If you try to start an ion pump when the vacuum in the chamber is in the mid 10-4 range, the gas load will be high enough to produce a visible ion plasma. Normally you don’t start the ion pumps until the vacuum is pulled down to the low 10-5 range by the turbo pump. But, sometimes you want to deliberately generate an ion plasma to help clean the ion pump elements.
There are two ways to do this.
- Just start the ion pumps when the vacuum reaches the mid to low 10-4 Torr range. You may see that the pressure in the chamber rises to the 10-3 Torr range when the ion pump high voltage is turned on. That is OK; keep the ion pumps on while pumping the chamber with the turbo pump. You can leave them on for 5 minutes or so, then shut off the ion pump supply and let them cool down for 5 minutes. Then repeat the process. After a number of cycles, vacuum will be in the low 10-5 range and the ion pumps will start. You know when the ion pumps start because the vacuum goes into the 10-6 range and keeps improving slowly. By forcing the ion pumps to start in the high 10-4 range the resultant ion plasma helps to clean the ion pump elements.
- If the pumps are loaded with argon or contaminated with hydrocarbons, you want to use oxygen to produce the ion plasma because oxygen will react with the contaminants. Assuming the ion pumps are started, back fill oxygen into the vacuum chamber to 5 X 10-5 Torr. Turn off the ion gauge and monitor the current on the ion pump control. Increase the oxygen until you get about 50mA of current on the ion pump control. Adjust the oxygen leak valve as needed to maintain 50mA or so of current. Maintain this condition for about 30 minutes, and then turn the oxygen off. As the pumps cool down the vacuum will recover and typically by the next day the ion pumps are happy once again.
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