Lightpipe proximity effects on Si wafer temperature in rapid thermal processing tools

Lightpipe radiation thermometers (LPRTs) are used as temperature monitoring sensors in rapid thermal processing (RTP) tools for semiconductor fabrication. In order to assure uniform wafer temperatures during processing these RTP tools generally have highly reflecting chamber walls to promote a uniform heat flux on the wafer. To minimize disturbances in the chamber reflectivity small, 2 mm diameter, sapphire lightpipes are often the temperature sensor of choice. This study was undertaken to measure and model the effect of LPRT proximity on the wafer temperature. Our experiments were performed in the NIST RTP test bed. We measured the spectral radiance temperature with the center lightpipe and compared these with the three LPRTs at the mid-radius of the wafer and the thin-film thermocouple (TFTC) junctions of a NIST calibration wafer. Depressions in the wafer temperature up to 25°C with the lightpipe at 2 mm spacing were observed. A finite-element radiation model of the wafer-chamber-lightpipe was developed to predict the temperature depression as a function of proximity distance and separation distance. The experimental results were compared with those from a model that accounts for lightpipe geometry and radiative properties, wafer emissivity and chamber cold plate reflectivity.