Temperature control and modeling of rapid thermal processing

A Rapid Thermal Processing (RTP) chamber capable of plasma-enhanced CVD was built with associated computer control system. The RTP chamber can handle up to a 6 inch wafer and is equipped with 37 tungsten halogen lamps positioned in three concentric zones to radiatively heat the wafer. Thermocouple instrumented wafers with K-type thermocouple junctions embedded at the proper temperature measurement positions acquired the temperature responses from center, middle and edge regions of the wafer. Two narrow band gap infrared pyrometers (λ = 3.3µm) were installed at the top of the lamp housing with water circulating light pipes accessing the wafer temperature signal points. In RTP, accurate temperature control during ramp and hold with fast ramp rate and good uniformity is necessary. The RTP chamber showed ill-conditioned behavior for a 3 x 3 lamp power and wafer temperature multivariable system, showing relatively large interactions between controlled variables, which yielded significant temperature differences across the wafer. A baffle ring was tested to obtain an improved condition number and uniformity of ±1◦C among three measured temperatures. Wafer effective emissivity and view factors were estimated from closed-loop identification of process parameters. Through the on-line estimation of the above parameters and simple closed-loop identification of the process parameters, such as time constants and process gains, one can detect wafer temperature variation from recipe set values when there is a unwanted deposition at the measurement side of the wafer. To accurately identify the ill-conditioned system, an iterative identification method was developed and effects of element uncertainties to control an ill-conditioned system was investigated. Furthermore, a multiloop control strategy was also developed. Finally, an iterative technique was introduced that allows a recipe-driven SISO(Single Input Single output) controlled system to automatically optimize the process recipe for optimal power ratios of each lamp zone.