An experimental investigation of Fang's Ag superlens suitable for integration

We report on experimental realization of the Fang Ag superlens structure [1] suitable for further processing and integration in bio-chips by replacing PMMA with a highly chemical resistant cyclo-olefin copolymer, mr-I T85 (Micro Resist Technology, Berlin, Germany). The superlens was able to resolve 80 nm half-pitch gratings when operating at a free space wavelength of 365 nm. Fang et al. used PMMA since it enables the presence of surface plasmons at the PMMA/Ag interface at 365 nm and because it planarizes the quartz/chrome mask. If the superlens is to be integrated into a device where further processing is needed involving various organic polar solvents, PMMA cannot be used. We propose to use mr-I T85, which is highly chemically resistant to acids and polar solvents. Our superlens stack consists of a quartz/chrome grating mask, a 40 nm layer of mr-I T85, 35 nm Ag, and finally 70 nm of the negative photoresist mr-UVL 6000 (Micro Resist). A 50 nm layer of aluminium on top of the quartz/chrome mask reflected all light that did not penetrate through the mask openings thereby reducing waveguiding in the top resist layer. The exposures took place in a UV-aligner at 365 nm corresponding to the excitation wavelength of the surface plasmons at the mr-I T85/Ag interface. Supporting COMSOL simulations illustrate the field intensity distribution inside the resist as well as the presence of surface plasmons at the mr-I T85/Ag boundary. AFM scans of the exposed structure revealed 80 nm gratings.