This exercise shows that the reactor pressure is influenced about equally by changing inlet flow rate and outlet pumping speed.
[Caution: in this example precise quantitative tradeoff between inlet gas flow rate and outlet pumping speed is influenced by the use of a turbo pump, whose pumping speed is not actually constant in the pressure regime approaching 1 torr. Click here for more information on turbo pump behavior ]
Given that either inlet flow rate (MFC) or throttle valve position can be used to establish and control process pressures, how would one decide which is preferable? This is a more complicated issue,which could be influenced by a number of factors, such as:
Resolution of the actuators (MFC
and throttle valve)
If one or the other has a higher resolution, that could make it the better
choice for pressure control, so that control could be achieved over a larger dynamic range
of pressures. MFC's can typically be set to about 0.1% precision, so a given MFC
which operates at 100 sccm full scale might be adjusted to a minimum value of 1 sccm with
10% accuracy. Throttle valve precision is closer to 1% typically. However,
when a processing system is designed, the pressure range of interest is usually already
identified, so that the maximum range of MFC flow rate and throttle valve open conductance
can be chosen readily.
Dynamic response time of the
actuators
It is advantageous to be able to adjust the pressure control element
rapidly, since in most cases this actuator will be used not only to set the pressure at a
process set point, but also to control the process pressure to that set point in the
presence of variations and drifts (this is regulatory pressure control). An MFC
system actually consists of a flow sensor and feedback control system as a regulatory
controller which maintains constant flow. Despite this complexity, MFC's respond
quickly (<1 sec) and can be used for rapid changes in gas flow. Throttle valves,
larger in size so as to accommodate large conductances comparable to that of the larger
pumps used on process equipment, require larger physical movement for resetting and
control, but their response times are still fairly fast (<1 sec).
Dynamic response of the system
The rate at which total pressure change can be made in the reactor depends
on the volume of the chamber and the flow rate with which gas is introduced or removed.
Throttle valves in series with large capacity pumps provide fast response,
determined mainly by the speed of the pump, except when the throttle valve is nearly
closed; in this situation, modest changes in reactor pressure take longer because the
effective removal conductance is low as a consequence of the small throttle valve opening.
MFC's can be slow to make large changes to higher pressure, because of low gas
inlet flow.
In practice, both the capacity of the MFC and the throttle valve are optimized for achieving the desired process pressure, and regulatory pressure control is achieved by a total pressure measurement (e.g., a capacitance manometer gauge) which drives the position of the throttle valve to maintain constant reactor pressure.