Lithography is the enabling technology behind almost all geek pursuits. Making chips that have more processing power, use less power, or have a smaller footprint depends on being able to form a sharp image of the small features on a silicon wafer.

The problem is that images are always blurred at some level, creating a lower limit to the size of the smallest feature. In a sort of Hail Mary approach to science, researchers began turning to the world of quantum mechanics for assistance. Amazingly enough, it turned out that entangled photons could help. In principle, imaging with groups of n entangled photons will give an image that is approximately n times sharper than possible using standard approaches. As with all bits of theory, though, this had a number of assumptions and potential gotchas. Now it seems that one of those gotchas is a show-stopper.

Lithography is the process of using an image to expose certain areas of a silicon wafer so that, when the wafer is exposed to chemicals, the exposed and unexposed areas react differently, creating the features that make up transistors. But images, at some level, are always blurred. The reason is that the lenses used to create the image have a finite size, and, to put it simply, have only a limited power to bend light. As a result, the light waves that pass through a lens are not perfect, suffering diffraction that blurs the focus.

The amount of blurring depends on how big the lens is, the power of the lens to bend light rays, and finally, on the wavelength of the light. In the end, it works out to a rather simple order of magnitude relationship: light waves focus to a disk that is about half the wavelength of light in diameter.

Faced with physics like that, engineers have taken the obvious approach: use shorter wavelengths of light. But this has run into a wall lately, where we are using the shortest wavelength of light that is still long enough to be transparent in the materials used to make the optics. And although manufacturers like ASML and Nikon have roadmaps and products that will overcome this barrier, researchers have been exploring alternatives.

One such alternative is quantum lithography, where we make use of correlations between photons to enhance the resolution of an image. Take the simple case of two slits in a piece of metal—this is the mask of the image we want to project—entangled photons that are incident on the slits pass through, interfere with each other, and are then detected. Now, if we only count photons that arrive simultaneously, we end up with an image that is sharper than simply detecting all the photons.

The key question is: should we expect entangled photons to always arrive at the same detector, or do they behave independently and have the chance to arrive at different detectors?

In the original conception of quatum lithography, people expected that, because the photons are entangled, they are highly correlated in both space and time. In other words, they would always arrive at the same detector at the same time. Later work questioned the validity of this assumption—after passing through the slit, they may remain entangled but propagate independently of one another. In other words, no matter where we place our detectors, we still observe a regular pattern of coincident photons, but they arrive at different detectors.

Does this matter? Very much so. For quantum lithography to work efficiently, both photons have to be absorbed simultaneously at the same location. If the photons are certain to propagate together, then the requirement for simultaneous arrival and absorption means that the exposure takes no longer than the exposure of a normal lithographic process (assuming all else is equal).

But if the photons can go anywhere, we have to wait for the simultaneous photons to occur by chance, meaning that the exposure time goes up as the square of the number of entangled photons. In other words, if you want to reduce your feature size by one half, you need entangled photon pairs and four times the exposure time; for one quarter sized features, you require four-photon entangled states and 16 times the exposure time. You can see that this scaling is very unfavorable.

The big question, then, is who is right? It turns out that an experiment that tests this had been done in a slightly different context, one where people investigated the arrival time coincidences as a function of spatial position after a beam splitting optic. The results indicate that the photons are not certain to arrive simultaneously at the same detector, meaning that exposure times do go up dramatically as resolution improves.

Does this sound the death knell for quantum lithography? Probably not. The truth is that the idea is too good to throw away immediately. Certainly, there may be ways around this and other problems, like the development of polymers that have the right properties for absorbing entangled photons efficiently and generating entangled photons efficiently. These are probably going to make it all worthwhile.

New Journal of Physics, 2011, DOI: 10.1088/1367-2630/13/4/043028

Listing image by Brookhaven National Laboratory