In situ ultrasonic measurement of photoresist glass transition temperature

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Abstract
We have developed a method to measure the glass transition temperature of photoresist in situ. The purpose of photoresist prebake is to evaporate excess solvent from the photoresist and to allow the polymer chains to relax into an ordered matrix. The phase of a high frequency ultrasound signal was monitored as it was reflected from the silicon/photoresist interface during the photoresist prebake. There was an initial decrease in phase followed by a rapid increase once a given temperature was reached. It is believed that this repeatable temperature is the glass transition temperature of the resist/solvent mixture and that the rapid increase in phase following it is a result of solvent evaporation. The glass transition temperature (Tg) for a freshly spun 2.2 μm Shipley 1813 resist was measured to be 50 °C. The Tg measured increased from 50 to 66 °C as the time delay between coating and prebaking was increased from 0 to 20 min. It is believed that this change in Tg is a result of solvent evaporation.