Application reports
No. 25 – The relation of a pressure decay measurement and a Helium lea
The relation of a pressure decay measurement and a Helium leak rate
Question: I am working in semiconductor production. After a tool maintenance and prior to process release we perform a pressure decay leak test. Can I relate the achieved pressure decay value with a Helium leak rate somehow ?
Answer: Yes – if you know the volume of the vacuum system you can convert the pressure decay reading into a helium leak rate and compare directly with the signal of a helium leak detector.
Additional information: Let’s take an example: in a vacuum system a pressure rise of 10 mTorr is measured after a time of 5 minutes. In order to estimate the corresponding helium leak rate you have to convert the two signals into the same unit.
Helium leak rates commonly are measured in „millibar * litre per second“ (mbar l/s or mbar l s1). In this unit there are a pressure in mbar, a volume in litre and a time in seconds. Prior to process release the pressure decay is measured in mTorr over time. If we compare this to mbar l/s we also have a pressure and a time but we lack a volume. We have to consider the volume which is available for the leak flow which enters the vacuum system via the leak. This is the chamber volume of your vacuum system. In addition we have to convert the pressure unit mTorr into mbar. We can do this with unit conversion tables (see tip # 21) or unit conversion tools in the world wide web.
The pressure decay of 10 mTorr corresponds to 0.013 or 1.3 x 10-2 mbar. Five minutes are 300 seconds. We assume a chamber volume of 5 litres. Now we can enter the data into the formula. Q is the abbreviation for leak rates and gas flows. Our pressure decay corresponds to a leak rate of 2.2 x 10-4 mbar l/s. If the leak detector is the only pump on the system you can directly compare the values with each other. If the leak detector is operated in parallel with a process pump one has to determine the respective flows to pump and leak detector. This is measured with a calibration leak which is connected to the chamber.
Ultimate precision during leak location during service is not very important since chamber volume is an estimate only. So we neglect the different gas behaviours during leak (air) and test (helium). We also neglect a correction of potential changes of the pressure gradient over the leak channel during leak test and real process conditions. In most cases this is OK for leak location but for a proper quantification one would have to make the corrections.
Please note: in our example we have chosen a small chamber and a short period for pressure decay measurement.Please find below a table which summarises some data from real life:
