Turbo Molecular Pumps

A turbo pump cannot start against atmosphere. A backing pump must bring the foreline into the turbo's start-up range, usually below 1 mbar. Oil-sealed rotary vane pumps are the cheapest choice; dry scroll or diaphragm pumps are used where hydrocarbon backstreaming would contaminate the process.

Grease-lubricated pumps can be mounted in any orientation and need no oil level checks, which suits mobile and multi-position installations. Oil-lubricated pumps generally carry higher gas throughput and longer bearing life but must be mounted within the specified orientation. Magnetic-levitation models remove the bearing question altogether at higher cost.

In order of likelihood: the foreline pressure is too high (backing pump worn or undersized), the chamber has a real leak, surfaces are outgassing because the system was never baked, or the seal at the inlet flange is damaged. Check the foreline pressure first — it is the fastest measurement and the most common cause.

Only in a corrosion-resistant version with purge gas. Standard pumps have aluminium rotors that halogen chemistries attack, and the bearing lubricant degrades. Etching and CVD lines use pumps with protective coatings, purge ports and, upstream, an isolation valve that closes on failure.

Mechanical-bearing pumps are usually specified for 25,000 to 40,000 running hours before bearing service, provided the pump runs clean, at rated foreline pressure and without repeated hard vents. Frequent venting to atmosphere at full speed and particulate ingress are what actually shorten bearing life.