Platinum/palladium thin-film thermocouples for temperature measurements on silicon wafers

A platinum versus palladium thin-film thermocouple system has been established for measuring temperatures on silicon wafers in a rapid thermal processing (RTP) tool. The application includes a silicon wafer with an array of thin-film thermocouples welded to wire thermocouples, used to calibrate radiometric temperature measurements of the RTP tool. The thin-film thermocouples have advantages over present technology using wire thermocouples because the films cause less disturbance of the heat transfer to and from the silicon wafer during periods of high heat flux. High-purity platinum and palladium films have been chosen because of their stability, their resistance to oxidation at high temperatures (to 900 degrees C), and their compatibility with Pt/Pd wire thermocouples. The research effort has concentrated on developing solutions to the adhesion problems for platinum and palladium on silicon; measuring the change in resistance of the films after exposure in air to high temperatures (up to 1050 degrees C); quantifying the drift and hysteresis of the thin-film thermocouples in calibration tests; and determining the effects of composition, stress, and morphology on the Seebeck coefficient. The results of the testing indicate that the Pt/Pd thin-film thermocouples on silicon can perform satisfactorily up to 850 degrees C. We present a discussion of the factors that affect the stability of the films; these include diffusion, chemical reactions, and thermal-mechanical stress, as well as the limitations of this material system. (C) 1998 Elsevier Science S.A. All rights reserved.