The pressure an assembled system settles at after a defined pump-down, often 12 or 24 hours, with all process gas off. It reflects the whole system: pumps, conductances, leaks and outgassing from every surface. Fabs check base pressure before runs or at each shift as a quick health test, and a creeping base is usually the first sign of a leak, a wet chamber or a tired pump.
Base came back at 8E-7 this morning instead of the usual 2E-7, I'm leak-checking before we release the tool.
When someone asks what base is, they want the number and how many hours it has pumped; a base pressure without a pump time says little.
Also heard as
- base
Do not mix it up with
- Ultimate pressure: Ultimate pressure is a pump's data-sheet limit on a clean test dome, while base pressure is what your own chamber actually reaches.
Related terms
Ultimate pressure
The lowest pressure a pump can reach on a sealed, clean test dome, as stated on its data sheet.
Pump-down curve
Plot of chamber pressure against time during a pump-down, used as the fingerprint of a healthy system.
Rate of rise
Test that isolates a pumped chamber and tracks how fast the pressure climbs, to estimate leaks and outgassing.
Outgassing
Slow release of gas from the surfaces and bulk of materials inside a vacuum system.