Ultrasonic sensor for photoresist process monitoring

An ultrasonic sensor has been developed to monitor photoresist processing i n situ during semiconductor manufacturing. Photoresist development, pre-exp osure bake, and postexposure bake were monitored for the Shipley 1800 serie s 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 measu ring thickness changes in the resist as it was removed. Data regarding depe ndence of development rate on exposure dose was obtained for the I-line res ist with exposure doses varying from 20 to 68 mJ/cm(2). Measurements showed an increase in average development rate from 0.04 to 0.155 mu m/s, with th e rate leveling off at around 55 mJ/cm2. Pre-exposure bake monitoring resul ts demonstrated the ability of the sensor to measure the glass transition t emperature of the resist film during prebake as well as the ability to inve rt out the elastic constants of the film using reflection theory. The glass transition temperature (T-g) is an important parameter in both the pre- an d post-exposure bakes and therefore could be useful in monitoring these pro cesses. Results of pre-exposure bake T-g measurements are presented for bot h I-line and DUV resists, The glass transition temperature during prebake w as 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 T-g of 118 degrees C for the I-line novolac resin, T he multiple uses of this sensor make it suitable for integration into a man ufacturing setting.