By Mike Fitzpatrick and Ken Goldstein, Ph.D.

Mike FitzpatrickClick here to enlarge image

Over the past two months, we considered the subject of leaks in high-purity gas systems. We discussed the concept of partial pressure differences as being the primary driving force that moves gas-phase contaminants (and small particles) from the low-pressure ambient into high-purity, higher-pressure systems. We also closely examined two leak-testing techniques commonly used in residential, commercial and industrial applications: bubble testing and static pressure decay testing.

Ken Goldstein, Ph.D.Click here to enlarge image

While these methods are perfectly acceptable for low to medium purity applications, they are entirely inadequate for the higher purity systems commonly used in our cleanrooms. The bubble test is quick but it cannot be used to detect very small leaks, and the static pressure decay test is not sufficiently sensitive to detect small leaks in a realistic time frame.

This brings us to the subject of helium leak testing using a mass spectrometer. The helium leak test comes in two flavors: inboard and outboard. Both approaches use helium as a tracer gas and benefit from the ease of detecting ionized helium using a mass spectrometer.

Since helium is chemically inert, it's nearly ideal for our use—there are few safety concerns and there's no possibility of chemical interaction with any of our system components. Let's look at these two test methods.

Inboard testing employs the following procedure:

1. “Bag” all potential leak sites (welds, valves, mechanical fittings, etc.) and seal the bags with tape (vinyl tape if in a clean environment);
2. Open all internal isolation valves and close off all external valves;
3. Pull a hard vacuum on the system and connect the helium mass spectrometer to one end;
4. Spray helium into each bag.

Any leak will gradually allow helium to migrate from the bag into the system and slowly work its way down to the mass spectrometer, where it will be ionized. This activates the detector alarm and provides an accurate reading of the helium concentration, thereby indicating the leak rate. (Not everyone uses polyethylene bags to physically “contain” the helium during testing. While dispensing with the bags makes the testing go faster, it also makes it less sensitive. This trade-off applies to all helium leak tests.)

This test has two major advantages: it's both sensitive and accurate. Leak rates as low as 10-10 cc/sec can be readily detected with instruments that can be purchased off-the-shelf. Unfortunately, the test has one major disadvantage. It's slow, which makes it difficult to complete the leak-checking of large systems. This is because helium tends to move slowly in a vacuum. Depending on the length of the system, its configuration, and diameter, it can be a matter of a few minutes to hours from the injection of the helium into the bag and its arrival at the mass spectrometer.

Since typical systems have hundreds of possible leak sites that must be checked, it's not feasible to inject the helium into each bag one-at-a-time and then wait to see if a leak develops, before injecting helium into the next bag. Instead, all of the bags are injected at about the same time, and we hope that there are no leaks. With most tests, this is the case; however, if there are any leaks, finding them can be slow, complex and frustrating.

Our testing has only informed us that there are leaks. It has not told us where the leak(s) are located. Finding the actual leak location(s) can be extremely time-consuming. A rational search strategy must be employed.

Outboard testing follows a similar procedure:

1. “Bag” all potential leak sites (welds, valves, mechanical fittings, etc.) and seal these with tape (vinyl tape if operating in a clean environment);
2. Open all internal isolation valves and close off all external valves;
3. Pressurize the system with helium or a helium/argon or helium/nitrogen mixture;
4. Slide an atmospheric-pressure probe into each bag to sniff for helium, thus locating the leaks and “reading” the leak rate from the instrument.

This test has two significant advantages. It is extremely fast, and the presence of helium in a given bag tells us where the leak is located, which makes leak repair relatively simple.

But this method has some significant disadvantages, too. To begin, we have flooded our system with helium that will have diffused into and saturated all plastic system components, such as valve seats. As a result, the background level of helium in the system can remain relatively high for an extended time—perhaps years. The system might be “contaminated” with helium to the extent that we may never be able to perform an inboard test on it.

For bulk gas systems, this may not be a significant problem, but for cylinder (specialty) gas systems, it can present problems. So, we invariably use the inboard helium leak test on these systems.

Another problem with the outboard test is that the mass spectrometer works best when operating under hard vacuum conditions. Its accuracy and sensitivity are greatly reduced when using the atmospheric pressure “sniffer” probe. The outboard test is several orders of magnitude less sensitive than the inboard.

The outboard test also has difficulty in accurately determining the concentration of helium in the bag. This concentration is a function of several variables: the leak rate, the volume of the bag, how long the helium has been leaking into the bag and how tightly the bag is sealed.

All in all, the outboard helium leak test is both less sensitive and less accurate than the inboard test, although it is orders of magnitude better than the bubble or static pressure decay tests for high-purity systems.

You might ask whether a given leak can behave differently in terms of our ability to detect it under vacuum, as opposed to under pressurized conditions. The short answer: it depends. Pinhole leaks due to improper welding technique, while extremely rare, should be readily detectable in either direction if using helium mass spectrometry.

But improperly tightened mechanical fittings may indeed leak in one direction but not the other. Fortunately, this is rare and the inboard test will pick up most of these.

Determining which test to use for a particular system often requires balancing time and money with the need for sensitivity and accuracy. Specialty gas systems almost invariably are subjected to the inboard test. Some owners will require the inboard test for all systems and others will compromise, using the inboard test for specialty gas systems and the outboard for bulk gas systems.

As with most things, there are pros and cons to each approach.

Michael A. Fitzpatrick has participated in the design and construction of semiconductor facilities for over 24 years and is a Senior Member of the Institute of Environmental Sciences and Technology (IEST). Mike can be reached at [email protected] KEN GOLDSTEIN is principal of Cleanroom Consultants Inc. in Phoenix, Ariz., and is a member of the CleanRooms Editorial Advisory Board. He can be reached at [email protected]