Resolution improvement review for the immersion lithography

The lithography technology has been developed to keep shrinking resolution to sustain Moore's Law. Rayleigh formula indicates wavelength and NA are two key factors. Since it is more and more difficult to further reduce laser wavelength, increasing NA is a feasible way to improve resolution. As it is known to all, the effective numerical aperture (NA) is depended on the minimum refractive index (RI) factors among the lens material, fluid material, and photoresist in the immersion lithography. This paper reviewed those key factors limited to NA with simulation data, and discussed the possibility of further improvement. Besides, solid immersion optical lithography is also discussed in this paper as an alternative way to extend resolution for simple periodic features.