How can structures smaller than one micrometer be created? And how can even smaller structures of less than 100 nanometers be produced without great effort? Scientists at the Fraunhofer Institute for Laser Technology ILT have developed several technologies to answer such questions. They can use them to simulate, produce and measure periodic microstructures. They use phase-shifting transmission masks that can efficiently generate nanostructures down to 28 nanometers.
The EUV source at the Fraunhofer ILT delivers 40 W at 13.5 nm (+/- 1 percent bandwidth) andthe density of transistors on integrated circuits has doubled approximately every two years for over 50 years. Compliance with this law formulated by Gordon Moore is made possible by constant advances in microlithography. The technical progress behind this is immense. However, the system technology required to produce the smallest structures is now also immense. A microlithography system of the latest generation costs well over 100 million dollars and weighs 180 tons.
"We are developing technologies for the production of nanostructures that even start-ups and SMEs can afford," says Dr. Serhiy Danylyuk, Team Leader EUV and DUV Technology at Fraunhofer ILT in Aachen, describing the strategy of his working group.
Sub-micrometer structuring of surfaces with deep UV lasers
The basic idea is to generate periodic structures via interference effects of coherent radiation, such as the achromatic Talbot effect. This creates an intensity distribution in the near field, i.e. less than 500 µm behind a mask, which can be used to generate microlithographic structures.
Using a KrF excimer laser at a wavelength of 248 nm, structures with a period of several hundred nanometers can be produced. This was tested at the Fraunhofer ILT using a LEAP150K laser system from Coherent. Lines 180 nm wide can be generated in photoresist with a period of 600 nm. With higher energies of 250 mJ/cm², silicon can also be ablated on glass in these structure sizes. This technology is also well suited for ablating PET plastic surfaces on a 300 nm scale.
Discharge-based EUV source for structures smaller than 100 nm
The nanostructures with 300 nm (left, DUV) and 28 nm (right, EUV) half-pitch (HP) - the world's smallest structures, generated with a laboratory-based EUV source.the principle also works with wavelengths in the extreme ultraviolet (EUV). The Aachen-based company has developed its own beam source for this purpose, the "FS5440". Based on a gas discharge, the necessary radiation is generated at 13.5 nm. It is considerably more compact than the laser-based EUV source used in large industrial systems. Nevertheless, its performance is more than sufficient for many applications in the production or measurement of nanostructures.
The EUV source is equipped with power and spectrum monitoring. It generates up to 40 W at 13.5 nm in +/-1 percent bandwidth. The available intensity in the mask plane is more than 0.1 mW/cm². Wafers with a diameter of up to 100 mm can be processed in the Fraunhofer ILT system. In the test, structures with a structure size of 28 nm (half pitch) were produced using the achromatic Talbot effect. This is a world record for such a small system. In future, the resolution is to be increased even further to 10 nm.
Focus on medium-sized companies
Periodic nanostructures have various applications. For example, they are ideal for testing new photoresists in the EUV range. The technology developed at the Fraunhofer ILT also makes it possible to create complex geometries and structures. Computer-aided lithography methods are used for this purpose. In this way, nanostructured coatings can be built up on broadband reflective mirrors for high-power lasers or nanoantennas for special plasmonic structures.
The aim of the experts at Fraunhofer ILT is to achieve sustainable technology development: "It is important for us to be able to offer the complete process chain," says project manager Danylyuk. "That's why we have set up the processes in-house, from simulation and mask production to the measurement of surfaces and layers." This gives start-ups or interested SMEs access to this cutting-edge technology at reasonable investment costs. Further details will be presented at the online conference SPIE Photomask Technologies + EUV Lithography from September 21 to 25.
