Dear All,
Presently, I am doing the 'bake-out' process of our ultra high vacuum (UHV) system. It consists of turbo pump (backed by a rotary pump), ion pump and titanium sublimation pump (TSP). Our system was in ambient condition (in air) for a very long period (more than six months).
With aim to get back UHV, we have started 'bake-out' of the whole system. During bake-out process at 110 deg C for 12 hours, UHV chamber has been pumped out by turbo pump (ion pump was off). After baking is over, we achieve extremely good base pressure of the chamber as ~ 1.8 * 10 -8 Torr. Then, as the ion pump is switched on, we observe the excessive degassing of the ion pump. When bias voltage in ion pump is reached 7.0 kV, the ion pump gauge shows the pressure as 6.0 * 10 -7 Torr. It is not improved significantly after waiting for several hours also. While other gauge of the UHV chamber (which is far away from ion pump) shows the base pressure as ~ 5.0 * 10 -8 Torr.
It indicates, due to exposure of UHV chamber in air , inner wall of the ion pump has been excessively contaminated.
Can you kindly suggest to resolve this issue ?
Thanking you.
Anjan
Dear Mr. Qili Li,
In our case, the ion pump has been contaminated excessively by water vapor, dust, organic compounds due to exposure for long time in air.
In this scenario, is there any special method to degas the ion pump ?
Thanks.
Anjan
When you baked out the chamber, was the body of the ion pump also heated? Sometimes there is an internal heater, or sometimes you wrap heating bands around it (if possible, remove the outer magnets or if not, don't go above 100degC).
As Qili Li said, when you finish the bakeout, use the degas function of the ion pump, while the chamber is still hot.
You say you have a TSP. Is it positioned behind the ion pump? If so, run the TSP a couple of cycles, and the walls of the ion pump gets coated in new, non-contaminated metal. That should improve your pressure.
12 hour of bakeout is not enough for UHV system. 24 hours is minimum, 48 hours for a UHV after a long time ambien is preferred. The ion pump should be bake-out too with an internal heater. TS pump could be switched on after backeout together with ion pump at 1e-8 mBar.
1. Bakeout temperature should be 120C to completely degass water, 160C to degass most of common hydrocarbons, and 400C to completely clean the system. If you are aiming for UHV in the area of low 10E-9 to low 10E-10 Torr and if system is "UHV clean," then 120C chamber temperature is sufficient, deeper targeted vacuum level would require higher temperature.
2. Ion pump must be heated! Ideally temperature of the ion pump body should be the same, or 10C - 20C lower then the chamber. If chamber is heated to 120C then ideal temp for ion pump body is 100C - 110C. Heating below 100C is ineffective, as bakeout times become unreasonably long.
3. Make sure that there are no "cold spots" on chamber or flanges, some flanges could need local heating.
3. Bake with both chamber and ion pump heated and ion pump "off" while pumping with turbo through bypass valve for at least 12 hours. Note vacuum level at the end of 12 Hrs of the bakeout.
4. Switch ion pump "ON" while everything is *hot* with bypass valve still open to continue pumping with turbo pump. You would see decrease in the vacuum level because of ion pump outgassing, this is normal. Continue heating and pumping with both ion pump "on" and turbo pump through open bypass valve until vacuum improves to the same level as you had at the end of the initial 12Hrs period. At that point close bypass valve and continue pumping with ion pump, while still heating both ion pump and the chamber.
4. Pump for at least another 6 hours, or longer if needed - until vacuum stops improving, i.e. until there is no improvement in vacuum reading on ion pump controller over the period of an hour or so. Note that at this point vacuum reading will fluctuate because of multiple reasons, so "filter" the readings.
5. Switch off heater of the ion pump body, while continue heating the chamber. Let ion pump cool while pumping the hot chamber (gases tend to "condense" into ion pump) for at least three, better 6 hours.
6. Switch off heaters on the chamber and let whole system cool down. It should be possible to reach low 10E-10 Torr range with this procedure, if there are no leaks and no hydrocarbon contamination in your system.
If you decide to bakeout temperatures higher then 140C would require removal of magnets from ion pump. If needed, then do high-temp bakeout over first 12 Hrs, then reduce temperature to 120C, re-assemble the magnets (heat-resistant gloves!) and then continue with the above procedure.
Bypass valve must be "UHV" type with metal/metal seal, with elastomer seal UHV is likely to degrade very quickly.
I hope your whole system is using either Conflat, or other type of metal gaskets...
Good luck!
Dear All,
Thank you very much for your valuable answers. After baking of the ion pump in our UHV set up, at present we are getting base pressure of the system as ~7.0 * 10 ^ -10 Torr. Now such excellent base pressure is being maintained by ion pump and TSP (with its performance in periodic interval). Hopefully, after 24 hours, we will be able to get back low 10^ -10 Torr base pressure.
With best regards,
Anjan
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