Nanofabrication and Evanescent Near Field Optical Lithography

In the fifty years since Richard Feynman's eloquent assertion that there is Plenty of Room at the Bottom the world of nano-engineering has matured so that sophisticated systems can now be manufactured with nano-scale dimensions in large quantities at low cost-flash memory chips are a classic example, with their multi-billion-bit storage capacity on thumbnail-sized slivers of silicon.

Optical lithography has driven many of the advances in nano-scale manufacturing, with its ability to print ever smaller features as the technology matures. Lens-based optical systems are typically used, with improved resolution usually being achieved by lowering the wavelength-current systems use 193nm illumination from ArF excimer lasers. By applying some additional tricks, these sophisticated projectors can be pushed to print dense lines narrower that 45nm, which is less that 1/4 of that wavelength.

But these systems are highly advanced and expensive, so are only suited to high-volume manufacturing. And getting the resolution below 20nm introduces additional complexities and expense. There is an inverse law of nano-manufacturing that seems to be at play here, where the cost of the printing tool goes up inversely with the resolution. There is no better example of this that with the extreme ultraviolet (EUV) systems that are being tested for 20nm-scale chip manufacturing-these use 13nm wavelength x-ray illumination and state-of-the-art vacuum-based reflective optical systems, and the development costs to date for the engineering test systems has been many billions of dollars.

Is this inverse law immutable, or are there other ways to optically print nano-scale patterns in moderate volumes at low cost? One technique that has been explored by Richard Blaikie and his team at the University of Canterbury in New Zealand is Evanescent Near Field Optical Lithography (ENFOL). As a simple extension of contact lithography (Fig. 1), the expensive lenses are thrown away and the mask pattern is directly transferred onto the substrate-by keeping the mask and substrate in intimate contact we capture the evanescent light (Fig. 2) that is locked within the near-field region of the mask, and nanoscale resolution is possible even with relatively long wavelengths. Using the 436nm line from a mercury lamp Blaikie's team proved the concept by printing sub-100nm features and, more recently, Ito and co-workers at Canon Inc. have printed 32nm lines using a wavelength of 365nm.

Figure 1. The ENFOL process. A patterned mask is held in intimate contact with an ultra-thin photoresist layer. UV illumination generates high-resolution, near-field (evanescent) light that is captured by the resist.

Figure 2. The difference between propagating light and evanescent light. Diffraction of a propagating wave from a sub-wavelength grating produces exponentially decaying (evanescent) near-fields trapped within a wavelength of the grating. These contain the high-spatial-frequency information about the structure of the grating.

The contact requirement for ENFOL means that it will not be a direct plug-in replacement for the mature projection printers, but it certainly adds another viable technique to the nano-engineer's toolbox when it comes to knowing how to manufacture niche products at moderate volumes. It sits alongside techniques such as nano-imprint lithography (NIL) and dip-pen nanolithography (DPN) in this toolbox, which are already being used extensively for prototyping and manufacturing sophisticated nano-scale photonic, electronic or biosensing systems.

And there is more to be explored for ENFOL and related techniques. Recently Blaikie and others have shown that adding a silver 'superlens' to the mask can provide a much-needed protective layer above the substrate, whilst maintaining resolution. And techniques for using reversible bleaching in dye layers to project the high-resolution evanescent fields onto the substrate have recently been pioneered by Rajesh Menon at MIT (now at Utah), to combine the best features of ENFOL with the convenience and maturity of traditional far-field projection optical lithography.

So 50 years on, the words of Dr Seuss may be as prophetic as Feynman's here, with the line From near to far, from here to there, funny things are everywhere ... in his classic One Fish, Two Fish, Red Fish, Blue Fish (Beginner Books, 1960). Near or far, funny things are everywhere in the world of nano-scale optics, where the wavelength is no longer seen as fundamentally limiting the length scale at which light can be used for nano-engineering.