Gas molecules desorbing from chamber walls and internal surfaces. Dominant below ~1 mbar. Decays with time (approximately as t−1). Reduced by bakeout and surface cleaning.
Atmospheric gas diffusing through solid materials (especially elastomer seals). Small, constant rate. Cannot be eliminated — only reduced by material selection (e.g., Viton over Buna-N).
Gas entering through a physical gap — damaged O-ring, loose fitting, cracked weld. Produces a constant, linear pressure rise in rate-of-rise tests. Detectable by helium leak testing.
Trapped gas escaping slowly from blind holes, O-ring grooves, or double seals. Looks like a leak but has no external path. Produces a decaying rate-of-rise (gas reservoir empties). Fix: vent holes, proper groove design.
Pump oil vapour migrating into the chamber against the pumping direction. Significant with oil-sealed rotary vane pumps at low pressures. Mitigated by foreline traps or oil-free pumps (scroll, diaphragm).
| Pattern | Curve Shape | Diagnosis | Action |
|---|---|---|---|
| Linear rise | P ╱╱╱ (constant slope) | Real leak — constant gas inflow from outside | Leak-check all seals, fittings, welds. Use helium spray method. |
| Decaying rise | P ╱‾‾ (fast then slow) | Outgassing — surface desorption decreasing over time | Extended pump time or mild bakeout (60–120 °C). Clean surfaces with IPA. |
| Plateau + decay | P ╱‾─ (rise, level, slow decay) | Virtual leak — trapped volume releasing finite gas reserve | Inspect for blind holes, double seals, trapped volumes. Add vent grooves. |
| No rise | P ─── (flat) | System is tight — gas load below detection threshold | System passes. Proceed with process. |