Enabling enhanced EUV lithographic performance using advanced SMO, OPC, and RET

Abstract

The current industry standard tantalum-based mask absorber (60 nm TaBN) gives strong 3D electromagnetic field (EMF) effects at wafer level, such as shadowing and pitch-dependent best focus shifts. A thinner mask absorber with higher EUV extinction coefficient or a phase shifting mask can mitigate 3D EMF effects [1]. The alternative mask absorber materials would enable further downscaling to foundry 5nm node using state-of-the-art EUV scanners (with 0.33 numerical aperture “NA”) and facilitate future high NA imaging using single exposure. Here we evaluate insertion options on the patterning roadmap for alternative EUV mask absorbers, including high-k absorbers and attenuated phase shifting masks (attPSM) [1-2]. All studies are using relevant designs from foundry N5 logic node. Two alternative mask candidates are compared with the standard TaBN mask. We bring theoretical proof of concept that alternative mask absorber materials generate significant imaging gain in terms of established success criteria. On a set of predefined types of clips (with variations of 1D/2D, horizontal/vertical, dense/isolated patterns), we seek for higher depth of focus (DoF), higher image log slope (ILS), high illumination efficiency (ideally it would be equal to 1), lower pattern shift through focus (i.e., lower tele-centricity errors), lower mask error enhancement factor (MEEF). Source mask optimization (SMO) on N5 logic clip shows a more balanced source and larger common process window for high-k absorber over Ta-based absorber. Using the optical proximity correction (OPC) engine with high-k mask absorber, shows significant gain on overlapping process window (PW), process variation (PV) band, and less line end shortening. Applying advanced Resolution Enhancement Techniques (RET), sub-resolution assist features (SRAFs) on N5 designs demonstrated an improved process in terms of common depth of focus (cDoF), and image shift through focus. It was also observed that the process not using SRAFs with the high-k absorber had superior process window and image shift compared to the Ta-based case with SRAFs. Therefore, adoption of such high-k absorbers could potentially postpone the need for SRAFs.