INFICON MDC-260 Thin Film Deposition Controller - Manual (Page 94)

MDC-260 DEPOSITION CONTROLLER
TUNING THE MDC-260 CONTROL LOOP 6-4
manual power mode and adjust the source power using the remote handset to
establish the power ramp parameters. Set the Predeposit Power level at or slightly
below the power needed to get the desired deposition rate.
With the power ramp parameters defined, the next step is to start the dummy
process to see how well the MDC-260 controls the rate. If the rate is too high or
low when the shutter opens then make a note to go back and adjust the Predeposit
Power level. Watch the rate graph and the power display. If the rate is different
from the target rate then you should see the MDC-260 adjust the power
attempting to achieve the target rate. If the rate is close to the target, then you
should temporarily change the rate to see how the MDC-260 reacts. Ideally the
MDC-260 will adjust the power so that the rate goes right to the target rate
without overshooting it. If it does then no further adjustments are necessary.
If it seems like the MDC-260 is reacting too slowly, press the Program key to get
back to the material screen and increase the Proportional Gain parameter. Begin
with changes of about 10 to 20%. Changes of this magnitude are a good starting
point because they are large enough to show the effect of the parameter and small
enough that you won't greatly overshoot the ideal setting. Remember that too
much Proportional Gain will make the system unstable and too little will make the
MDC-260 slow to react. An unstable system is evident by the rate oscillating
around the target value. A general rule of thumb is the faster the source, the
larger the Proportional Gain. And conversely, the slower the source the smaller
the Proportional Gain.
With the Proportional Gain at an acceptable value, the next step is to adjust the
Derivative Time if necessary. Disturb the system again by changing the target
rate. Watch the rate graph as the rate approaches the target. If the rate overshoots
the target then increase the Derivative Time and change the target rate again to
see the effect. Repeat these steps slowly increasing the Derivative Time until the
rate goes right to the target without overshoot.
In very slow systems such as large filament boats, the Proportional Gain
parameter may have to be set so 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 parameter. 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.