Interested in linking to "Ultimate Limits to Performance"?
You may use the Headline, Deck, Byline and URL of this article on your Web site. To link to this article, select and copy the HTML code below and paste it on your own Web site.
Greg: What are the temperature measurement requirements?
Mark: Besides the need for contactless sensors, the resolution must be better than 0.1 °C. Optical pyrometers without any emissivity errors have a resolution that ranges from 0.1 °C for standard devices to 0.02 °C for special devices. The response time must be faster than sample time. For rapid thermal processing, the response time must be less than 10 ms. Optical pyrometers can be as fast as 2 ms.
Stan: What about measurement noise?
Mark: The prongs holding the rotating wafer can cause a 40 °C to 60 °C drop. The EMI noise from a zero-crossing SCR can also cause noise of 1% to 3%. Intelligent filtering is needed, as well as an incredibly fast model-based control system to smooth out noise and push out the bandwidth.
Greg: Besides reducing the effect of process and EMI noise, why is model-based control important?
Mark: Model-based control reduces the adverse effect of resolution on the signal-to-noise ratio by relying less on feedback control. The multivariable nature of model-based control provides optimal coordination that eliminates interaction in wafer zone control. The radial zones must be controlled at different temperatures. The outermost radial zone must be kept at the highest temperature due to heat loss at the edge.
Stan: What type of model-based control technology do you use?
Mark: We use the best controller for the system under control given the process characteristics and performance requirements. This includes the sensors, actuators and the process dynamics. The controller might be a multivariable PID or one of the many possible model-based controller methodologies with a Kalman filter to overcome system noise issues.
Greg: We conclude with the first half of my "Believe It or Don't" list.