The provision for doing complete closed loop control with an MFC and pressure transducer is limited, I am quite sure, to the MKS 1159 and 1179 MFCs. They have a special feature for this which results from their use in wafer backside cooling systems.
With small high vacuum pumps (30 to 80 l/s at the inlet), you only need a couple or three sccm to control at 5 to 10 mTorr. MFCs usually have a hard time controlling below a few % of full scale. You can minimize gas use by having a small butterfly valve above the pump inlet. In operation this valve can be mostly closed. This also increases the pressure drop from chamber to pump making it less likely that you will stall the pump.
Steve
Created on Monday, March 05, 2001 3:25 PM EDT by Steve Hansen