Impact of EUV SRAF on Bossung tilt

Abstract

Mask 3D (M3D) effects remain a significant challenge affecting EUV lithography (EUVL) imaging performance due to the comparable sizes of the mask and the EUV wavelength. Pre-compensation with the insertion of sub-resolution assist features (SRAFs) has been proposed as a solution to compensate M3D effects and improve the process window for advanced technology nodes. In this paper, we discuss the possible positive impact of SRAFs on Bossung tilt, and provide physical insight into the optical mechanisms at play enabling M3D effect mitigation. In particular, we consider an example isolated 2-bar (CD = 16 nm) pattern imaged under delta function dipole illumination. We compare the scattered order distribution and Bossung tilt with and without SRAFs. The results show that SRAFs actually introduce stronger effective single pole aberrations in the imaging process. However, the opposite impacts on Bossung tilt from each pole results in an overall improvement for dipole illumination. Reduced Bossung tilt and a 21% improvement of the overlapping process window are achieved by the insertion of asymmetric SRAFs into the 2-bar mask design.