Modern life would be very, very different if it weren't for photolithography, a very simple step in the processes for making an integrated circuit. Essentially, you use a slide projector to project an image of the circuit pattern on a wafer. Exposing the wafer to a light pattern modifies a chemical layer on top of the wafer, creating a mask. The mask allows selected parts of the wafer to be processed to create the circuit.

Despite its relative simplicity, photolithography is the limiting step that governs the rate at which power consumption drops, speed goes up, and the number of transistors increases. As you might imagine, a lot of people have spent a lot of time trying to improve or replace photolithography with a history of success.

There are worries those heady days may be nearing an end, however. Current photolithographic systems are linear optical systems, so the minimum feature that they can project is limited by the size and precision of the optics. Because all optics have a finite size, there is a minimum feature size that can be created, called the diffraction limit.

Current systems go beyond that with a number of tricks, such as changing the refractive index of the material the light travels through after it leaves the projection system, or by limiting the exposure time and double patterning, so each wafer is exposed twice with a slight offset between exposures. These tricks have given us factors of two to three in resolution improvement.

Unfortunately, there seems to be little room left for tricks now, and radical new technology must be introduced. A dark horse option is something near and dear to my heart: a modification to stimulated emission depletion imaging (STED).

STED imaging

STED imaging, as it is currently implemented, is totally useless for photolithography, but it does allow the diffraction limit to be beaten. In a STED microscope, one stains a sample with a fluorescent dye. To image the sample, you illuminate it with two lasers. One laser puts all the dye molecules in the focal point of the laser in excited state. If left to themselves, the dye molecules will relax by emitting a photon, leaving you with an image with diffraction-limited features.

But, the second laser doesn't leave the dye to itself. This laser has a special spatial profile that contains a dark area. When it illuminates the sample, it stimulates the dye molecules to emit, leaving only the dye molecules in the dark area in the excited state. They will emit after a bit of a delay. By scanning the dark spot over the sample, an image with a resolution that is the same as the size of the dark spot is created.

The best thing is that the size of the dark area is not governed by the diffraction limit—instead, it is governed by the intensity of the light beam: the brighter the laser, the smaller the dark area. In other words, if you want to see smaller features, crank up the laser power.

It didn't take people long to realize that this process might be applied to photolithography.

No one took the idea too seriously for two reasons. One problem is that, although you might be able to make features that are 6nm in size, those features will be separated by the diffraction limit. In other words, the feature density is too low. To overcome that, the wafer must be stepped a few nanometers at a time and the different parts of the circuit imaged at each step. If that doesn't sound like a nightmare, you have never tried to do precision mechatronics before.

The second problem is that you need two masks: the first mask is the standard circuit mask, while the second provides the nodes that thin out all the features to provide the resolution. Alignment of the masks and laser systems would be very, very difficult.

That is what makes this latest bit of research so interesting. It works with just a single mask, making the alignment procedure no more difficult than with current machines. The basic idea is the same, but instead of fluorescent dyes, they use a resist coating that will oxidize in the presence of ultraviolet light. The resist, however, has an unsual electronic state, which I will call a dark state, from which the UV cannot induce the resist to oxidize.

STED meets the dark state

This means that if you illuminate the wafer with UV and with a laser that puts the resist into the dark state, then only those resist molecules that are not in the dark state will oxidize. Indeed, if the laser is powerful enough, oxidation is prevented entirely. The mask, then, is simply designed to introduce dark spots into the laser illumination. The combination of the two light sources results only in exposed features where the laser light is dark.

There are still, unfortunately, a couple of catches. For instance, the mask cannot be an ordinary mask. It has to be produced in such a way that the dark areas get smaller as the laser intensity increases. In principle, it is possible to make arbitrary patterns that do this, but all practical implementations have been either lines—line patterns were used as the example in the work described here—or dots. So, I think there will have to be some thought given to the mask design.

The second problem is that the line spacing is still limited to something like half a wavelength. In the researcher's example, the closest line spacing was about 300nm, while their linewidth was less than 80nm. To get closer linespacing, you have to move the wafer, which is what the researchers did. When you think about 6nm features—6nm is the best resolution reported by STED so far—then, we are talking about 25 wafer steps just to expose one part of the wafer. If you consider the accumulation of positioning uncertainty, this represents a major technical challenge. For instance, a positioning accuracy of 0.06nm corresponds to an overall line position accuracy of 0.3nm in a 25 step process. That is likely well beyond the error budget allowed in current processes, let alone one that has a feature size of just 6nm.

In this proof-of-principle demonstration, the researchers showed a 78nm feature size—or more than twice that of the current processes. However, in this case, the resolution scales as the square root of the laser intensity, so increasing the laser intensity by a factor 6 (and that is not much to ask), will get the researchers to the same resolution as the current node. The final thing to note is that the oxidation process was a combination of a light controlled, voltage controlled electrochemical process. At present, it simply isn't very fast, with exposure times in the range of hours. The take home message: there is a lot of optimization to be done, but this looks hopeful.

Physical Review Letters, 2011, DOI: 10.1103/PhysRevLett.107.205501