Gas Injection Solutions for Oil and Water Contamination Problems

Contamination issues during operation can be solved or prevented through special gas purging of the system and/or pumping lines, or avoided entirely. Contamination is a persistent trap that catches those applying vacuum technology. If you define contamination as something that can interfere with or is responsible for interfering with an ongoing process or activity, you can easily understand that the prevention of contamination and contaminants is a whole. Nearly every process or vacuum system has its own specific contamination issues that must be considered when attempting to understand or troubleshoot problems, and only a few affect many of the processes. For example, an excess helium (He) background will have no real effect within a thermal evaporation system due to its inert nature, but He can be a major problem in a leak detector, as it can cause sensitivity and/or response time issues. A contaminant is anything that interferes with a vacuum process or the internal equipment being used. This means there are so many possible contamination sources that generalization over sources and solutions is impossible, but there are certain specific issues relating to particular pumping systems and commonly accepted contaminants to be focused upon.

Momentum Transfer Pumps

Generally, high vacuum pumps can be separated into trapping and momentum transfer pumps. For example, trapping pumps such as ion sputtering, gettering, and cryogenic pumps pump gas into a "trash can" where it is temporarily or permanently held. Momentum transfer pumps, on the other hand, pump gas by allowing it to enter the pump inlet, compress it through a mechanical effect, and transfer it to another volume at a pressure lower than atmospheric pressure under high pressure. This group includes diffusion, molecular drag, turbomolecular, and turbo/drag pumps. In each case, they require a backing pump to reduce their inlet pressure to a low enough level to allow operation of the high vacuum pump. At this point, the backing pump changes its role to ensure that sufficiently low pressure is delivered to the high vacuum pump exhaust for compression to be effective. After the pressure is reduced from atmospheric to the specified inlet pressure, the backing pump's role becomes "rough pumping" and while providing low exhaust pressure, its role is "backing." The degree of compression a pump can produce depends on the ratio between inlet and exhaust pressure and the molecular weight of the gas being pumped. The line connecting the exhaust port to the backing pump inlet is called the vacuum line. The limiting vacuum line pressure subsequently becomes the vacuum line tolerance, and this limit varies considerably between pump type and individual design, so a fixed general specification is not possible. During this pumping process, a series of potential contamination scenarios arise, and each must be evaluated separately.

Water Vapor

Water vapor is a major problem for many vacuum processes because it is slowly outgassed from the internal surfaces of all vacuum chambers. Since pumping is conducted at pressures below 10-3 torr, it becomes a significant residual gas, and as this transition enters the drying zone, it is properly identified. In the drying zone, the momentum transfer pump must either completely pump out the outgassed water or continuously exhaust it through pumping to provide a very low partial pressure. This means the entire pumping line from the inlet to the backing pump exhaust is continuously exposed to water vapor. If, as is generally the case, the chamber is being roughly pumped with a high vacuum pump, then all the water vapor in the form of moisture in the chamber air will move across the entire pumping line. Consider that at room temperature, one liter of air at 50% relative humidity contains roughly 20 torr liters of water vapor. The internal surfaces of a member of the turbo family can be larger, and these surfaces will outgas water vapor as it passes through the pump. Additionally, condensation will occur as a result of compressing wet air in the rough pumping/backing pump, and some of this condensing vapor will re-enter the vacuum line as the pump reaches lower pressure and higher temperatures. At this stage, the pumping system will have difficulty pumping more water vapor than the chamber, because the compression process will become less efficient due to further accumulation of water vapor in the vacuum line. At this point, rather than struggling with gas laws, we can turn them to our advantage. Oil-free mechanical pumps and rotary vane pumps tend to have higher pressures that reach the molecular flow regime. By definition, molecular flow occurs when the mean free path or distance between molecule-to-molecule collisions is greater than the diameter of the chamber or pumping line. Since there is no other molecule subsequently affecting water molecules, they can move freely back from the momentum transfer pump, and water exiting the backing pump is not pumped out.

Oil Vapor

When an oil-free mechanical pump is used for rough pumping/backing, oil vapor is likely to come from behind the pump. As the pump oil heats due to mechanical agitation, its vapor pressure increases sufficiently to allow it to enter the vacuum line. If, as is usually the case, the inlet pressure is within the molecular flow regime during extended operation, there will be no collision from the pumped molecule to the oil molecule to stop oil vapor from entering the momentum transfer pump where it will pass through the pump and enter the process chamber. There are various traps for backstreaming oil vapor that can help solve this problem, but they must be used and maintained very carefully. The same gas flow solution already defined for water vapor contamination will also solve this problem. When the vacuum line operates at viscous flow levels, the molecules of the flowing gas will increase the collision rate and stop backflow. This technique is also quite useful when the backstreaming traps are being regenerated. Oil molecules trapped on cold surfaces or within the pores of a molecular sieve will escape during regeneration; either as the cryogenic traps are heated or as gas escapes from the molecular sieve traps. When the trap and vacuum line are purged with dry gas at viscous flow pressures, oil vapor seepage from the trap will be prevented.

Light Gases

All momentum transfer pumps have low compression ratios for helium (He) and hydrogen (H2), and as a result, compressed light gases in the vacuum line flow back over the pump or slow down the pump speed. The result of all this can be a relatively high partial pressure of light gas in the chamber. When a heavier dry gas flows into the vacuum line, the lighter gas will be sent to the backing pump.

Diameter Method

When the vacuum line tolerance pressure is not high enough to allow the vacuum line pressure to expand toward the viscous flow regime, you can try to solve the problem by changing the diameter of the vacuum line. Remember that flow regime is a function of both pressure and diameter, so it is possible to increase the diameter of the vacuum line to a point where viscous flow conditions can be met without raising the vacuum line pressure above the momentum transfer pump's vacuum line tolerance level.

Conclusion

The disadvantages of contamination problems that arise in commonly used vacuum systems can be eliminated by careful application of gas flow to the vacuum line of systems with momentum transfer pumps. Pressure regulation can be manual or can be accomplished with the aid of gas flow controllers. When thermal conductivity measurement instruments such as heat gauges or Pirani gauges are used in the vacuum line, ensure that you apply sensitivity correction for the gas in question; this is particularly true for argon. Ali Cihan Şengül / Industrial Engineer - Sales Representative - Gücüm Pompa Makine Sanayi ve Ticaret A.Ş.