Assessment and Future Directions of Nonlinear Model Predictive ...

Application of the NEPSAC NonlinearPredictive Control Strategy to a SemiconductorReactorRobin De Keyser 1 and James Donald III 21 Ghent University, EeSA-department of Electrical energy, Systems and Automation,Technologiepark 913, 9052 GENT, Belgiumrdk@autoctrl.UGent.be2 ASMA, Phoenix, Arizona, USA1 IntroductionIncreased requirements of flexible production have led to the development ofsingle-wafer processing equipment for integrated circuit fabrication. For commerciallyfeasible throughput, it is substantial to minimize the process cycletime by heating only the wafer surface, in an extremely short time period. Thisis only possible using radiation heating, leading to RTP systems - Rapid ThermalProcessing. Under such circumstances the system is no longer isothermaland temperature uniformity control becomes an issue of considerable concernand technical difficulty. Commercial RTCVD reactors (Rapid Thermal ChemicalVapor Deposition) have been in use for more than a decade, but the technologystill suffers from some limitations [6]. One of these is the inability to achieve withcommercial control equipment an adequate temperature uniformity across thewafer surface during the rapid heating phases (e.g. from room temperature upto 1100 o C in the order of 1 minute). Deposition of silicon should be performed ina manner which minimizes crystalline growth defects, such as lattice slip. Suchdefects are induced by thermal gradients in the wafer during high temperatureprocessing. For example, while gradients of about 100 o Cacrossawafermaybetolerable at a process temperature of 800 o C, respective gradients of only 2 − 3 o Care allowable at process temperatures of 1100 o C. Due to the radiant type ofheating, these semiconductor reactors represent a highly nonlinear interactivemulti-input multi-output system.The problem of RTP-control has been extensively dealt with in many researchprojects during the 1990’s [6]. The current paper presents (partial) results ofan extensive research project. This project ran during the 2 nd half of the 90’sbetween ASM America Inc. (manufacturer of RTP equipment) and Ghent University(developer of EPSAC Model based Predictive Control technology). TheEPSAC (Extended Prediction Self-Adaptive Control) strategy is further referredin [2, 3, 4, 5]. The paper is organized as follows: section 2 introduces the underlyingcontrol problem; section 3 reviews briefly the EPSAC/NEPSAC strategy;R. Findeisen et al. (Eds.): Assessment and Future Directions, LNCIS 358, pp. 503–512, 2007.springerlink.com c○ Springer-Verlag Berlin Heidelberg 2007