Cryogenic Vacuum Pump Storage
Technical Bulletin – Cryogenic Vacuum Pump Storage
The best approach to maximize the lifetime of your refurbished cryopump is to install as soon as possible following the refurbishment. If the pump must be stored, PTB recommends not exceeding 18 months. Below are some essential technical characteristics to consider.
Vacuum Sealing – Moisture Absorption by Arrays
It is highly recommended that the cryopump be placed under a vacuum of ~200 mtorr if being stored for extended period of time. This will ensure that the 15K charcoal array does not absorb moisture. Moisture is very difficult to remove from the cryopump during the regeneration process and may result in aborted regeneration during the rough to base pressure and rate-of-rise (ROR) phases do to the high concentration of moisture in the 15K array.
The Gifford McMahon (GM) refrigerator compresses and expands the helium gas to produce cold temperatures. During this process, numerous seals and O-rings are encapsulated within the GM refrigerator. When the refrigerator is stored for a long period of time, typically greater than 18 months the seals and O-rings will degrade and/or dry out resulting in inefficiencies in the GM refrigerator’s ability to produce cold temperatures.
A cryopump’s motor encases drive bearings as well as motor bearings and O-rings. The bearings are lubricated and encapsulated with grease. Bearing grease will over a time harden and/or separate during dormancy.
Bearings do have a shelf life. Bearing manufacturers are vague in referencing the “shelf life” of bearings, which vary between 2 to 5 years. The shelf life only refers to a bearing that is still in the manufacturer’s vacuum-sealed package. PTB’s recommendation is not to exceed an 18-month shelf life for bearings that have been removed from the manufacturer’s original package.
Once a bearing has exceeded its shelf life, the reliability is significantly reduced and will manifest itself in one of two failure modes. When there is a force (torque) applied to the bearing it will either not spin resulting in a DOA (Defect on Arrival). Alternatively, if it does spin the lubricant is severely reduced, resulting in excessive heat being generated and will ultimately fail. Unfortunately, this type of failure is classified as a latent defect and typically occurs while the cryopump is under vacuum resulting in unscheduled tool downtime.
Document # PTB-T35-0011 Rev A