Tiny computer chips are making computers smaller and more powerful. The conductor tracks of the most modern chips are only twelve nanometers apart. Researchers have now broken this lower limit with a new process. To do this, they modified the photolithography technique, in which conductor tracks are applied using ultraviolet light. By reflecting the light, the researchers created conductor tracks with a distance of just five nanometers. However, practical chip patterns are not yet possible with the process.
A single smartphone is now more powerful than mainframe computers, which filled entire rooms just a few decades ago. This is possible because the microchips for computers are becoming smaller and more powerful. Today, around 60 billion components can fit on the surface of a fingertip. The conductors that make up the circuits are placed close together – in the most modern chips, they are only twelve nanometers apart. In a single human hair, thousands of conductors could fit next to each other.
Tighter conductor tracks
To produce these fine structures, a process called photolithography is used in industry. The basis is the so-called wafer, a thin disk of silicon that acts as a semiconductor. A layer of photoresist is applied to this. If this is irradiated with light, its properties change and it becomes soluble or insoluble in certain solvents. In this way, the exposed or unexposed areas can be removed as required. In this way, the chip patterns can be “printed” with light. The shorter the wavelength of the light used, the finer the structures possible. The chip industry currently uses extreme ultraviolet radiation (EUV) with a wavelength of 13.5 nanometers.
A team led by Iason Giannopoulos from the Paul Scherrer Institute in Villigen, Switzerland, has now developed a technique that allows the conductor tracks to be arranged even more closely than before: at a distance of just five nanometers. The team also used EUV light with a wavelength of 13.5 nanometers. However, instead of directly irradiating the wafer, the researchers used mirrors. They used them to direct two parallel beams of light onto the wafer so that they overlapped and created a specific interference pattern. Depending on how the mirrors were aligned and the angle at which the light fell, different patterns of exposed and unexposed areas were created – twice as fine as would have been possible without the mirror technology.
No fundamental limit in sight
Electron microscopy images showed that the conductor tracks were well separated from each other even at a distance of just five nanometers. “Our results show that EUV photon lithography can produce extremely high resolutions, which suggests that there are no fundamental limits yet,” says Giannopoulos’ colleague Dimitrios Kazazis. “This broadens the horizon of what we think is possible and may also open up new avenues for research in the field of EUV lithography and photoresist materials.”
Theoretically, the new process could halve the current chip size. In practice, however, this method cannot yet produce complex patterns such as those required for computer chips. Since the interference of the light rays only produces simple, periodic structures, the winding, three-dimensional conductor tracks of the microchips cannot yet be produced using this method. The technology is therefore not yet directly relevant for industry. “However, this progress is of crucial importance for scientific and industrial research, as it takes into account the increasingly demanding requirements of nanoscience and future technology hubs,” says the team.
Source: Iason Giannopoulos (Paul Scherrer Institute, Villigen, Switzerland) et al., Nanoscale, doi: 10.1039/D4NR01332H