Bauke Vermaas is a freelance science journalist.

23 January 2017

The Advanced Research Center for Nanolithography was established to investigate the advanced physics associated with EUV lithography. This summer, it published its first peek into the complex physics of firing lasers at tin droplets to generate EUV light.

ASML’s state-of-the-art nanolithography machines use microscopically small tin droplets to generate EUV light. Firing intense laser pulses at these droplets results in a very hot plasma that emits the desired light. ‘A laser-produced plasma is a good source for EUV light,’ says Oscar Versolato of the Advanced Research Center for Nanolithography (ARCNL). ‘The laser heats the droplet to a million degrees Celsius, to the point where the tin atoms shed off a large fraction of their electrons. Energetic processes, such as collisions, involving these electrons and their parent tin atomic ions lead to the emission of the desired EUV light.’

The light-generating sequence is not at all straightforward, however. ARNCL was established in January 2014 at ASML’s initiative for exactly that reason: to carry out fundamental research that has relevance for key technologies in nanolithography. The centre forms a public-private partnership between the Foundation for Fundamental Research on Matter (FOM), the Netherlands Organisation for Scientific Research (NWO), the University of Amsterdam, the Vrije Universiteit Amsterdam and ASML.

ARCNL, led by Dutch scientific heavyweight Joost Frenken, now employs about eighty people, and is still growing towards around a hundred researchers and support staff. ‘We investigate fundamental questions about processes and machines that ASML is currently producing or developing,’ Frenken says. ‘We also look at long-term possibilities and perspectives. The fact that we approach their R&D questions in a more fundamental and scientific way makes it a very fruitful collaboration. Our complementary expertise has already led to several patent applications.’

As one of the leaders of ARCNL’s EUV Plasma Dynamics group, Versolato set up experiments to understand the creation of EUV light from tin droplets. Working with research scientist Hanneke Gelderbom at the University of Twente and ASML, Versolato’s experimental study led to the first peek into the delicate yet violent process of generating EUV light from droplets. Their work resulted in the first scientific article to come out of ARCNL, published this summer in Physical Review Applied.

BCe24 save the date
ARCNL Druppelgenerator en plasmabron
ARCNL’s plasma EUV source housed in a stainless steel vacuum chamber. In the centre of the picture we see a plasma being generated from a tin droplet, producing both EUV and visible light.

Knockout

The formation of a laser-produced plasma from tin droplets in next-generation lithography machines occurs in two stages. Two laser pulses are fired at each droplet. The first pulse hits the droplet, creating a small plasma and causing the droplet to accelerate: in just a few dozen nanoseconds, the droplet’s speed increases from zero to a hundred metres per second (causing a g-force of a billon g). The acceleration is so powerful that the droplet also deforms radically into a disk-like shape. This deformation happens at a much longer, microsecond timescale.

A second, far more powerful laser pulse subsequently delivers the knockout punch: the deformed droplet disintegrates and transforms into an EUV-emitting plasma. ‘The first pulse needs to be very precise to produce the correct deformation, because that determines the ultimate efficiency of the entire process of generating EUV,’ says Versolato.

EUV ARCNL droplet
Shadowgraph images of tin droplets seen from the front (left section) and from the side (right section), when hit by the laser pulse (impinging from the right in the right section). The laser pulse induces a plasma. This plasma expands and pushes the droplet away from the laser. Consequently, the droplet acquires speed and deforms: it becomes as flat as a pancake.

Beautiful

Versolato’s group designed an experiment to study the physics underlying the propulsion and deformation of tin droplets in detail. The experiment approximates industrial conditions as accurately as possible for immediate relevance. ‘We generate tin droplets in much the same way as in an EUV lithography machine and hit them with a laser pulse with a focus width large enough to ensure the right pressure profile to deform the droplet into a thin flat sheet,’ Versolato explains. ‘We use a photographic technique called shadowgraphy to track the size, shape and velocity of the droplet.’

The researchers also tested a recently developed analytical model that describes the deformation of droplets when they are hit by a laser pulse. They demonstrated that the deformation of a tin droplet is very similar to that of a droplet of water under the influence of laser light.

‘There are two types of physics at play,’ explains Versolato. ‘In the first few nanoseconds, plasma physics describes the small plasma that accelerates the droplet and starts to deform it. This deformation is a thousand times slower, lasting for several microseconds, and is described by fluid dynamics. The fact that we’re unravelling fundamental physics in a system with direct applicability makes this work very exciting.’ Versolato now leads a new group at ARCNL – Atomic Plasma Processes – that dives even deeper into the plasma formed by a laser punch, in addition to his work in the EUV Plasma Dynamics group.

‘Although the physics of the acceleration differs, the fluid physics is completely scalable from water to tin,’ Versolato says. ‘Furthermore, the acceleration of the tin droplet was found to exhibit a beautiful dependence on the intensity of the laser pulse. These initial scientific results from ARCNL contribute to understanding the first step in the formation of laser-produced plasma for EUV light.’

Edited by Paul van Gerven