2 tuning a slow source – INFICON XTC/C Thin Film Deposition Controller User Manual
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XTC/C - XTC/2 Operating Manual
CTL GAIN . . approximate the process gain by dividing the increase in
deposition rate (Å/sec) by the increase in source power (%). Set
this parameter to this computed value. Optimize this value by
changing the value in use. Remember that increasing the value
of this parameter reduces the controller change for a given error
in the deposition RATE.
NOTE: If satisfactory control cannot be established using only CTL GAIN the
source is probably not a "fast" source.
The response of a system with too little controller gain (its CTL GAIN value is
too large) is characterized as over damped as shown in
. Decrease
the CTL GAIN value until the system oscillates as is shown by the under
damped curve. Proper control is established by an intermediate value that
approximates the critically damped curve.
Figure 4-6 Examples of Damped Curves
4.5.2 Tuning a Slow Source
A slow source, for the purpose of this discussion, is a deposition source that
has more than a one second delay (lag) between the control voltage change
(into the source’s power supply) and the measurement system’s ability to sense
that change has taken place. Most thermal sources are slow sources. A typical
fast source is an electron beam heated type that does not use a hearth liner.
If the source response has been characterized as "SLOW" (as suggested in the
NOTES in
and then
set the PID control parameters as follows:
CTL GAIN = K
P
, enter this value into the parameter
CTL TC = T
1
, enter this value into the parameter
CTL DT = L, enter this value into the parameter