Ultrasonic monitoring of photoresist processing

Abstract

A high frequency ultrasonic technique has been developed to monitor photoresist processing in situ during semiconductor manufacturing. Photoresist pre-exposure bake and development have been monitored using the sensor, and the post-exposure bake has been studies as well. The in-situ glass transition temperature (T g ) was determined during the prebake for I-line films down to 0.6micrometers as well as for chemically- amplified DUV resist of similar thicknesses. Using classical reflection theory, photoresist properties such as the density, thickness and acoustic velocity were determined during processing. This in situ parameter inversion method can be used to determine process endpoint if the optimal density, velocity, and thickness are predetermined. The T g for post-exposure bake of I-line resists is expected to be the T g of the novolac resin alone, without solvent present. Measurements using the described sensor have confirmed that the resin T g during postbake is 118 degrees C, the value of T g provided by Shipley. This provides a measurement of postbake as well as a confirmation that the sensor is measuring T g accurately. The development process was also monitored using this sensor. Results prove the usefulness of this sensor for in situ measurements of resist thickness changes during development. This was verified for different exposure doses and for resist coated on a wafer with circuit topography.