Pumping speed and throughput: understanding Q = S × P
How volumetric flow, pressure, and pumping speed relate in vacuum systems—and how to size pumps and throttles for process gas loads.
By Semiconductor Tools Editorial Team · Last updated: 2026-06-14
The constitutive vacuum relation Q = S × P balances gas throughput Q (load) against pumping speed S at pressure P. At steady state, Q from MFC process gas, leaks, and outgassing equals S×P at the chamber. Every pressure-control troubleshooting path eventually uses this equation.
Units and conversions
- S: L/s (volumetric speed at specified flange)
- Q: Torr·L/s, mbar·L/s, or Pa·m³/s
- P: must be consistent with Q/S definition point
MFC volumetric flow converts to Q via STP: 1 sccm ≈ 0.01267 Torr·L/s in our Pumping Speed Calculator.
Worked example — CVD pressure setpoint sizing (Pumping Speed Calculator)
Recipe: N₂ 400 sccm + NH₃ 80 sccm = 480 sccm total. Target pressure 3 Torr. Throttle + turbo system—what Seff is required at the chamber?
- Enter flow 480 sccm, pressure 3 Torr.
- Q ≈ 6.08 Torr·L/s. Required S = Q/P ≈ 2.03 L/s.
- This is modest—medium-vacuum CVD is throttle-limited, not turbo-limited. Verify throttle valve conductance provides the dominant restriction intentionally.
Worked example — Low-pressure RIE
Total flow 150 sccm, target 12 mTorr:
- Pumping Speed Calculator: S ≈ 158 L/s required.
- Run Conductance Calculator on actual foreline → if Seff = 90 L/s, equilibrium P = Q/Seff ≈ 21 mTorr—recipe cannot hold 12 mTorr until conductance or pump path is improved.
- Cross-check Q in mbar·L/s via Throughput Converter: 150 sccm ≈ 2.53 mbar·L/s.