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
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.
In EUV Lithography, mask shadowing effect and photon shot noise effect are the main sources of patterning limit,
critical dimension (CD) non-uniformity and low imaging properties. In this paper, the patterning performance of a 6%
attenuated phase shift mask (PSM) is valuated, and the results show that this can be used for half-pitch (hp) down to 14 nm with 0.33NA due to the improved stochastic patterning properties. The proposed PSM consists of 26.5 nm of TaN as an absorber layer and 14 nm of Mo as a phase shifter on 2.5 nm thick Ru capped Mo/Si multilayers. This structure has ~6% reflectivity at the absorber stack and 180° phase shift. The improved stochastic resist patterning properties of PSM were compared with those of conventional binary intensity mask (BIM) with a 70 nm-thick TaN absorber for the 14 ~ 22 nm line and space (L/S) 1:1 dense pattern with 0.33NA off-axis illumination conditions with a EUV generic resist model.