INFICON MDC-370 Thin Film Deposition Controller - Manual (Page 94)

MDC-370 DEPOSITION CONTROLLER
The first step in setting the PID parameters for a new material or source is to enter
the recommended starting values listed above. Be sure and choose the PID values
for the type of source you're using. Next, create a dummy process with the first
layer set for the new material. Start and abort the dummy process to load the new
material as the active material. You should now see the material's name in the top
line of any Status Screen. Next, open the shutter and put the 370 in the 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 370 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 370 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 370 reacts. Ideally the 370 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 370 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
370 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.
Tuning the MDC-370 Control Loop 6-4