Metal nanoparticle arrays for near-field optical lithography

We have recently proposed a new approach to optical lithography that could be used to replicate arrays of metal nanoparticles using broad beam illumination with visible light and standard photoresist. The method relies on resonant excitation of the surface plasmon oscillation in the nanoparticles. When excited at the surface plasmon frequency, a resonantly enhanced dipole field builds up around the nanoparticles. This dipole field is used to locally expose a thin layer of photoresist, generating a replica of the original pattern in the resist. Silver nanoparticles on photoresist can be resonantly excited at wavelengths ranging from 410 nm to 460 nm, allowing for resonantly enhanced exposure of standard g-line photoresist. Finite Difference Time Domain (FDTD) simulations of isolated silver particles on a thin resist layer show that broad beam illumination with p-polarized light at a wavelength of 439 nm can produce features as small as 30 nm, or λ/14. Depending on exposure time lateral spot sizes ranging from 30 to 80 nm with exposure depths ranging from 12 to 45 nm can be achieved. We discuss the effect of particle-particle interactions in the replica formation process. Experiments on low areal density Ag nanoparticle arrays are discussed. Resist layers (thickness 75 nm) in contact with 40 nm Ag nanoparticles were exposed using 410 nm light and were subsequently developed. Atomic Force Microscopy on these samples reveals nanoscale depressions in the resist, providing evidence for plasmon-enhanced resist exposure.