Optical nanolithography using evanescent fields

Abstract

In the optical near field region the well understood resolution limits for projection optical lithography can be overcome. This offers the possibility of performing optical nanolithography without the need to use expensive, deep ultraviolet light sources. For exposures that utilise evanescent fields close to metallic amplitude masks sub-diffraction-limited resolution has been achieved experimentally, and the theoretical resolution limits have been explored using vector electromagnetic near field simulations. Resolution down to 20nm using exposure wavelengths greater than 400nm is predicted. It is also found that the exposure wavelength is of secondary importance in this regime, and that the properties of the mask are much more significant. Scaling to smaller feature sizes requires better resolution and control during mask manufacture, rather than the conventional (and costly) approach of driving the exposure wavelength deeper and deeper into the ultraviolet. Near field interference effects have also been explored, and the characteristics of spatial frequency doubling using Evanescent Interferometric Lithography (EIL) have been determined by simulation. Sub-diffraction-limited resolution can be achieved with increased exposure intensity compared with conventional interferometric lithography. The tradeoff is against the depth of field in the resultant interference pattern. Finally, the use of negative refraction and surface plasmons have been investigated to improve further the resolution in the evanescent near field, and to produce novel, three dimensional near field patterns.