Optical Nanolithography Using Evanescent Fields
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Optical Nanolithography Using Evanescent Fields Richard J. Blaikie, Sharee J. McNab1 and Maan M. Alkaisi MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Electrical and Computer Engineering, University of Canterbury, Christchurch, NEW ZEALAND 1 Present address. IBM Thomas J Watson Research Center, PO Box 218, Yorktown Heights, NY 10598, USA ABSTRACT In the optical near field region the well understood resolution limits for projection optical lithography can be overcome. This offers the possibility of performing optical nanolithography without the need to use expensive, deep ultraviolet light sources. For exposures that utilise evanescent fields close to metallic amplitude masks sub-diffraction-limited resolution has been achieved experimentally, and the theoretical resolution limits have been explored using vector electromagnetic near field simulations. Resolution down to 20nm using exposure wavelengths greater than 400nm is predicted. It is also found that the exposure wavelength is of secondary importance in this regime, and that the properties of the mask are much more significant. Scaling to smaller feature sizes requires better resolution and control during mask manufacture, rather than the conventional (and costly) approach of driving the exposure wavelength deeper and deeper into the ultraviolet. Near field interference effects have also been explored, and the characteristics of spatial frequency doubling using Evanescent Interferometric Lithography (EIL) have been determined by simulation. Sub-diffraction-limited resolution can be achieved with increased exposure intensity compared with conventional interferometric lithography. The tradeoff is against the depth of field in the resultant interference pattern. Finally, the use of negative refraction and surface plasmons have been investigated to improve further the resolution in the evanescent near field, and to produce novel, three dimensional near field patterns.
INTRODUCTION The resolution of conventional projection optical lithography is limited by diffraction, making it extremely challenging to fabricate sub-100 nm structures even using deep UV light sources and advanced wavefront engineering. However, by working in the optical near field the conventional diffraction limit can be overcome and nanopatterning can be achieved. Whilst this idea is not new [1,2], it is only relatively recently that detailed studies have been performed. The first demonstrations of sub-wavelength resolution near field nanolithography used serial scanning techniques [3,4], however for the rest of this review paper we will concentrate on parallel-exposure techniques. Two near field lithography techniques have been developed using waveguiding or interference effects in transparent masks. Light-coupling masks (LCM) [5], using a conformal elastomeric material, consist of a surface relief pattern on a transparent mask. Flood exposure through this relief pattern induces near-field optical modes, and this waveguiding action amplifies the intensity i
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