In situ simultaneous measurement of temperature and thin film thickness with ultrasonic techniques

A novel technique to measure in situ, simultaneously, temperature and thin film thickness during semiconductor processing is described in this paper. The measurement technique is based on the principle that the velocity of an ultrasonic Lamb wave propagating in a silicon wafer is a function of both the wafer temperature and the thin film coating on the wafer surface. We are able to obtain the processing temperature and film thickness simultaneously with two sets of sensors operating at two distinct frequencies, 0.5 MHz and 1.5 MHz. This technique is demonstrated in an aluminum sputtering system. We have achieved a temperature measurement accuracy of ±0.15°C and an aluminum film thickness resolution of ±170 Å. The measurement does not depend on the optical or the electrical properties of either the wafer or the film material, and is insensitive to the processing environment. With its high measurement accuracy and setup simplicity, this sensor system carries great potential in semiconductor process monitoring and control