Many manufacturing environments require that gaseous nitrogen, argon, helium, hydrogen, methane, ethylene, and halo-carbons be kept free of contaminants. Trace levels of contaminants such as oxygen, moisture, carbon dioxide and nitrogen can wreak havoc on process yields and product quality.

The first challenge for users is determining the required gas purity levels and actually getting gas having the correct impurity levels from the gas supplier. The second challenge, one that is often overlooked, is taking gas having known impurity specifications and transporting, storing, consuming, processing or using it without introducing contaminants along the way.

Gas delivery systems, the network of piping, valves, and fittings used to deliver a gas from the point of origin to the point of use, can be very reliable in maintaining gas purity. Their components, however, do wear out or break down. Human error is always a possibility; an operator who accidentally leaves a piping connection loose can cause an atmospheric leak resulting in the contamination of large quantities of gas.

Numerous components can provide a path for atmospheric contamination. The sources of atmospheric leaks can include bad valve seats, worn compression fittings, or diaphragm seals or capped service taps that are accidentally left loose.

Contamination caused by atmospheric leaks often goes unnoticed until the damage has been done. Unfortunately, in-process quality checks of product—not the gas—are the most common method for determining that contaminants have entered a gas. When detected in this manner, it is often too late … the damage has been done.

 Helium Leak Testing

The classic approach to checking the integrity of a gas delivery system is helium leak testing. There are two methods:

• The outboard leak integrity check requires helium to be flooded into the piping branch to be checked. A high sensitivity mass spectrometer is used to “sniff” around every joint, valve, and fitting. If the mass spectrometer detects helium, then that joint, valve, or fitting may be leaking.
• The inboard leak integrity check requires the use of a mass spectrometer fitted on a piping branch that is under vacuum. Helium must be flooded around the outside of every joint, valve and fitting. If helium is detected, then the source of leakage may be pinpointed and addressed.

Both methods are time consuming, labor intensive, and cannot be done on a continuous basis. In order to helium leak check a gas delivery system, the piping branch has to be isolated, and that means downtime. The entire piping branch must be shut down, and each joint, valve, and fitting must be checked. Also, after each branch is tested, it must be well purged to ensure that all contaminants have been evacuated.

Oxygen Analyzers

An oxygen analyzer, a tool that users of high purity gases often overlook, can analyze a gas on a continuous basis to detect for atmospheric leaks. When an oxygen analyzer is used, piping branches do not have to be shut down to be leak tested. Continuously monitoring a gas carried through a piping network for oxygen content ensures that a breach in the gas delivery system is detected before poor quality gas contaminates the process environment. Monitoring for oxygen is advantageous even when a manufacturing environment is insensitive to oxygen contamination, but may be sensitive to other atmospheric contaminants, because an increase in oxygen concentration is indicative of an increase in other atmospheric contaminants.

Oxygen is an ideal species to monitor in high purity gases because:

• Oxygen is often present at low trace levels in high purity gases.
• Oxygen is readily available in the environment surrounding a leaking joint, fitting, or valve at 20.9% concentration.
• Oxygen is not “clingy” like moisture. It will find the leak path and readily diffuse through it much faster than moisture would.
• Once oxygen is inside a pipe, it will travel with the prevailing flow or diffuse rapidly to a sampling location where an analyzer can detect its presence.

The main benefit of using an oxygen analyzer for leak detection is that it can be used without interrupting the flow of gas through a piping network. This provides users with the means to perform non-invasive leak detection.

The goal of non-invasive leak detection is to be proactive and prevent contaminated gas from causing problems in the manufacturing process. The technique requires the use of a data acquisition system that can record oxygen concentration data points at 1-minute intervals.

The oxygen concentration levels within the gas delivery system must be trended over time. The historical data must then be analyzed and “warning oxygen concentration” alarm set points determined. The “warning oxygen concentration” alarm set point must be below the gas specification limits, but above the typical oxygen concentration levels in the gas delivery system. If and when the oxygen concentration exceeds the “warning oxygen concentration” alarm set point, further investigation to assess potential sources of contamination must take place. If the oxygen concentration ever exceeds the gases’ specification limits, indicative of a breach in the system, additional measures must be taken immediately.

When a leak exists, oxygen from the air surrounding the leak will mix with the gas from within the piping network via diffusion. If the flow rate/pressure of the gas is dropped, the oxygen concentration within the gas increases. By lowering the flow rate past the leak, the concentration of contaminants introduced into the gas will increase, causing the oxygen concentration within the gas to increase. This is what allows the oxygen analyzer to be used as a leak detector.

Gas Leak Detection in the Semiconductor Industry

The oxygen analyzer selected for noninvasive leak detection needs to have detection capability two to five times better than the gases’ oxygen impurity specification. The semiconductor industry happens to be one of the more demanding industries in terms of purity requirements. Gases used in that industry are typically required to have less than 10 ppb oxygen and sometimes less than 1 ppb oxygen.

Non-depleting electrochemical analyzers have the lowest detection limits of any commercial oxygen analysis technology and are available in ranges from 0-10 ppm all the way up to 0-25%. The oxygen analyzer used for leak detection must be relatively insensitive to flow rate changes and maintain its accuracy when sample gas is flowed through the sensor between 0.5 slpm and 2 slpm. To check for leaks, the flow rate must be dropped from 2 slpm by reducing the pressure as close to the source as possible so that the flow rate through the analyzer drops down to 0.5 slpm. If the reading from the analyzer increases when the flow rate is dropped, then a leak is present. If the oxygen concentration remains relatively unchanged, then the system may be deemed leak-free.

 Leak Mechanisms

There are two types of leak mechanisms: actual leaks and virtual leaks. Both will act differently under the leak detection procedures detailed above.

• An actual leak is a direct leak path from atmosphere on a flowing gas line. An actual leak has a fixed rate of atmospheric contaminant diffusing into a gas contained within a pipe, across a valve, fitting, or other system component.
• A virtual leak, on the other hand, is a trapped pocket of air or contaminated gas. This pocket is usually in a dead leg and has high purity gas flowing past it. A virtual leak causes small amounts of contaminants to continuously flow out of the dead leg.

In Figure 1, there was no downward trend in the readings from the analyzer after it reached a value of 360 ppb. The gas was purified and should have had <1 ppb of oxygen. At approximately 6:41 a.m., the flow rate upstream of the analyzer was dropped from 2 slpm to 0.5 slpm. Immediately after this occurred, the analyzer shot upward and read ~580 ppb. A VCR fitting upstream of the analyzer was determined to be leaking and was tightened. The oxygen concentration trended down to zero immediately after the fitting was tightened.

In Figure 2, a very slight downward trend was noticed when analyzing historical data over several hours (not shown). The first data point shown in the chart is at approximately 5:15 p.m.. At 5:50 p.m. a pressure regulator upstream of the analyzer was used to reduce the flow rate through the analyzer to 0.5 slpm. The reading from the analyzer shot upward and read ~485 ppb. The flow was increased to 2.0 slpm and the readings equilibrated to around 90 ppb. The flow rate was dropped again, the readings went up, and when flow was reestablished the oxygen concentration was close to 10 ppb.

The trend depicted in Figure 2 is indicative of a virtual leak–i.e. a dead leg containing a trapped pocket of contaminated gas. By cycling the pressure within the lines, the contaminated gas in the dead leg readily mixed with the prevailing gas flowing past it.

The data show that leaks can significantly impact contaminant levels in a gas. A purified gas having <1 ppb O₂, or other atmospheric contaminants, can be transported in a piping network having a leak and see its concentration of contaminants increase to ppm levels by the time it gets to the point of use. Manufacturers who use oxygen analyzers for noninvasive leak detection can detect these contaminants before they negatively affect process yield or product quality. Those who do not may be left without a suitable analytical tool to indicate that an atmospheric leak is affecting process yield/product quality.