Ultrasonic cure and temperature monitoring of photoresist during pre-exposure bake process

A system of in situ ultrasonic sensors has been developed that can be used to monitor the photoresist prebake process. A high frequency phase measurement monitors the resist film properties while a lower frequency time of flight measurement monitors the corresponding wafer temperature. The high frequency measurement involves calculating the phase of an ultrasound signal as it is reflected from the silicon/photoresist interface. As the photoresist film changes in thickness and viscoelastic properties, the phase of the reflected signal will change. In this way, it is possible to follow how the photoresist film changes as it bakes; the solvent evaporates from the resist, decreasing the thickness and increasing the density. Results indicate that there is a phase minimum at a repeatable temperature, believed to be the softening or glass transition temperature (Tg). The lower frequency (200kHz) time of flight measurement employs PZT-5H piezoelectric transducers bonded to a quartz buffer rod. The transducer generates a Lamb wave in the wafer which is then detected at another location by an identical transducer. The time of flight of the Lamb wave through the wafer depends linearly on temperature. Using these two sensors, we can measure the wafer temperature and the photoresist properties during prebake; providing us with the information necessary for in situ process control.