Metal layer single EUV expose at pitch 28nm: how bright field and NTD resist advantages align

Abstract

We evaluated the printability of patterns relevant for Logic Metal at P28nm (L/S and T2T) on wafer using EUV single expose. We compare illumination sources with and without fading correction as well as Bright field / Dark field mask tonalities and NTD MOR / PTD CAR resist. In simulations, Bright field (BF) imaging gives better image quality than Dark field (DF) at small pitch/CD. It also enables smaller T2T. To avoid tone inversion (assuming dual damascene processing), BF imaging requires the use of a NTD resist. On wafer, exposure latitudes increase for a BF/NTD choice, concurrent with simulations, even after correcting out SEM shrinkage. Also, T2T CD is reduced. In terms of illumination, we compare dipole sources to fading corrected sources. As fading correction, we have both induced aberrations (Z6-corrected dipole) and monopoles. As expected, a fading correction significantly reduces best focus differences of L/S through pitch and T2T. Moreover, the Z6-corrected dipole is optimal to print small T2T with better uniformity. Finally, we observe that PTD and NTD MOR resist utilize the same aerial image differently. NTD resist can leverage pupil shapes with high exposure latitude, but low depth of focus, better than PTD resist. Fading correction via induced aberrations naturally produces such sources. In summary, the preferred option is a Z6-corrected dipole for best focus alignment and sharp T2T, together with BF imaging to allow higher L/S exposure latitudes and small T2T. Combining this choice with NTD MOR resist avoids tone inversion and leverages the illumination source optimally.