Full-wafer thermal imaging in ultrahigh epitaxy tools

The surface temperature distribution of a GaAs wafer, heated under vacuum, has been measured using a digital camera. A method is proposed to remove parasitic signals from the image. The accuracy of the thermal image is validated by comparing the results with a separate measurement from absorption band-edge spectroscopy (ABES). The thermal imaging data are observed to be within the experimental error from the ABES technique for the entire surface of the wafer. We observe a radial temperature profile with a center-to-edge difference that varies as a function of the central temperature. A difference of 25 °C is observed for a central temperature of 565 °C. This difference increases with the wafer temperature, confirming that it is due to a net heat flux escaping the wafer by its edge, which is in contact with a graphite holder. Based on these results, a solution is proposed in which the graphite wafer holder is replaced by a ceramic version.