April 2011 - Control Global

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C O N T R O L T A L K scale units that have only heaters. Large bioreactors have jackets with temperature-water systems that can provide a cooling rate commensurate with the heating rate. Even here, the tuning for no overshoot is not easy. The dynamic reset limit PID option is essential to prevent the integral action of the primary temperature loop from changing the setpoint of the secondary loop or the final control element too quickly. (See my February 18 blog post, “Dynamic Reset Limit, www. modelingandcontrol.com/2011/02/dynamic_reset_limit.html) Stan: What are the final control elements that are used in the semiconductor industry? Mark: The temperature controllers are typically manipulating power to a heater. Pulse-width modulation is used to vary the amount of on time in a cycle. The typical power controllers used today and their performance values from the least to the most expensive are shown in Table 1. The electromechanical contacts are only suitable for very slow systems with very loose control requirements. Solidstate relays offer tighter control, but are not fast enough for many wafer temperature control systems used in silicon wafer processing. Silicon control rectifiers (SCR) offer the fastest and most reliable control with the longest life. The phase-fired SCR commands true power regardless of line voltage or heater resistance. It also offers the tightest control, but filtering must be employed to reduce EMI noise. Greg: What are some of the faster rates of temperature transitions? Mark: The temperature of semiconductor wafers goes from room temperature to 1150 °C. The ramp rate might be 100 °C/ min. Yet customers think they can use a 1-sec controller cycle time that corresponds to a 1.6 °C error. For rapid thermal processing, the ramp rate could be Type Cycle Time Resolution Noise Heat Generation Electromechanical Contactors 30 sec 3% Low Low Solid State Relays (SSR) 1 sec 1% Moderate Moderate Silicon Control Rectifier (SCR) Zero-Crossing 16 msec 2% Low High Phase-Fired 16 msec 0.1% High High power controller performance Table 1. Typical power controllers and their performance values are listed here from the least to the most expensive, with electromechanical contracts costing the least. 200 °C/sec. A sample rate of 100 Hz is then needed. Faster-than-required sample rates cause unnecessary wear of contactors and relays, and reduce the signalto-noise ratio because the true change in the process temperature becomes a smaller percentage of the noise from the process, EMI and resolution limits. Stan: What about the precision of control needed? Mark: The temperature of the wafer for some applications needs to be held within 0.1 °C in order for the gases to react and the substrate layers to grow. For example, the temperature must reach 999.9 °C to exceed the activation energy for the reaction, but must be kept from exceeding 1000 °C. An improper temperature causes not only an inconsistent uniformity, but also different layer thicknesses that create problems for feature size and other key electrical properties. 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. Go to www.controlglobal.com/1104_ControlTalk.html to see Greg’s “Believe It or Don’t” list. 60 www.controlglobal.com A p r i l / 2 0 1 1
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April 2011 • Volume XXIV • Numb
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April 2011 • Volume XXIV • Numb
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April 2011 - Control Global

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