Abstract

ABSTRACT A system of in situ ultrasonic sensors has been developed that can be used to monitor the photoresist prebakeprocess. A high frequency phase measurement monitors the resist film properties while a lower frequency time offlight measurement monitors the corresponding wafer temperature. The high frequency measurement involvescalculating the phase of an ultrasound signal as it is reflected from the silicon/photoresist interface. As the photoresistfilm changes in thickness and viscoelastic properties, the phase of the reflected signal will change. In this way, it ispossible to follow how the photoresist film changes as it bakes; the solvent evaporates from the resist, decreasing thethickness 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 flightmeasurement employs PZT-5H piezoelectric transducers bonded to a quartz buffer rod. The transducer generates aLamb wave in the wafer which is then detected at another location by an identical transducer. The time of flight of theLamb wave through the wafer depends linearly on temperature. Using these two sensors, we can measure the wafertemperature and the photoresist properties during prebake; providing us with the information necessary for in situprocess control.Keywords: Photoresist, prebake, ultrasonic sensor, temperature, Lamb wave, time of flight, process monitoring,semiconductor manufacturing.

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