Feasibility study of double exposure lithography for 65nm and 45nm node

Abstract

There are many challenges ahead to use ArF for printing 45nm node device. High numerical aperture (NA) exposure tool and double exposure technique (DET) are the promising methods to extend ArF lithography manufacturing to 45nm node at k₁ factor below 0.35. Scattering bars (SB) have already become indispensable as chipmakers move to production in low k₁ factor. The optimum SB width is approximately (0.20 to 0.25)*(λ/NA). When SB width becomes less than the exposure wavelength, the Kirchhoff scalar theory is no longer accurate. When the optical weight of the SB increases, they become more easily printable. In order to ensure more robust lithography, both foundry and IDM prefer to choose higher NA exposure tool to maintain higher k₁ (>0.35). The chipmaker is currently using 0.85 NA, ArF, scanners for 65nm development and early device qualification. With immersion, the NA can be greater than 1.0. Under such a hyper NA (>1) condition, SB scalability and optical weight control are becoming even more challenging. Double exposure methods using either ternary 6% attenuated PSM (AttPSM) for DDL, or applying CPL mask with DDL, are good imaging solutions that can go beyond 45nm node. Today DDL with binary chrome mask is capable of printing 65 nm device patterns. Transmission tuning combined with OAI can achieve best-known imaging contrast for low k₁ lithography. In this work, we investigate the use of DDL with 6% ternary AttPSM to target 45nm node features. SB scalability issue can be addressed in both schemes since the SB can be exposed away using the combined dose from double exposures. Dipole with linearly polarized light can significantly improve the image contrast. The key to take advantage of polarization is to convert the layout into the corresponding x y patterns. We have developed model-based layout conversion method to generate both 6% AttPSM that can consider polarization effect. In this study, we share our initial findings through simulation and to report the initial binary mask printing result of DDL with linearly polarization.