Moore's Law is... well, a law of sorts. It's more of an observation, but one that has held true for quite some time. It's now so embedded in the psyche of the industry that failing to satisfy Moore's Law is thought to be the second sign of the apocalypse (the first was Mac OS X on x86).

Moore's Law is the observation that the number of features on a silicon chip doubles every 18 months. Silicon chips are made using photolithography, which is, at its heart, a process that involves making a photographic image of the circuit on a silicon wafer. The smallest feature is limited by how small a dot of light you can make with your imaging system, which faces a fundamental limit called the diffraction limit. Using a normal optical system, there is no getting around it. But, that doesn't mean you can't do an end-run around the diffraction limit, and a recent bit of research suggests how that might be possible.

Let's start with a basic photolithography recipe. Take one silicon wafer, coat liberally with a light sensitive chemical. Create a negative image of a circuit and use a slide projector to project that image onto the silicon wafer. The light causes a reaction that changes the solubility of the chemical. Use an acid wash to remove the unexposed parts of the wafer and start etching.

The trick that the researchers make use of in the new work is that the light-induced reaction can be manipulated. Essentially, the light causes the molecule to transition from an unexcited state to an excited state, whereupon the molecule becomes unstable and falls apart (or cross-links to form polymers). In doing so, its solubility changes. This is the basis of putting the circuit on the wafer.

But those excited states are not the only possible excited states. Others don't cause the molecule to fall apart. If you choose a wavelength of light that puts the molecule in a stable excited state, then an interesting phenomena can occur: Rabi cycling.

The light field hits the population of molecules and starts putting them into the excited state. If the light field is intense enough over the right length of time, all the molecules will end up in the excited state. If the light continues to shine, the molecules all start to cycle back down and back up, and on and on.

There are two important properties of Rabi cycling. First, if you choose a light pulse with just the right intensity and pulse duration, all the molecules end up sitting in the excited state. Second, the lower the intensity of the light, the longer the pulse duration needs to be. If you are using a laser beam, the intensity is highest at the center and fades away towards the edges. This means that if you choose a pulse duration and intensity that is correct for the center of the beam, then the entire population is in the excited state only at the center of the beam—at the edges, the population is not fully excited.

We can make use of this by taking a second light field that has the right color to take molecules from this stable excited state to the unstable excited state, where the molecule falls apart. How do we use that to provide smaller features? Umm, it's a bit complicated at this point.

We want to use this to create very small regions of excited molecules that we then hit with a second light source that causes them to react. To do this, we take the Rabi-cycling light beam, split it in two, and recombine it at an angle. This interference creates small stripes and, in a thin line down the very center of the stripe, there's a population of excited molecules. Hit that with the second light source, and we get features that are much smaller than the ones set by the diffraction limit.

Stripes don't sound very impressive, but this is no big deal. This sort of interference technique can be used to create any pattern at all, provided you split up the beam sufficiently many times.

The downside to the paper is that it is only calculations at this point. But they are important calculations, because as the researchers demonstrate, the resolution becomes independent of the color of the light. You could use microwave radiation to get patterns with features the same as those available from the light used in today's standard wafer steppers.

Of course, the imaging process is more complicated, involving two light sources, many-beamed interference patterns, and different photoresists. But, irrespective of its eventual practicality, wafer stepper makers are still contemplating the "next generation" lithography—using extreme ultraviolet radiation, vacuum systems, and other not-very-nice things—with a certain amount of distaste. Something new is going to have to be developed, so it might as well be a Rabi-cycling photoresist system.

On a more personal note, I like this research because it is very similar to something we published on imaging earlier this year. We still think our use is a bit crazy and a bit unlikely to work out, but, for photoresists, it makes a lot more sense. In any event, its nice to know that if we are crazy, we have company.

Physical Review Letters, 2010, DOI: 10.1103/PhysRevLett.105.183601

Listing image by Patryk Buchcik