No abstract

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 cm 2 , respectively, whereas the outgassing rate of RRR, scaled to a 0.4 W cm -2 power density, was estimated as 1.3×10 15 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, [σ abs /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.

Neutral outgassed species from polymethylmethacrylate (PMMA), a model resist (denoted RRR), and three underlayer thin films have been characterized and their absolute outgassing rates determined using a quadrupole mass spectrometer following irradiation at 13.5 nm. The radiation, which was in the extreme ultraviolet (EUV) energy range at 13.5 nm, was delivered from the 08A1-beamline at the National Synchrotron Radiation Research Center in Taiwan. The side chains of acryl-type polymers that are vulnerable to fragmentation and outgassing upon EUV irradiation were characterized and the important outgassed species (H 2 O and H 3 O þ ) were identified from the Si-containing photosensitive materials. The authors propose that the Si-OH formed on the Si-containing surfaces upon EUV irradiation likely undergoes subsequent incremental H 2 O and H 3 O þ outgassing upon cumulative EUV irradiation. A benchmark comparison revealed that the absolute outgassing rates of PMMA, RRR, and a Si-containing underlayer sample determined in this work were consistent with values previously reported in the literature. Thus, we have effectively demonstrated that the Taiwanese facility is capable of evaluating absolute resist outgassing rates, making it one of a limited number of EUV resist evaluation sites worldwide. The authors have provided direct evidence that the extent of ionic and neutral outgassing from 80, 100, and 125 nm PMMA and RRR films is not thickness-dependent. The authors have further demonstrated that the absolute outgassing rates of PMMA and RRR films of various thickness and two oxy-hydrocarbon underlayer samples correlated strongly with the 13.5 nm photoabsorption (r abs ) and the structural toughness [double-bond-equivalence-per-backbone (DBEPB)] of the polymers. The authors revealed that the formula r abs /DBEPB could be used as a generic metric to predict the resistance of photosensitive films to EUV irradiation for the oxy-hydrocarbons in this work and four previously studied photoresists.