Optical properties, including refractive index, photoabsorption coefficient, and film thickness, were derived with a
specular extreme ultraviolet reflectivity (SEUVR) method at 13.5 nm using a newly built ultrahigh-vacuum reflectometer.
Light at 13.5 nm was delivered from the 08A1BM-LSGM beamline at the National Synchrotron Radiation Research
Center in Taiwan. Samples of thin-film polymethylmethacrylate (PMMA), round-robin resist (RRR), and underlayer
materials were investigated. We observed an evolution in the reflectivity curve of most samples, which was ascribed
mainly to a loss of thickness in the film caused by photoabsorption followed by an ablation effect. The thickness-loss
rates of PMMA and RRR were (0.32±0.15) and (0.12±0.02) nm mJ-1 cm2, respectively, whereas the outgassing rate of
RRR, scaled to a 0.4 W cm-2 power density, was estimated as 1.3×1015 molecules cm-2 s-1. The loss of thickness was
further proven by examining the thickness profile of overexposed samples with a profilometer. From these results, we
have established a satisfactory correlation between the thin-film thickness losses with a structural metric, [σabs/doublebond
equivalent per carbon atom]. The reflectivity curve stopped changing when the sample was overexposed. EUV
photochemical reaction mechanisms were proposed, including outgassing by first-order exposure kinetics from the
surface, which corresponded approximately to the linear ablation rate, and a slower diffusion process for outgassing from
underneath the surface, which was observed for the overexposed sample as the stop of changes in the thickness.