Ultrasonic monitoring of photoresist processing

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 studied as well. The in situ glass transition temperature (Tg ) was determined during the prebake for I-line films down to O.6im as well as for chemically-amplified DUV resists 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 Tg for post-exposure bake of Iline resists is expected to be the Tg of the novolac resin alone, without solvent present. Measurements using the described sensor have confirmed that the resin Tg during postbake is 1 18CC, the value of Tg provided by Shipley. This provides a measurement of postbake as well as a confirmation that the sensor is measuring Tg accurately. The development process was also monitored using this sensor. Results prove the usefulness of this sensor for in situ measurement of resist thickness changes during development. This was verified for different exposure doses and for resist coated on a wafer with circuit topography.