Scaling of T-g and reaction rate with film thickness in photoresist: A thermal probe study

thermal probe technique, local thermal analysis, was used to measure the gl ass transition temperature (Tg) and reaction rate as a function of film thi ckness in chemically amplified photoresists. Using this technique, heat los s into a resist film was monitored as the temperature of the probe was ramp ed from ambient to temperatures as high as 200 degreesC. The thermal events , glass transition temperature or heat evolved during reaction, were record ed 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 ultrathi n films approximately 50 nm thick. The measured T-g in ultrathin resist fil ms was 4-22 degreesC higher relative to that measured in thick films. We al so investigated the behavior of polyhydroxystyrene films, and found that cr osslinking to the substrate can increase T-g by a large amount. The photore sist films were then exposed with x-ray radiation at the same dose (950 mJ/ cm(2)) for both thick and ultrathin films to ensure a constant photogenerat ed acid concentration with thickness. The exposed areas of the films were h eated with the thermal probe, and an increase in heat flow into the exposur e area, attributed to the heat of reaction, prior to the glass transition t emperature was measured. Kinetic rate constants were estimated with data fr om the power supplied to the probe as a function of temperature using a fir st order reaction model. The results indicate that the rate of reaction in ultrathin resist films is smaller than in thicker Trims for resists process ed at the same postexposure bake temperature. We find that T-g and reaction rate depend on film thickness in ultrathin photoresist films; the differen ces in these properties are expected to lead to large changes in the proces sing conditions used for ultrathin films relative to thick films. (C) 2000 American Vacuum Society. [S0734-211X(00)16406-6].