Different methods are available for locating vacuum chamber leaks, based on the vacuum level of the leak. Leaks can be classified into three types—gross leaks, high vacuum leaks, and mid-vacuum leaks.
Handling Gross leaks
During gross leaks, the chamber will just pump down approximately between the atmosphere and the low 10−2 Torr range. In this type of leak, the vacuum is not sufficient for an ion gauge to switch on.
The primary thing that technicians have to do with any leak is to ensure that all the flanges users worked on have been tightened properly and look uniform. If a flange has a bigger gasket gap on one side than the other, then that flange may not be seated correctly.
Then, users will have to ensure that the pump (a turbo pump in general) can reach vacuum when pumping is not taking place in the vacuum chamber. While checking the vacuum on the turbo pump or rough pump with a TC gauge, a little methanol or isopropanol should be sprayed on suspected flanges while keeping an eye on the vacuum gauge.
The vacuum will usually decrease (pressure will rise) when the methanol or isopropanol makes its way into the leak. Then, venting should be carried out, and the suspected gasket must be changed.
The other beneficial method for gross leaks is to use nitrogen to pressurize the system slightly with nitrogen and then spraying the suspected flanges with Snoop (or soapy water). For the majority of vacuum chambers, users do not have to pressurize them beyond 3 PSI.
The reason that users should not over-pressurize a vacuum chamber is that viewports are somewhat concave and built to hold vacuum, but not engineered to hold pressure. If a vacuum chamber is over-pressurized, it can result in blowing out of the viewports—this is not good and is a safety risk. When users locate the leak, they can see bubbles forming.
Handling Mid-Vacuum Leaks
In the case of mid-vacuum leaks, the ion gauge will work and the vacuum will be anywhere between the low 10−3 Torr and the high 10−8 Torr range. For such vacuum leaks, the ion gauge must be tracked while spraying a little methanol or isopropanol onto the suspected flanges. In this vacuum range, when the methanol or isopropanol finds its way into the leak, the vacuum may enhance or degrade, but users will see a sure change in the vacuum.
Occasionally, the methanol or isopropanol will plug up the leak temporarily and the vacuum will increase remarkably. Users can use a heat gun to evaporate the methanol or isopropanol on the flange where the vacuum change was noticed. Once the flange cools down, the process has to be repeated to establish the location of the leak.
Handling High Vacuum Leaks
During high vacuum leaks in the low 10−8 Torr to the low 10−9 Torr range, spraying the flange with methanol or isopropanol will generally not help. In those circumstances, users may have to use a residual gas analyzer (RGA) to locate the leak.
To use an RGA, users would have to vent the chamber and set up an RGA on one of the flanges. They then have to pump back down and perhaps even bake the chamber out based on the level of the leak. For instance, if the leak is in the low 10−9 Torr, then users will have to bake out the chamber so as to get down that low.
Once the base vacuum is reached, users can set up the RGA in the leak check mode or simply scan over the 4 AMU helium peak. Then, using helium that is connected via a regulator to a small tube, a small amount of helium is bled over the suspected flanges and feed-throughs while tracking the helium peak on the RGA.
Helium is a tiny molecule and hence can enter the leak. Then, it can be detected by the RGA. For ideal results, it is recommended to close off the pumps (or switch off the ion pump) while checking leaks by using helium.
It is important to keep an eye on the vacuum and ensure that the chamber is pumped out regularly to maintain the vacuum at least in the 10−6 Torr or better. When users locate the leak with helium, an instant and intense change in the RGA scan can be seen. It will be easy to establish the leak location by pumping out the helium and then doing the experiment again.
Large industrial vacuum chambers come with portable leak checkers so that the chamber does not have to be vented to set up the RGA.
In the case of high vacuum leaks, if users are not able to locate a leak, there may not be a leak. Occasionally, what seems to be a leak is really just plenty of water vapor, a virtual leak, or maybe hydrocarbon contamination. In these scenarios, a prolonged bake out—for 24 or 36 hours at 150 °C to 200 °C—should solve the issue.
This information has been sourced, reviewed and adapted from materials provided by RBD Instruments, Inc.
For more information on this source, please visit RBD Instruments, Inc.