Previously we investigated pressure control by adjusting inlet gas flow for fixed pumping speed. The throttle valve adjusts the conductance between the pump system and the reactor. When the throttle valve is closed sufficiently, it becomes the limitation to the speed at which gas may be removed from the reactor to the pumps. This is essentially the point at which the throttle valve conductance becomes equal to or notably less than the speed of the pump (recall that pump speeds can be measured in the same units as conductances, though they may change with the pressure regime in which the pump is working).
In order to see how the throttle valve can control reactant pressure, we need to have the SiH4 delivery system admitting gas into the reactor, as will be done in the following exercise.
This exercise shows that the reactor pressure can be controlled by the adjustable throttle valve between the pumping system and the reactor.
Furthermore, it demonstrates that, in this regime of system design parameters, the reactor pressure varies continuously with throttle valve opening, increasing as the throttle valve is further closed (for fixed gas inlet rate).
[Caution: the near-linear behavior of pressure with throttle valve conductance 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 ]