Evanescent Wave Lithography. a New Tool in Nanofabrication

Conventional optical lithography is based on UV-light sources and illuminating photoresists through masks. Currently, the author is working on the use of evanescent waves to perform lithography, and to the best of our knowledge not used before. Evanescent Wave Lithography has some key advantages and singular capabilities as we will try to demonstrate. In particular, it can reach lower resolution limits and simultaneously make continuous 3D surface profiles in an easier manner. The depth profile capability comes from the special property of exponential decay in the field amplitude of an evanescent wave, contrary to conventional photolithography where the resist is whole-thickness exposed. Only binary profiles are possible in a single step classic lithography basis. The lower achievable resolution limit (~50nm) comes from the well known fact that the effective wavelength in a propagating media is n times less than in vacuum propagation, being n the refractive index of the propagating media. To achieve evanescent wave regime we need total internal reflection at a resist-prism interface (Fig.1) and this needs of an incident medium with a higher refractive index than the resist, typically higher than 1.65, so for example, the effective wavelength when using vacuum i-line (365 nm) should be less than 220 nm. Therefore, by certain means we can achieve the resolution of VUV-EUV lithography but based on near-UV light-sources and conventional resists.