The entire process of vacuum deposition rests on the foundation of efficient working of Diffusion pump. Good quality Diffusion pumps are like work horses: they are capable of functioning effortlessly for long periods of time provided they are fed nicely and taken care of. For diffusion pumps, the fodder is diffusion pump oil.
Diffusion pump oils are of two types – hydrocarbon and silicone.
Most diffusion pump users these days prefer silicone oil to hydrocarbon because of many reasons; one of them being – easy maintenance of the pump in case of silicone diffusion pump oil. Unlike hydrocarbon oil, silicone diffusion pump oil does not leave any deposits – carbon or otherwise – on the jets of the diffusion pump. Hydrocarbon oil is chosen only where cost cutting is required.
Supervac Industries manufactures both types of diffusion pump oils – Ultra high vacuum Silicone diffusion pump oil (SV-Sigma) and Hydrocarbon Diffusion Pump oil (SV-Delta).
Silicone oil should never be mixed with hydrocarbon oil. In case a pump has been running on hydrocarbon oil but now needs to be run on silicone oil, the diffusion pump chamber should be opened up, cleaned thoroughly with alcohol, other solvents, soapy water and de-ionized water. After that the opened chamber of the pump should be left to dry completely in a closed, clean environment for at least 3-4 days.
WARNING: There should be no residual alcohol or moisture left in the pump. Reason?
- This can contaminate the oil that will be charged after the cleaning.
- Residual vapors increase the vacuum time.
In normal circumstances, if a good quality Silicone Diffusion Pump oil is being used then diffusion pump needs to be opened up for cleaning only once every 4-5 years. Unnecessary opening up of the pump (as some freelance ‘maintenance experts’ tend to advise gullible customers) can damage the pump as they are not meant to be opened repeatedly. Also, this is an absolute waste of useful machine time and money.
All that is needed to be done for normal oil change is –
- Open the drain cock of the diffusion pump AFTER it has cooled down and let the oil flow out completely.
- Close the drain cock.
- Charge the new diffusion pump oil.