Plasmonic lithography

As the next-generation technology moves below 100 nm mark, the need arises for a capability of manipulation and positioning of light on the scale of tens of nanometers. Plasmonic optics opens the door to operate beyond the diffraction limit by placing a sub-wavelength aperture in an opaque metal sheet. Recent experimental works [1] demonstrated that a giant transmission efficiency (>15%) can be achieved by exciting the surface plasmons with artificially displaced arrays of sub-wavelength holes. Moreover the effectively short modal wavelength of surface plasmons opens up the possibility to overcome the diffraction limit in the near-field lithography. This shows promise in a revolutionary high throughput and high density optical lithography. In this paper, we demonstrate the feasibility of near-field nanolithography by exciting surface plasmon on nanostructures perforated on metal film. Plasmonic masks of hole arrays and "bull's eye" structures (single hole surrounded by concentric ring grating) [2] are fabricated using Focused Ion Beam (FIB). A special index matching spacer layer is then deposited onto the masks to ensure high transmissivity. Consequently, an I-line negative photoresist is spun on the top of spacer layer in order to obtain the exposure results. A FDTD simulation study has been conducted to predict the near field profile [3] of the designed plasmonic masks. Our preliminary exposure test using these hole-array masks demonstrated 170 nm period dot array patterns, well beyond the resolution limit of conventional lithography using near-UV wavelength. Furthermore, the exposure result obtained from the bull's eye structures indicated the characteristics of periodicity and polarization dependence, which confirmed the contribution of surface plasmons.