Effect of Source Pupil Shape on Process Windows in EUV Lithography

According to the International Technology Roadmap for Semiconductors report, extreme ultraviolet (EUV) lithography is a key lithography tool for manufacturing devices at the 22-nm node and above. The current findings suggest that the lithographic image affects the critical dimensions (CDs) of wafers used in EUV lithography differently from those used in conventional lithography because of off-axis illumination and reflective optics design. The process windows of line/space (L/S) were explored, using a target CD of 22 nm and illuminating target contact hole (CH) features of a CD of 35 nm by using conventional, annular, dipole, and quasor source shapes. The diffraction amplitudes of the L/S and CH EUV masks and corresponding aerial images were calculated. The results indicate that the dipole is the favorable illumination source shape for printing the L/S features and the quasor is favorable for printing the CH features. In addition, the optimal dipole and quasor illumination settings are reported for the L/S and CH features, respectively. The exposure latitude was 4% and the depth of focus (DOF) was 100 nm to illuminate the L/S feature by using the dipole to print the target at a CD of 22. The exposure latitude was 24% and the DOF was 300 nm to illuminate the CH feature by using the quasor to print the target CD at 35 nm.