In 1965 Gordon Moore carefully examined an emerging trend in electronics industry and predicted that the number of transistors in an integrated circuit would double approximately every two years. Moore’s law became the driving force in the semiconductor industry and resulted in the shrinkage of transistors lengths from 10 micrometers (size of a red blood cell) to tens of nanometers (size of a protein) over the last 40 years ensuring smaller, faster and less expensive chips used in various devices like PCs, smartphones and tablets. Such a tremendous progress in the computer industry wouldn’t be possible without light.
To modify surface layers of material at the atomic level to make transistors as tiny as possible one can exploit various methods of nanolithography to transfer the required patterns from a mask to a medium. In particular, optical nanolithography uses light to print features with nanoscale dimensions since according to the Rayleigh’s criterion, the minimal size of an object that can be printed with sufficient quality scales with the wavelength of light. To obtain finer chip patterns, the next-generation nanolithography employs extreme ultraviolet light (EUV) at 13.5 nm. In comparison with ultraviolet light used in most lithography machines today (near 200 nm, invisible to the eye), the resolution for EUV based lithography can in principle be higher by at least one order of magnitude, which could possibly enable printing objects below 10 nm.
However, photons at 13.5 nm have very high energy (92 eV), meaning that they are easily absorbed by any form of matter. Therefore, to design and build a 13.5 nm light source with sufficient output power is not an easy task. There are several methods to produce EUV light, but at ARCNL (Advanced Research Center for Nanolithography in Amsterdam) our focus is on so-called laser-produced plasmas (LPP). In brief, a high power laser is focused on a small droplet of tin atoms, which due to the interaction with the laser light shed their electrons and become highly ionized. These tin ions subsequently emit photons with numerous wavelengths including the required 13.5 nm light, which is collected by a highly reflective mirror and directed into the lithography system.
Many questions presently still remain about the fundamental processes underlying the laser-driven plasma formation, ionization and plasma dynamics, and subsequent EUV emission from the hot plasma. Nevertheless, the aforementioned process of EUV generation from tin atoms can reach a conversion efficiency of several percent. Laser-produced plasmas are of particular interest for EUV lithography and one of the goals at ARCNL is to understand and optimize the 13.5 nm wavelength production from LPPs.
To increase the conversion efficiency and to control the plasma we are going to use novel state-of-the-art laser sources, which combine high-power laser amplifier systems (optical parametric chirped pulse amplification), high-harmonic generation and pulse shaping methods. The motivation for making use of noncommercial, home-built laser systems originates from the desire to investigate laser-produced plasmas under unfathomable conditions. For example, we are interested in the light-matter interaction occurring on different time scales. By making use of ultrafast lasers generating pulses as short as tens of femtoseconds (10-15 s) we are looking at the onset of the ionization process in tin atoms. On the other hand by employing nanosecond pulses, which are a million times longer, we are investigating the plasma dynamics. The understanding of the fundamental processes underlying the plasma generation and its evolution will allow us to control the EUV production.
EUV lithography is a new and cutting-edge technique that is just now finding its way into the semiconductor industry. We are convinced that this technology will be essential for pushing the transistors size down to several nanometers.
Aneta Stodolna graduated from the Technical University of Gdansk and received her PhD in atomic physics from the FOM Institute for Atomic and Molecular Physics in Amsterdam. Her PhD research on atomic orbitals was listed in the Top 10 breakthroughs in physics in 2013 by Physics World magazine. Currently she works as a postdoctoral fellow in the EUV Generation and Imaging group at ARCNL.
Stefan Witte has a PhD in atomic and laser physics from VU University Amsterdam. He presently is group leader of the EUV Generation and Imaging group at ARCNL. His research interests include ultrafast and high-power laser development, EUV generation, plasma physics and coherent lensless imaging methods.
Kjeld Eikema has a PhD in atomic physics from VU University Amsterdam. He is a professor at the VU University (Ultrafast Laser Physics and Precision Metrology) and part-time group leader of the EUV Generation and Imaging group at ARCNL. His research interests include fundamental tests through precision spectroscopy, ultrafast and high-power laser development, EUV generation and lensless imaging methods.