OP: Research Zheng Xi and Aurele Adam on Tweakers

Our researchers have found a way to figure out whether a mask and wafer are aligned. Their findings are of importance for the chip production and may help to reduce the overlay problem.

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More information about research project can be found below:

New research makes ‘parking’ nanostructures on a chip easier

For decades, the building blocks that make up computer chips have been getting smaller and smaller. Interestingly, creating these nanostructures is now comparatively easy. ‘Parking’ them in the right spot is the difficult part. New research by Zheng Xi of Imaging Physics promises to help solve this problem.

Do you want a faster, more energy efficient smartphone? Yes, of course you do. Unfortunately, improving smartphones is getting increasingly more difficult. The main reason for this is that their performance is tied to the number of transistors we can cram onto a chip. And while there used to be plenty of room at the bottom, it’s getting pretty packed down there.

In fact, we are close to reaching the limits of Moore’s Law, which states that the number of transistors we can put on a chip doubles approximately every two years.

Etching with electrons

The challenge researchers and chip makers are facing has everything to do with the way transistors are made at the smallest scale. This is achieved by means of a process called lithography. Think of it as etching structures onto a surface using a tiny pen. Only instead of a pen, scientists use electrons or light. The video below explains the process in more detail.

In order to build the structures they need, chip makers use a so-called ‘mask’ that lets through electrons in some places and blocks them in others, as you can see in the video. The electrons that are allowed to penetrate the mask react with a polymer layer deposited on a wafer of silicon.

‘If you then attack this layer with aceton or another such chemical, the part that was hit with electrons will stay in place,’ says Dr. Aurèle Adam, Assistant Professor at the Optics group. ‘The rest of the polymer dissolves.’

The result: a series of nanostructures on which you can build another layer of building blocks. And then another. And another. Rinse and repeat, and you ultimately end up with a wafer full of functional chips. That is, if you managed to place the tiny building blocks in their exact designated spots.

Aligning the mask

The most difficult part of this process is aligning the masks in such a way that the nanostructures end up precisely where they need to be. This is called the ‘overlay problem’, and it is daunting indeed.

Adam: ‘The structures we are talking about are very small, say 32 nanometres. As a consequence, putting them in the right spot requires an unprecedented level of precision.’ Compare it to parking your car. The car itself may be two metres long, but parking it without bumping into your neighbour’s car requirescentimetre-precision.

The car in this metaphor is the wafer on which the nanostructures are built, since the optical system never moves. So how do you move a platform that has a wafer on it one nanostep at a time? It seems impossible, but this part, at least, has been figured out. Some materials expand when exposed to an electric charge. This property, called the piezo-electric effect, is used to move a wafer as little as one nanometre at a time. Keep in mind, a nanometre is a billionth of a metre.

But this method, though very precise, does have its limitations. ‘There is an uncertainty of  several nanometres when the mask moves back and forth with respect to the wafer’, explains Xi. You can tell a computer to move the wafer by ten nanometres. But after the piezo-electric effect has done its work, the platform could have moved fifteen nanometres, or maybe just five. Who knows?

Research comes in

Well, as it turns out, Dr. Zheng Xi knows. He has made use of a specially designed optical beam that has a tiny black dot at its centre. This special kind of beam has been employed in many so-called ‘optical super-resolution schemes’, the most famous example of which is the ‘donut beam’ used by the winners of Nobel Prize in Chemistry in 2014.

Xi simulated placing two golden nanorods on top of a mask and then used those rods as markers. When you aim the aforementioned laser beam in between these markers, electromagnetic radiation is produced in the far field. The amount of radiation on both sides of the field can be measured.

Aiming the laser beam at the very centre of the nanorods yields the same amount of radiation on the left and right side of the rods. By slightly moving the wafer that is positioned underneath the mask, the amount of radiation in the far field changes. ‘It’s like a scale’, says Adam. ‘We weigh how much light is left and how much is right, and by calculating the ratio between the two we know exactly how much we have moved the wafer.’

Math needs practice

Xi has done the math. Now his method needs to be tested in practice. Scientists of the Shenzhen University in China possess the equipment needed to set up a test and they will do so in collaboration with the ImPhys Optics group of the TU Delft.

Accurate Feeding of Nanoantenna by Singular Optics for Nanoscale Translational and Rotational Displacement Sensing by Zheng Xi Lei Wei, A.J.L. Adam en H.P. Urbach was recently published in Physical Review Letters.