Calibration wafer for temperature measurements in RTP tools

Abstract

Rapid thermal processing (RTP) is a key technology that is used to produce integrated circuits at lower cost and reduced thermal budgets. One of the limiting factors in expanding the use of RTP is the accuracy of temperature measurements of the wafer during processing. We are developing a wafer for calibrating radiometric temperature measurements in RTP tools. The calibration wafer incorporates thin-film thermocouples with platinum/palladium (Pt/Pd) wire thermocouples welded to thin-film pads at the periphery of the 200 mm wafers. We have reduced the uncertainty of the temperature measurements up to 1200 K with this system. This has been accomplished by reducing the uncertainty due to the thermocouple itself and due to reduction of heat transfer near the junction.We report results of NIST calibrations of radiometers using Pt/Pd wire thermocouples welded to the thin films on the wafer and of calibrated type K thermocouples. The thin-film thermocouples were sputter deposited from high purity Pt, Pd and Rh. These thin-film thermocouples were calibrated by comparison with Pt/Pd wire thermocouples in a specially designed test cell at temperatures up to 1150 K. Radiometric temperature measurements were made on the calibration wafer in the NIST RTP sensor test bed, using a commercial radiometer, and compared to those obtained from the thermocouple measurements. A model is presented to account for errors in the radiometric measurements due to stray radiation from the heating lamps, reflection of wafer emission from the chamber walls, and wafer emissivity. The calibrated type K thermocouples indicated temperature measurements within 4 K of both the Rh/Pt and Pt/Pd thermocouples on the 200 mm calibration wafer between 1000 K and 1150 K. The Pt/Pd thin films proved less durable than the Rh/Pt thin films and the limitations of these systems are discussed.