Abstract: Ultrasensitive leak detection
Authors: Winkelman Claude R. , Davidson H. G. 
Published: July 2002

When it becomes necessary to perform leak detection in the ultrahigh sensitivity decades, leak rates of 1.0×10−12 std cc/sec or smaller, the major requirements are a qualifiable standard for system calibration and equipment that has satisfactory sensitivity.

There are no commercially available calibrated leaks with flowrates smaller than 10−10 std cc/sec. This is primarily because of the techniques and equipment used to qualify these standards. Usually, the accumulation method is used in conjunction with an analytical mass spectrometer. This is certainly one of the more accurate methods. 1 The primary disadvantage is the time requirement. For example, assuming normal sensitivities, approximately 2 h are required to accumulate enough gas to detect a 10−9 std cc/sec leak, and much longer periods are necessary as the flowrates decrease.

Some of the other factors to be considered and on which the ultimate system sensitivity is also dependent, is the choice of a calibration gas. Ideally, the base or parent peak should be as free of interference as possible. One example that has recently been reported is the use of freon 23 (CHF3) with a base peak at M/E 51 and a 1.1% 13C fraction at M/E 52.

An additional consideration in the choice of a calibration gas is molecular size. In the ultrasensitive regions, leak sources of only a few Angstroms in size will be encountered. 2 There is sufficient basis to indicate that in order for the molecule to have free ’’passage,’’ the hole must be several times larger than the molecule, 3 approximately ten times for water and between 3 and 5 times for N2, Ar, CHF3 and H2.

In our original efforts to construct a qualifiable standard, we concentrated on the pore−type leak and produced several leaks in the range of interest (10−13 std cc/sec). All these leaks were of similar construction; a knife edge was forced into a soft metal and maintained under stress. After a few months’ use of these original leaks, we realized that there were satisfactory commercial components available and a second generation leak was constructed from commercial parts; the knife is forced into a gold seat. Leaks of this type must be constructed with a protective vacuum or they will clog within a few hours due to atmospheric contamination.

At these very low flowrates, stability of both the leak and the detector are important. Temperature sensitivity of the leak, due to the stainless steel and gold components, was considered, and a heating package was designed to maintain the leak assembly at 100°C; however, we have found it to be unnecessary. For detection of extremely small leaks, a dynamic transfer technique is normally used with a residual gas analyzer measuring the appropriate partial pressures. The stability of the RGA is, therefore, important. In quadrupole type RGA’s, changes of a few microvolts in the voltages on the rods can cause errors in the ppm range. Sensitivity is the most important characteristic of the RGA in this application and, in determining specifications, actual measurements should be made rather than depending on a calculated sensitivity. Currently, digital techniques offer the greatest sensitivity. Since any good quadrupole RGA will have at least unit resolution, instrument resolution is not normally a problem.

From digital RGA measurements using CHF3 as the calibration gas and a standard leak rate of 1.3×10−13 std cc/sec, we have determined the system sensitivity. Figure 1 shows RGA measurements of the M/E 51 peak with the leak off and on. Minimum detectable leak rates were typically 1×10−15 std cc/sec.