The ability of nanotechnology to manipulate systems down to the scale of individual atoms and molecules offers exciting potential for novel devices and materials. It is expected that significant progress in science and technology will be achieved. As a result, considerable interest in developing new nanofabrication techniques for manufacturing nanoscale structures has emerged worldwide. Techniques such as nano-imprint lithography, microcontact printing, immersion lithography and electron projection lithography offer alternatives to conventional electron beam nanolithography or recently developed EUV lithography. Many of these alternatives offer advantages of low cost and high throughput for mass production of nanostructured materials and devices.
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 nanoscale patterning can be achieved. Whilst this idea is not new 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 which is not suitable for high throughput lithography process, however in this chapter we will be presenting the implications of using evanescently decaying components in the near field of a photomask as a parallel lithography tool for the fabrication of nanoscale structures.
Both experimental and simulation results for evanescent near field optical lithography (ENFOL) will be discussed. The ENFOL technique allows for sub-diffraction-limited resolution to be achieved with optical lithography by keeping a shadow mask in intimate contact with an ultra-thin photoresist layer during exposure - this ensures that the photoresist receives exposure from the evanescently decaying diffracted orders in the near field of the mask. Features as small as 70 nm on a 140 nm period have been patterned using broadband UV illumination (365-600 nm), and subsequently transferred these patterns into silicon using reactive ion etching (RIE) (Blaikie et al. 1999; Alkaisi 2001). This resolution is below the conventional diffraction limit for projection optical lithography and illustrates the promise for extending optical lithography into the sub-100 nm realm. Simulation studies have shown that resolution down to 20 nm should be possible with this technique (McNab and Blaikie 2000), an enticing prospect for the nanolithographer.
The rest of this chapter will be structured as follows. We will first briefly review the historical development of near field contact lithography, outline the principles of the ENFOL technique and give details of the techniques required for the fabrication of conformable near field masks. Experimental results demonstrating the resolution that can be achieved with ENFOL exposure into ultra-thin photoresists are presented. Details of the subtractive and additive pattern transfer processes that have been developed are presented. Simulation results are also shown. This indicates that the resolution limits for ENFOL have yet to be reached experimentally. Finally, advanced near field lithography techniques such as evanescent interferometeric lithography, planar lens and surface plasmon lithography will be discussed.
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