Model-based emissivity correction in pyrometer temperature control of rapid thermal processing systems

Single-wavelength pyrometers are most often used to infer wafer temperature in rapid-thermal-processing (RTP) systems. A constant wafer emissivity is assumed with a pyrometer, but a variation in the wafer´s surface emissivity can result in an error in the inferred temperature which affects the temperature control of the RTP system. A time-dependent variation is evident in rapid thermal chemical vapor deposition where the emissivity is a function of the film type and thickness. An approach which uses a physically based model of the emissivity variation as part of the feedback control loop is described. The technique employs a first-order model of the emissivity as a function of film thickness from which a projected actual wafer temperature is inferred. The film thickness is approximated using a valid growth-rate expression and temperature as a function of time. These models are then incorporated into the feedback loop of the RTP control system