Ultrasonic cure monitoring of photoresist during pre-exposure bake process

We have developed a sensor to measure the glass transition temperature (T_g) of a photoresist during its pre-exposure bake. During prebake, the resist must reach this temperature in order for significant solvent evaporation to occur. To determine T_g we measured the change in phase of a high frequency pulse as it was reflected from the interface between an 8" silicon wafer and a photoresist film. This measurement was performed before and after a prebaked Shipley 1813 resist film was removed from the wafer as well as during the prebake of a subsequent film. The phase change measured when a film of resist was removed from the wafer agreed well with theoretical calculations. The phase change was also monitored during photoresist cure and was found to decrease to a repeatable minimum at about 48 degrees Celsius. We believe that the characteristic temperature that resulted in the phase minimum was the glass transition temperature for the resin/solvent mixture, and that the nonlinear increase in phase that followed this minimum was due to solvent evaporation