Control strategy for temperature tracking in rapid thermal processing of semiconductor wafers

A multivariable control strategy is developed and applied to a three-zone lamp, three-point sensor rapid thermal processing (RTP) system. The strategy is based on a physics-based nonlinear model of wafer heating. A feedforward mechanism is used to predict temperature transients, and a feedback mechanism is used to correct for errors in the prediction and reduce spatial temperature nonuniformities. Experimental results are presented that show a controlled ramp-and-hold from 20°C to 900°C at a rate of 45°C/s with less than 15°C nonuniformity during the ramp and less than 1°C nonuniformity of the time-averaged temperatures during the hold as measured by three thermocouples placed at the center, 1-in radius, and 1.75-in radius on a 4-in-diameter wafer. RTP design issues are also discussed in terms of control authority or the ability to generate a wide range of energy flux profiles to achieve temperature uniformity for different processing conditions