Ultrasonic Sensor for Photoresist Process Monitoring

An ultrasonic sensor has been developed to monitor photoresist processing in situ during semiconductor manufacturing. Photoresist development, pre-exposure bake, and postexposure bake were monitored for the Shipley 1800 series I-line resists, and the pre-exposure bake of Shipley APEX-E deep-uv (DUV) resist was monitored as well. Development monitoring was achieved by measuring thickness changes in the resist as it was removed. Data regarding dependence of development rate on exposure dose was obtained for the I-line resist with exposure doses varying from 20 to 68 mJ/cm2: Measurements showed an increase in average development rate from 0.04 to 0.155 m/s, with the rate leveling off at around 55 mJ/cm2: Pre-exposure bake monitoring results demonstrated the ability of the sensor to measure the glass transition temperature of the resist film during prebake as well as the ability to invert out the elastic constants of the film using reflection theory. The glass transition temperature (Tg) is an important parameter in both the preand post-exposure bakes and therefore could be useful in monitoring these processes. Results of pre-exposure bake Tg measurements are presented for both I-line and DUV resists. The glass transition temperature during prebake was found to be higher for the DUV resist than for the I-line series. The I-line resist post-exposure bake measurement of glass transition temperature confirmed the reported Tg of 118 C for the I-line novolac resin. The multiple uses of this sensor make it suitable for integration into a manufacturing setting.