EUV structuring with minimized expense

For EUV lithography, chip manufacturers are using high-end systems. The Fraunhofer Institute for Laser Technology ILT in Aachen is working on EUV solutions for small and medium-sized companies.

Getting started with EUV lithography is associated with considerable investment for chip manufacturers. It costs €140 million for the systems of ASML, so far the only manufacturer. For this price, customers receive a lot of high-end technologies: The systems are as large as buses, weigh 180 tons and are fitted with hundreds of thousands of highly precise individual components. The laser systems from TRUMPF alone comprise 450,000 individual components. Four high performance lasers connected in series collectively generate an output of 30 kilowatts.

Due to the high costs, access to nanostructuring with extreme ultraviolet (EUV) radiation remains obstructed for medium-sized enterprises and start-ups in the long term. From a financial perspective, only a handful of semiconductor groups are able to make the technological leap from the previous 193 nanometers (nm) to 13.5 nm wavelengths and thereby realize structures only a few nanometers large on microchips and storage media.

Extreme and deep UV with greatly reduced cost

Researchers at the Fraunhofer Institute for Laser Technology ILT in Aachen are pursuing the goal of making EUV and deep UV (DUV) solutions for nanostructuring accessible to small and medium-sized companies. “We are developing technologies for the manufacturing of nanostructures that start-ups or medium-sized enterprises can afford,” says Dr. Serhiy Danylyuk, project manager for EUV and DUV Technology at Fraunhofer ILT. To achieve this, his team is creating periodic structures using the interference effect of coherent rays – similar to the principle of the achromatic talbot effect. British scientist Sir William Henry Fox Talbot discovered the effect in 1836: When light is cast through a periodic grid, its exact structure is visible behind in the near field in the distribution of brightness. The ILT researchers replaced Talbot’s grid with masks, through which they could send the rays of an excimer laser. In the near field – barely 500 µm behind the mask – this results in an intensity distribution of the light that is suited to the creation of nanolithographic structures.

In their experiments, the team used a LEAP150K laser system from Coherent. With a repeti-tion rate of 150 hertz at its 150-watt maximum output, the excimer laser creates pulses in the region of DUV wavelengths (248 nm). Using this, they were initially able to yield 180 nm-wide lines each 600 nm apart in photoresist with their Talbot approach. Background: The semiconductor industry uses nanolithography to expose photoactive paint layers on wafers. According to ILT, the procedure is also suited to the use of higher pulse energies in order to structure silicon on glass and on the surfaces of PET plastics that is precise to the nanometer.

EUV from a gas discharge beam source

In order to be able to yield even finer nanostructures on surfaces, the ILT team is also using EUV wavelengths of 13.5 nm. For this purpose, they developed their own very compact beam source (“FS5440”) that creates EUV radiation with an output of up to 40 watt based on gas discharge. As in ASML procedures, plasma hotter than 200,000°C is created, but without the laser bombardment of tin droplets. Instead, the discharge-produced plasma (DPP) process is based on the rapid discharge of large capacitors. In this process, the gas is inserted between two or more electrodes. According to ILT, the available intensity in the mask levels is more than 0.1 mW/cm². With this beam source, the team has since conducted experiments underlying the approach of the achromatic Talbot effect. In this series of experiments, they were al-ready able to achieve structure sizes of 28 nm for the exposure of wafers with diameters up to 100 mm. “This is a world record for such a small system,” report the Aachen-based researchers. Now they are striving to increase the resolution to as much as 10 nm.

Of course, this minimalist EUV lithography isn't considered competition to the high-end sys-tems in the semiconductor industry. Rather, the ILT systems are intended for use in testing new photoresists for EUV processes or incorporating nanostructures into broadband reflective mirrors for high performance lasers. The processes are also suited to the implementation of nano antennas for plasmonic structures. For this kind of computer-based nanolithography, the institute has drawn up a complete process chain from simulation to mask manufacture to the measurement of surfaces and coatings. In doing so, the team headed by Danylyuk is also providing start-ups and medium-sized enterprises affordable access to nanostructuring with EUV radiation.

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