INFICON SQM-242 Thin Film Deposition Controller Card Operating Manual - Manual (Page 83)

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SQM-242 Operating Manual
Figure 8-4 Example of Over Damped Curve
8.5.2 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. In very slow systems such as large filament boats, the P term may
have to be set low to maintain stability where the rate smoothly levels off but
remains below the target value. In this case, you will need to adjust the integral time
constant. This parameter works in reverse meaning the smaller the value the larger
the effect. So, slightly decrease this parameter then watch the rate graph. The rate
should ramp up to the target without overshoot. If the ramp takes too long then
slowly decrease the Integral Time again and repeat these steps until you are
satisfied with the control. Most thermal sources are slow sources. For slow
sources, the D term is more involved. As illustrated in
Figure 8-5 , the derivative
time constant (D) is the time delay between a change in the source’s power setting
and a noticeable change in deposition rate.
Figure 8-5 Example of Delay Setting