3 loop tuning procedure – INFICON SQM-242 Thin Film Deposition Controller Card Operating Manual User Manual

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SQM-242 Operating Manual

8.5.3 Loop Tuning Procedure

NOTE: Please keep in mind, control loop tuning is a trial and error process and

there is no "best" procedure to accomplish this task.

Set System Parameters: In the SQM-242 Co-dep software, set a Period of
0.25 seconds as a good starting point. Set Tooling parameters to 100% for now.
Initially set the Rate Filter to 1.00 (no filter) to see the noise of the system.
Simulate should be OFF. Keep in mind that Simulate mode is a tool for testing
process layers. It is not likely to match the control response of your vacuum
system.

Create a One-Layer Test Process: In View >> Input setup, input the Z-Ratio
and density of the material you are depositing. On the main dialog box, set the
desired rate and leave the other parameters at their default values.

Test the Setup: Set the power to manual mode, then press Start. Slowly
increase the power to 10%, and verify that your power supply output is about
10% of full scale. Continue to increase power until a rate near your desired rate
is achieved. Again, verify that the power supply output agrees with the
SQM-242 Power (%) reading. If the readings don’t agree, check your wiring and
verify that the Edit>>Output menu, Full Scale voltage agrees with your power
supply’s input specifications. Log the data for a few minutes. Plot the data, if the
system has significant short term noise at a fixed power (maybe >10%), the
control loop will be very difficult to adjust, especially at low rates. It is better to
eliminate the source of the noise before attempting to set the PID values.

Select a Filter Alpha: On the View >> Card Setup menu, slowly decrease the
filter Alpha from 1to a lower value until the rate display noise is minimized. If
you set Alpha too low, the display will lag the true system response and may
hide significant problems. A value of 0.5 equally weights the current reading
and the previous filtered readings.

Determine Open Loop Gain: Record the Power reading at the desired rate as
PWR

. Slowly lower the power until the Rate (Å/s) reading is just at (or near)

zero. Record the zero rate Power reading as PWR

Determine Open Loop Response Time: Calculate 1/3 of your desired rate
(RATE

1/3

), and 2/3 of the desired rate (RATE

2/3

) for this layer. Slowly increase

the power until Rate (Å/s) matches RATE

1/3

. Get ready to record the loop’s

response to an input change. Quickly adjust Power (%) to PWR

. Measure the

time for the Rate (Å/s) reading to reach RATE

2/3

. You may want to do this

several times to get an average response time. Twice the measured time is the
step response time, TIME

. TIME

is typically 0.2 to 1 second for E-Beam

evaporation, 5 to 20 seconds for thermal evaporation.

Set PID Values: Set the power to zero. In the Edit>>Output menu set P=25,
I= TIME

, D=0. Set Max. Pwr to ~20% higher than PWR

. Exit the output

menu and select Auto to move to Auto (PID control) mode and observe the
Power graph. The power should rise from 0%, and stabilize near PWR

with