One of the things that has kept Moore's Law rolling along is our ability to fashion chips with ever-smaller features. The masks that produce the features of a chip are made by a process called photolithography, in which light is used to etch a pattern on a photosensitive surface. The size of features you can create are proportional to the wavelength of light, so the formula for shrinking chips has been pretty simple: use shorter wavelengths. Unfortunately, with today's chips, we're running into the UV end of the spectrum, and running up against our ability to manipulate light at these wavelengths. Three papers released to Science Express describe techniques that might get us past this roadblock.

Building features with donut-shaped lasers

For two of the papers, the technique isn't actually new. Instead, the basic approach is similar to one developed for capturing microscope images of biological samples called STED (stimulated emission depletion) microscopy, which our own Chris Lee has written about extensively. STED produces images using fluorescent molecules by exciting the molecules with a narrow laser beam, and then hitting them with a second, donut-shaped laser that forces the fluorescent molecules back to the ground state. The net result is that the only molecules left glowing are the ones in the middle of the donut hole, and it's possible to make that hole smaller than the wavelength of the light involved.

Basically, if you replace "fluorescent molecule" with "photoactivated polymer," you get the gist of the new technique. The researchers used a chemical reaction that is catalyzed by light to produce a stable polymer. Using this technique on its own, it's possible to create a pattern that's roughly proportional to the size of the laser you use to start the reaction. The two research groups simply found ways of using the donut-shaped laser to shut the polymerization reaction down.

For the first paper, the technique is pretty simple. The authors created a mixture of chemicals similar to one that would work for a standard, light-active polymerization: it included the monomer form of the target polymer, a chemical that can be photoactivated in a way that would trigger polymer formation. But the mixture also contained a third chemical, activated by a second wavelength of light, that shuts this reaction down. The reaction was activated by the narrow laser beam as normal, but that light was accompanied by a donut-shaped laser in the wavelength that activated the inhibitor, which severely limited the scope of the polymerization. The technique allowed them to produce features much smaller than this sort of photo-activated polymerization normally allows.

Because this technique uses two separate lasers and different chemical reactions, it can be used to scan a pattern fairly quickly. The second paper takes a somewhat different approach, one that trades speed for even better resolution. Instead of using a chemical reaction that would proceed once a single photon was absorbed, they relied on molecules that had to absorb two photons at the laser's wavelength before reacting, a technique called multiphoton absorption polymerization, or MAP. MAP is slower than a single-photon reaction, but confines the reaction to the highest-intensity portion of the laser beam.

The authors then identified a molecule that underwent a slow polymerization reaction after being hit by two photons from the central beam. This allowed the authors to hit the sample with a short, femtosecond pulse of the central laser, and then insert a delay before hitting them with a donut-shaped laser at the same wavelength. By this point, any molecules that absorbed a single photon had lost it already, while those that picked up two hadn't started polymerizing yet. As a result, the donut shaped laser couldn't start new reactions, but could block those molecules that might polymerize from continuing their reaction.

Although it was slower, the technique produced impressive refinements in spatial terms: the features produced in the tests went down to 1/20th of the wavelength of the laser. The laser itself produced fairly long wavelength light (800nm), so the features were only on par with current cutting-edge processes. But the authors suggest that, with the right combination of lasers and reactant, they might be able to get it down to 10nm.

Building an optical gate

Paper number three has nothing in common with the first two other than the use of lasers. In this case, the authors used a combination of an interference pattern and a reversible, light-activated chemical reaction to get features down below anything Intel is currently shipping.

The technique relies on the chemical reaction shown at the top of this page. When hit with the longer wavelength of light, the molecule would adopt the form on the left, at which point it would absorb light from the shorter wavelength, which is in the UV range; when it did, the reaction is reversed. By adjusting the relative intensities of the two lasers, the authors could control the amount of short wavelength light that got through.

The authors then put a layer of this chemical on top of a compound that can be etched away by light at the shorter wavelength. They then exposed this setup to a set of overlapping interference patterns, one for each of the two wavelengths (these look a bit like two sine waves, where peaks represent high intensity light). The presence of the chemical acted like a gatekeeper, as the long-wavelength light kept it in a form that prevented the UV light from reaching the substrate it could etch.

The only place that the UV light could get to that substrate was at the troughs of the long-wavelength interference pattern. Even in those locations, the UV light wasn't strong enough unless its peak intensity was located precisely at these troughs. As a result, there's only a very narrow space where the UV light can get through the chemical layer and reach the etching substrate, smaller than the wavelength of the light. With their setup, the authors were able to create features 36nm in size, which is roughly a tenth of the wavelength of the incident light.

It's important to emphasize that these techniques are both at the initial demonstration phases, so they're not going to be put into use immediately. But that also means that there's the potential for some significant refinement if either of the approaches get commercialized. In either case, they serve as a clear indication that there may be ways to sneak well below the limits imposed by a light's wavelength.

Science, 2009. DOI: 10.1126/science.1167610, 10.1126/science.1168996, 10.1126/science.1167704