An average of 40% of the MFC's received per month are seen with contamination ranging from moderate to severe. The gas types vary from reactive (such as Silane - SiH4, and Phosphine - PH3) to corrosive (Chlorine - Cl2, and Boron Trichloride - BCl3). Reactive gases are not typically considered to cause damage to the MFC itself, or to the associated plumbing. They can, however, leave crystallized or powdered gas residue which leads to MFC failure. Corrosive gases, over a period of time, have been known to eat away at the metal surfaces which eventually leads to in-operability or failure. When oxygen, water vapor, or other gases combine with highly reactive gas molecules, solid contaminants such as silicon dioxide or other residuals can create a blockage within measurement and control equipment.

Corrosive gas contamination takes a larger toll on gas systems and MFC's. Cl2 in particular (and Cl2 based gases) will corrode and pit the metal structures in both MFC and delivery system components. For obvious reasons these damaged or failed components should be repaired/replaced immediately.

Proper filtration reduces the risk of contamination, however, submicron size particles occasionally slip through. If enough of these contaminants escape filtration, the entire system can be at risk to failure. Often it is difficult to determine if there actually is a contamination problem.

Here are a few primary causes of contamination

1) IMPURE PURGE GASES

2) IMPROPER PURGING PROCEDURES

3) LEAKS

The following is a list of common symptoms associated with contamination:

• Immediately after installation the MFC functions properly. After a short period of time a notable decrease in performance occurs.
• A sticking valve is occasionally relieved by tapping on it.
• A heavy fluctuation is noticed immediately after installation.
• Valve flows wide open and has no flow reading.
• Valve will not shut down.

It is important to keep in mind that when any MFC in a reactive or corrosive environment is going to be removed from the system, a proper pump and purge procedure is performed. Before and after a reactive gas system has been exposed to air, it is necessary to purge the system with an inert gas such as N2 or Argon (with less than 10ppm O2 and water vapor). It is critical for purge gases to be dry and pure. Bottled gases are preferred, but house gases can be used after they have been properly certified.

Cycle purging alternates evacuating the gas system and pressurizing the gas system with an inert purge gas. This procedure causes absorbed molecules and trapped gases to be removed much more quickly than straight purging.

Contaminants can be trapped in plumbing dead space and micro cavities. The application of vacuum to a gas system can be very effective. While this option may not be available for an atmospheric system, some can be modified and adapted.

Inspect the MFC and associated gas lines. If there is any sign of contamination present after removal of an MFC, carefully inspect removable parts within the jungle. Make sure they are cleaned and free of all contaminants. Try to locate the source of any possible contamination before installing new MFC's.

There is zero tolerance for leaks in a pure gas system. It is commonly believed that if a gas line pressure is above that of ambient, the surrounding atmosphere will not be permitted to enter the system. This is not always the case. Any flow through an orifice or leak path is susceptible to back diffusion into the gas stream caused by the surrounding gas (such as air). The system can them become contaminated with oxygen or moisture.

AVOID ALL LEAKS AT ALL TIMES
Check all new or newly serviced systems. Periodic inspections are highly recommended to avoid catastrophic failures. Use a leak detector capable of detecting leaks in the range of less than 1X10-9 atm cc/sec of helium.

Immediate and proactive action is the best insurance for longer lasting and reliable MFC performance.