrolls. But dry scrolls may also need a particulate filter to handle possible contamination from the degradation of tip seals.
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