Reactant Pressure Control by MFC
It is essential to establish particular values of pressure of reactant gases such as that indicated for this system, in which silane gas (SiH4) is made available as a precursor for the deposition of polysilicon thin films for semiconductor manufacturing.  The reactant gas is introduced after the pumping system has evacuated other gases from the reactor, including the air or N2 vent gas which may have been there before the pumps were turned on, and any residual reactive gases from previous processes.

The MFC system provides a way to introduce a fixed flow rate of SiH4 into the reactor, in which the MFC setting determines the inlet gas flow rate and the downstream valve (MFC DSV) admits the gas to the reactor (caution: the MFC DSV must be open in order for the MFC setting to control gas inlet to the reactor).   Depending on the relative flow rate of SiH4 into the reactor and the corresponding rate of SiH4 out of the reactor to the pumping system, a steady-state SiH4 pressure will be established in the reactor.

Since the speed of the pump system is fixed by the system design, let's see how the SiH4 pressure in the reactor is influenced by the MFC setting for SiH4 inlet flow rate.

This exercise shows that the reactor pressure can be controlled by the SiH4 gas flow rate into the reactor, with fixed pumping system speed and a fixed throttle valve position.

Furthermore, it demonstrates that, in this regime of system design parameters, the reactor pressure is directly proportional to the input gas flow rate:

Pressure = constant x  ( inlet flow rate )

[Caution: this simple relationship breaks down for this system when the reactor pressure rises to the range of 1 torr or above, at which the turbo pump does not function effectively!  Click here for more information on turbo pump behavior ]