Evaluation of optical properties of EUV resist underlayer

The resist underlayer (UL) has been shown to beneficially impact the exposure latitude in photolithography techniques. As a result, the development of the resist UL is in progress for extreme ultraviolet lithography (EUVL) as well. Since the aspect ratio of patterns increases as the feature size decreases, a high-performance EUV UL is expected to be in high demand. In this study, we evaluated the optical properties of the EUV UL by using the lithography simulation tool PROLITH X5 (KLA-Tencor). We quantified the imaging properties of a 14 nm half-pitch (HP) line and space (L/S) pattern by varying the refractive index, extinction coefficient and thickness of the UL under 0.5 numerical aperture (NA) conditions with a conventional binary intensity mask. These simulations reveal that the number of photons absorbed in the photoresist increases as the refractive index of the UL decreases; this results from the increase in reflectivity from the UL/photoresist interface. Therefore, the line critical dimension (CD) mean value decreases and stochastic imaging properties improve in the observation plane. As the refractive index of the UL is reduced, however, the light intensity in resist and the distribution of photons is distorted by the standing wave effect, resulting in roughness and non-uniformity in the pattern sidewall. Therefore, the refractive index of the UL should be similar to that of the photoresist in order to get the optimized performance.