Conclusions

In the optical near field region the resolution limits for imaging are well below those for the far field. We have performed experimental and simulation studies of the implications of this for optical lithography, and in our ENFOL technique resolution below on fifth of the wavelength can routinely be achieved. Simulations indicate that there is scope to improve this resolution by at least another factor of two, which would allow sub-50 nm patterning without the need to use expensive deep-ultraviolet light sources; all of the experimental work presented here has been achieved with standard, low-cost mercury lamps.

The presence of surface plasmons on the metallo-dielectric interfaces in ENFOL-like exposures also opens a number of interesting possibilities. Firstly, the high electric fields associated with near-field plasmonic light confinement reverse the usual constrain of low intensity for near field imaging, showing that exposure times need not be sacrificed for resolution in the near field. Secondly, interference between evanescent orders can be used for interferometric lithography (EIL), resulting in exposed features smaller than the mask features; so near-field lithography is not solely limited to 1 x reproduction. Thirdly, the tuned plasmonic properties of a silver film exposed near the i-line of mercury can be used to 'project' a near field image (PLL); so the constraint of hard contact can potentially be relaxed. Finally, plasmonic effects in an underlying layer of a resist stack can also be used to improve the depth of focus. These techniques are still in their infancy, and further work is needed to explore their true potential.

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