Nanoscale optical patterning using evanescent fields and surface plasmons

Abstract

Patterning with sub-diffraction-limited resolution has been demonstrated using a simple photolithography technique. Evanescent fields and surface plasmons are critical to the image formation, which is investigated here using computer simulations and experiments. A regime exists in which surface plasmons are resonantly excited, which we have named Evanescent Interferometric Lithography (EIL); period halving and reduced exposure times characterize this exposure mode. Two other exposure modes have been investigated in which surface plasmons on a planar metallic film beneath the mask are used to improve pattern formation. In the first, Planar Lens Lithography (PLL), a planar silver layer excited near its plasma frequency is used to form a projected near-field image. For a 40-nm thick silver layer, we predict that resolution down to 40 nm should be possible. However, the image is affected by the loss in the silver layer, the mask period, duty cycle and surrounding refractive index. Experimental verification of PLL is presented for 1-micron period structures imaged through 120 nm of silver. Finally, simulations are used to show that surface plasmons on an underlying silver layer can be used to improve process latitude and depth of field. We have named this mode Surface Plasmon Enhanced Contact Lithography (SPECL).