The reflectivity of a multilayered structure on top of a silicon wafer has been measured in a rapid thermal processor at 0.6328 &mgr;m. This structure consisted of a Ge thin film sandwiched between SiO2and Si3N4films. The wafer has been submitted to thermal cycles around the melting point of Ge (938 °C). It has been shown that the onset of Ge melting which corresponds to an abrupt reflectivity change can be detectedinsituby monitoring the intensity of a reflected laser light. It has therefore been demonstrated that the detection of Ge thin‐film melting can serve as a tool for temperature sensor calibration in a rapid thermal processor.