High temperature materials for thin-film thermocouples on silicon wafers

Abstract

We have developed an instrumented calibration wafer for radiometric temperature measurements in rapid thermal processing (RTP) tools for semiconductor processing. The instrumented wafers have sputter deposited thin-film thermocouples to minimize the thermal disturbance of the wafer by the sensors. The National Institute of Standards and Technology (NIST) calibration wafer also employs platinum–palladium wire thermocouples to achieve a combined standard uncertainty of 0.4°C in the temperature measurement of the thin-film thermocouple junction at 900°C. The high temperatures of the wafer has required the development of new thin-film material systems. We have reported the results of our testing and characterization of sputtered platinum, palladium, rhodium, and iridium thin films using titanium bond coats on thermally oxidized silicon wafers. Depth profiling with secondary ion mass spectrometry was used to determine the diffusion profiles from the metal film to the silicon after heat treatments as high as 1000°C. Electron microscopy and optical microscopy were used to follow the reactions and the deterioration of the thermoelectric films. In addition, performance tests up to 1000°C in the NIST RTP test bed were used to determine the stability of the material systems. Failure mechanisms and limitations of the thin-film thermocouple materials have been discussed with data on hysteresis and drift in thermometry performance. The results of our evaluations indicated that Rh/Ir thin-film thermocouples have the best properties for wafer temperatures above 900°C.