Photolithography Computer chips can become even smaller

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Researchers at the Paul Scherrer Institute PSI have further developed the resolution of so-called photolithography. With their findings, they aim to help advance the miniaturization of computer chips.

The miniaturization of computer chips is a key factor in the digital revolution. It makes computers smaller and more powerful at the same time, enabling developments such as autonomous driving, artificial intelligence, and the 5G standard for mobile communications. Now, a working group led by Iason Giannopoulos, Yasin Ekinci, and Dimitrios Kazazis at the Laboratory for X-Ray Nanoscience and Technology at the Paul Scherrer Institute PSI has developed a novel technique that allows for even denser circuit patterns. The most advanced microchips currently have conductor paths only twelve nanometers apart—about 6000 times thinner than a human hair. However, the researchers created conductor paths that are only five nanometers apart. This allows circuits to be arranged much more compactly than before. "Our work demonstrates the potential of light to create patterns. This represents an important step both for industry and research," explains Giannopoulos.

Chips are created like the image in the cinema used to be

As recently as 1970, a microchip could accommodate only about 1,000 transistors. Today, it holds approximately 60 billion components on an area hardly larger than a fingertip. The production of these components is done using a method of exposure known as photolithography: A thin wafer of silicon is coated with a light-sensitive layer called photoresist. This is followed by an exposure that conforms to the chip's blueprint pattern, altering the chemical properties of the photoresist. This makes it soluble or insoluble in certain solvents. Subsequent processes then remove either the exposed areas (positive process) or the unexposed areas (negative process). In the end, the desired wiring pattern with the conductor paths remains on the wafer.

Crucial to the miniaturization and increasing compactness of chips is the type of light used. Physical laws state that the denser the structures can be packed, the smaller the wavelength of the light used. In the industry, "deep ultraviolet light" (DUV) has been commonly used for a long time. This involves laser light with a wavelength of about 193 nanometers. For comparison, the human-visible range of blue light ends at about 400 nanometers.

Since the year 2019, manufacturers have been using "extreme ultraviolet light" (EUV) for mass production with a wavelength reduced by more than ten times to 13.5 nanometers. This allows for the printing of even finer structures down to ten nanometers and below. At PSI, researchers use radiation from the Swiss Light Source SLS, which is tuned to the industry standard of 13.5 nanometers.

Photon-based lithography allows for the highest resolutions

The PSI researchers have expanded the conventional EUV lithography by not irradiating the sample directly but indirectly. In EUV mirror interference lithography (MIL), two coherent beams from two identical mirrors are reflected onto the wafer. These beams then create an interference pattern that depends on both the angle of the incoming light and its wavelength. The group achieved resolutions, meaning distances between the conductor paths, of five nanometers with a single exposure. The conductor paths revealed good contrasts with sharp edges under the electron microscope.

"Our results show that EUV photolithography can produce extremely high resolutions, suggesting that there are no fundamental limits yet," states Kazazis. "This is really exciting because it expands the horizon of what we consider possible and can also open new avenues for research in the field of EUV lithography and photoresist materials," says Kazazis.

From the end of 2025 in a new EUVL chamber

Currently, this approach is not of interest for industrial chip production because it is very slow compared to industrial standards and can only produce simple and periodic structures instead of a chip design. However, it does offer a method for the early development of photoresists needed for future chip production, with a resolution that is not possible in the industry. The team plans to continue its research with a new EUV tool at the SLS, expected by the end of 2025. This new device, in conjunction with the currently being upgraded SLS 2.0, will offer significantly more power and capabilities.