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overshoot. In this case the derivative element would be

fairly strong. In the case of “C” the system reaches the

setpoint quickly but overshoots and oscillates about the

setpoint for a few cycles. Increasing the proportional

term would probably drive the system into oscillation.

“D” also overshoots but the system settles fairly quickly.

Finally, “E” represents a very slow response system

with a persistent error that requires attention from the

integral element.

The type of response that is applicable to a given

vacuum system is highly dependent upon the

application. In some cases overshoot is not desirable but

on the other hand, time is money and getting to setpoint

quickly can be very important in terms of manufacturing

efficiency.

Upstream control is the simplest pressure control

mode and can permit a single gas (or a premix of gases)

to enter the system. Common examples of systems using

upstream control include freeze dryers and single gas

sputter deposition. Both require accurate pressure

control and a specific gas for the process - nitrogen for

freeze drying and argon for sputtering.

VI. RATIO UPSTREAM CONTROL

This configuration is required where it is necessary to

control pressure and also introduce multiple gases where

the mass flows of the gases have to be supplied in a

specific ratios due to reaction requirements

One of the most common examples of a process

that uses ratio control is reactive sputtering. The

parameters that must be controlled are the chamber

pressure and the ratio of argon to the reactive gas

(oxygen or nitrogen). A typical ratio for this application

might be 95% argon to 5% oxygen.

The solution is to replace the simple proportioning

valve with a pair of mass flow controllers. Each gas is

controlled by one of the MFCs in such a way that the

proper mass ratios are maintained.

Referring to Figure 7, the output of the pressure

controller is fed to a ratio control device. This simply

apportions the total signal required between the two

MFCs. If MFC “A” is defined as the master controller

that is delivering some time varying Q, then MFC “B”

will deliver some constant fraction of what “A” is

delivering. The net result is a constant split or ratio of Q

between the two MFCs but the total flow is adjusted in

response to the pressure controller.

the Bell Jar, Vol. 10, No. 3/4, Summer/Fall 2001

Vacuum Chamber

Pressure

Gauge

Throttle Valve

To Pump

Mass Flow

Controllers

Flow Ratio Controller

Q(t)

kQ(t)

Set Point Input

Pressure

Reading

Deviation

Error

Gas 1

Gas 2

Figure 7 - Ratio Upstream Control

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