Super-resolution imaging and performance optimization for single- and multi-layer silver superlenses
For conventional optical imaging and projection lithography the resolution is limited to approximately half the exposing wavelength. The trend in semiconductor manufacturing has therefore been to use shorter wavelengths, with 193 nm ArF laser sources being used currently. Another way to improve resolution is to go into the near field region, where conventional resolution limits no longer apply. We have used our Evanescent Near Field Optical Lithography (ENFOL) technique to demonstrate resolution down to λ/7 in a hard-contact lithography experiment , with λ/20 resolution predicted for thinner resists . A problem with ENFOL and related techniques is that they require intimate mask/resist contact, which may be undesirable in a manufacturing environment. Following Pendry’s proposal that a planar metal film can act as a plasmon-mediated near-field superlens  a near-field planar lens lithography (PLL) technique has been developed in which a planar metallic film is inserted between the mask and imaging photoresist . Using PLL we have provided the first experimental demonstration of sub-wavelength imaging using a silver planar lens illuminated at the i-line wavelength of a Mercury lamp . We have gone on to show that this technique is capable of producing sub-diffraction-limited near-field images , as have another group using a related technique . Further improvements in the imaging properties of silver superlenses have been proposed by going from singlelayer to multi-layer lens structures . If the same total thickness of silver and dielectric spacer layers is used, then the lamination of these materials provides resolution enhancements by the introduction of additional metal surfaces between the object and image planes; as the imaging in these silver superlenses is strongly mediated by surface plasmons, the incorporation of additional layers on which such plasmons can be generated allows higher spatial frequency components to be efficiently transferred through the system. The resolution enhancements provided in going to multi-layer superlenses have been experimentally tested very recently . This work showed that super-resolution can also be achieved through a double-layer silver superlens, with enhanced transmission compared to a single-layer lens. We review this work here and present the main experimental findings. Analytical and simulation results are then given to show how the performance of multi-layer superlenses can be optimized by changing the relative layer thicknesses in the lens stack.