Overcoming the resolution challenge using special illumination techniques to print 50/50-nm nested contact holes at 157-nm wavelength

Several approaches to printing 50/50 nm nested contact holes are described and compared using lithographic simulations (ProlithTM 7.1). The approaches used include: off-axis quadrupole illumination and attenuating phase-shift mask with optimized polarization of the illumination; chromeless alternating phase shift-masks (CAPSM) in conjunction with special polarization schemes; immersion lithography with extremely high numerical aperture (NA) at 157 nm wavelengths; and EUV lithography. We show how the limits of the off-axis illumination technique can be pushed with the use of radial polarization and how the mask bias (or background transmission) can be used to optimize the image. Resolution limits are further pushed with 2D chromeless alternating PSM combined with the radial polarization. We show that with radial polarization, high-contrast images can be obtained and high-quality contact holes at 100 nm pitch can be printed using negative photo-resist. It is shown that, with immersion in a liquid of refractive index equal to 1.5, standard attenuating PSM with quadrupole or quasar illumination with unpolarized light and positive photo-resist will allow the printing of 100-nm-pitch contact holes. We compare these findings with results obtained at an EUV wavelength to confirm that imaging at an EUV wavelength and low NA can also provide excellent conditions to print 100-nm-pitch contact holes.