Scaling of Tg and reaction rate with film thickness in photoresist: A thermal probe study

A thermal probe technique, local thermal analysis, was used to measure the glass transition temperature ${\mathrm{(T}}_g)$ and reaction rate as a function of film thickness in chemically amplified photoresists. Using this technique, heat loss into a resist film was monitored as the temperature of the probe was ramped from ambient to temperatures as high as 200 °C. The thermal events, glass transition temperature or heat evolved during reaction, were recorded as a function of the probe temperature. The $T_g$ of the photoresists UVN 30, UV6, UV3, KRS, and KRS-XE was measured for thick films and for ultrathin films approximately 50 nm thick. The measured $T_g$ in ultrathin resist films was 4–22 °C higher relative to that measured in thick films. We also investigated the behavior of polyhydroxystyrene films, and found that crosslinking to the substrate can increase $T_g$ by a large amount. The photoresist films were then exposed with x-ray radiation at the same dose (950 mJ/cm²) for both thick and ultrathin films to ensure a constant photogenerated acid concentration with thickness. The exposed areas of the films were heated with the thermal probe, and an increase in heat flow into the exposure area, attributed to the heat of reaction, prior to the glass transition temperature was measured. Kinetic rate constants were estimated with data from the power supplied to the probe as a function of temperature using a first order reaction model. The results indicate that the rate of reaction in ultrathin resist films is smaller than in thicker films for resists processed at the same postexposure bake temperature. We find that $T_g$ and reaction rate depend on film thickness in ultrathin photoresist films; the differences in these properties are expected to lead to large changes in the processing conditions used for ultrathin films relative to thick films.