Essential maintenance: Cryo pumps

Faebian Bastiman

Cryo pumps and ion pumps make an exceptionally good team for III-As MBE. Whilst the ion pump stoically and gradually maintains the UHV vacuum, cyro pumps provide that little extra pumping capacity to bring you from “good” high -10 mBar to “great” low -10 mbar. However the pumping capacity of a cryo pump degenerates with time, making them less and less effective. In fact from the moment they reach minimum temperature (~10K) every following moment sees them becoming less effective until their internal absorbing surfaces finally saturate. The efficient operating time will depend on the cryo pump volume and how aggressively you operate your MBE system. In a typical R&D III-As system the efficient operating cycle is usually 2-3 months.

But how to regenerate a cryo pump?

Well first of all you will need to shut the gate valve that connects the cryo pump to the growth chamber. After that you are free to turn off the compressor and power down the pump. However remember in so doing the pump will rapidly heat up and the internal pressure can exceed atmosphere by over 100 times. Clearly leaving the pump sealed on both ends is not a good idea. The pressure increase is of course not instantaneous, and happens more readily above 77K when N2 gas starts evaporating.

The next step involves connecting a dry scroll pump to the exhaust and opening the exhaust valve once the cyro pump reaches ~80K. The cryo pump can then be left in this state for several hours until the internal temperature equilibrates to the ambient. At this temperature most of the contaminants are highly volatile; however water and some of the other cryo absorbed matter may need a little more encouragement. The final step in regeneration is achieved by using heated dry N2 gas to raise the cryo pumps internal temperature to ~60°C. Indeed most cryo pumps have a double-exhaust to enable this function.

In order to perform this final step you will need (i) ultra-pure N2 from a cylinder (ii) a fully baked, ultra clean SS line from the cylinder to the cyro pump and (iii) a heat source. Since you are baking the line anyway, you can simply install heat wraps or tape on the line to heat the pipe to ~100°C and in so doing this will also heat the N2 passing through the pipe. A more elegant (and expensive) solution is to utilise an ultra-pure 316L SS inline N2 gas heater (from Heateflex). The N2 needs applying constantly during the ramp up and ramp down steps, with a hold time of about an hour.

Once the N2 heating cycle is completed the dry scroll pump can be operated to pull the cryo pump down to ~10-3 mBar. Whilst the cyro pump can be operated from this base pressure , you can significantly reduce early stage contamination by employing a turbo pumping station (like the Pfeiffer Hi-cube Classic N.B. avoid the Hi-cube Eco which does not have sufficient pump rate for this application) to bring the interior pressure down to ~10-7 mBar (N.B. It is the size of the roughing pump and not the turbo molecular pump that is the limiting factor).

A schematic of the regeneration system is shown below. Note the inclusion of the 3 way bypass valves for the pumping station to avoid flushing it with heated N2 gas during the final regeneration step. It is also possible to include an optional wide range gauge (WRG) on the cryo exhaust to monitor the internal pressure whilst refining the regeneration cycle’s timing.

Cryo pump regeration system

Then simply seal the exhaust, restart the compressor and wait for the internal temperature to reach ~10K. Once done you are free to open the gate valve and continue normal system operation. The entire regeneration cycle may take 5-6 hours. So rather than lose an entire day it is better to schedule it over night or over a weekend. How can you do this? Why by automating the process with the MBE control software of course. What is that you say? “Your software does not have this feature”. Why not?

One thought on “Essential maintenance: Cryo pumps

  1. Pingback: Essential Maintenance: The maintenance cycle | Dr. Faebian Bastiman

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