INFICON XTC/C Thin Film Deposition Controller - Manual (Page 115)

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XTC/C - XTC/2 Operating Manual
4.5 Tuning the Control Loop
The function of the control loop parameters is to match the instrument’s reaction
to an error (between the measured deposition rate and the desired rate) to the
time related characteristics of the deposition source and its power supply.
There are three adjustable parameters; CTL GAIN, CTL TC and CTL DT used
to accomplish this. It is convenient to think of sources as falling into two
categories "fast" or "slow". Fast sources use an integrating type controller while
slow sources are better controlled with a PID type. A more extensive discussion
of control loops is presented in
section 5.6 on page 5-12 . The tuning
parameters are affected by source level, rate, sweep range or beam density,
tooling and source condition.
NOTE: The use of a chart recorder, especially when beginning a new
application is highly recommended. Set the recorder output to "rate"
and use it to monitor the response to small changes in the DEP RATE.
NOTE: If you do not know if the source is fast or slow, it is straight forward to
measure the delay with the chart recorder. Using manual power control,
establish rate and allow it to become steady. When the chart recorder
pen crosses some convenient reference point, increase the source
power a few percent (~5% if possible). Allow the source to again
stabilize. Graph the delay time, as is shown in
Figure 5-7 on page 5-13 ,
to determine if the source is "fast" or "slow". Run the recorder at a chart
speed sufficiently fast to accurately measure time. Delay times greater
than 1 second characterize the source as "slow".
4.5.1 Tuning a Fast Source
A fast source, for the purpose of this discussion, is a deposition source that has
not 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. In general, fast sources are: all electron beam
types (unless a hearth liner is used), some very small filament sources and
sputtering sources.
If the source response has been characterized as "FAST", as suggested in the
NOTES in
section 4.5 , it is easy to set the INTEGRATING TYPE control
parameters as follows:
CTL DT. . . . . since this is a fast source, set this parameter to 0.1
and leave it there.
CTL TC. . . . . set this parameter to 0.1 and leave it there.