Effects of Mask Materials on Near Field Optical Nanolithography
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Effects Of Mask Materials On Near Field Optical Nanolithography Sharee J. McNab1 and Richard J. Blaikie Department of Electrical and Electronic 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 Simulations have been performed to study the effects of different masking materials (Cr, Au, W and Si) on near field optical contact lithography. For near field amplitude masks it is found that Cr is an excellent material at blue/UV wavelengths, due to its small skin depth. Au is too transmissive to be considered. W has a small skin depth like Cr, although its dielectric behaviour at blue/UV wavelengths leads to image reversal phenomena that may be undesirable. Image reversal is even more pronounced for Si. For Evanescent Interferometric Lithography (EIL) - a technique that relies on surface-plasmon-induced resonant enhancement of the evanescent fields beneath a grating - Cr and W are again superior to Au, and significant intensity enhancements are predicted beyond the grating cutoff. For Si the resonant enhancements can be very large and narrow band, which may be of great advantage. For example, near field exposure intensity approximately 100 times the incident intensity is predicted for a 270 nm period silicon grating illuminated at a wavelength less than 10 percent higher than cutoff. 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. Nanometre scale resolution has been achieved experimentally for near field contact lithography [3,4] and simulations predict resolution down to 10nm [5]. In addition, an evanescent interferometric lithography (EIL) technique has been proposed for patterning periodic nanostructures [6]. In these techniques the details of the contact lithography mask geometry and mask material are critical, as the evanescent fields around the mask play a large role in the exposure. There have been simulation studies reported on the effects of the mask geometry [7], with a recent comparative study showing that an embedded amplitude masks (EAM) [8] gives superior transmission and contrast to a conventional metal-protruding mask (MPM), with further improvement possible using a light-coupling mask (LCM). The effects of mask material are not as well studied, even though this is just as important as mask profile. Only Cr and Au [8] have been used in studies of near field contact lithography using amplitude masks, and the effects of varying the properties of the dielectric material in LCM lithographies have not been explored. In this paper we will compare four different mask
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