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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
MinorityCarrier wrote ...
This is one of the weaknesses of analog PID. A change can de-tune the loop enough to send the control into oscillations.
This is a nice example, but I wouldn't blame the PID controller. It's just a hideously non-linear plant. You have square-law going from heating demand to heat output, probably a Stefan-Boltzmann fourth power in there somewhere, and possibly worst of all, the lamps can only heat and not cool, so the loop gain has a big discontinuity at zero demand.
And to cap it all, because it's a "rapid" thermal processor you have a very aggressive spec to meet, so you basically have to tune the controller to the ragged edge. I think you'd have the same instability problems no matter what controller you used: if I had to improve this system I'd start thinking about some of Dr. Slack's space-age tricks like inverse dynamics, and give an input for the user to specify what wafer type it was dealing with.
Changing the wafer type probably increased the loop gain because it had more reflectivity, or emissivity or whatever.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
Emissivity.
Actually the wafer is cooled with argon at a flow rate sufficient to flush the volume of the process chamber several times a minute. This cooling is the #1 disturbance term the PID servo loop was being tuned for. The photo below (a tuning iteration gone terribly wrong) shows how fast the wafer temperature (blue line) falls when the lamps shut off.
For the record, this process tool was subject to drift requiring frequent re-calibration, and was ultimately deemed not capable for this process.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Minority Carrier...your system is so well defined and measurable that you could use a microcontroller to predictively control the temperature. Provided that each operation that changes (open/close valves etc.) 'informs' the processor, then the system is highly deterministic. I did this in the 70's using an 8048 - I've no idea what controller would be best for this - usually the best processor family is the one that you're most familiar with !
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