Shadowgraphy technique sheds new light on vapors

Published on December 22, 2023
Category EUV Plasma Processes

ARCNL researchers developed a new optical method to study vapors with a high spatial and temporal resolution. In a paper in Applied Physics Letters, which was selected as the editor’s pick, the scientists demonstrate the potential of their method by measuring the temperature and composition of a tin vapor, a topic inspired by industrial sources of extreme ultraviolet light.

State of the art lithography machines make use of extreme ultraviolet light (EUV) to print extremely small chip designs on semiconductor wafers for the production of computer chips. This EUV light is created by firing a laser onto a pancake-shaped molten slab of tin to generate a light-emitting tin plasma. One of the challenges is to increase the efficiency of this process and thus the power of the light source.

Luc Assink (left) Dion Engels (right) in the EUV Plasma Processes lab. Credits photo: Ivar Pel.

At ARCNL’s EUV Plasma Processes group, alternative ways are studied to generate EUV light from tin. One of these is to first vaporize the tin, without creating a plasma right away. ‘To get an idea of how to optimize the amount of light that can be generated this way, you first need to know what happens inside the vapor. We’ve developed an optical method to look directly into the vapor,’ says first author of the paper, PhD student Dion Engels, who started working on this project as a MSc student from TU/e based at ARCNL.

From monochromatic to color photos
Engels and his colleagues expanded on an optical absorption technique they use in the lab, called shadowgraphy. Engels: ‘Shadowgraphy comes down to taking photos: you shine a light on the vapor and then measure what part of the light has been absorbed. This technique worked well, but was rather limited, since we only used green light. By using a different, tunable laser system, we were able to do the same type of measurements at a wide range of wavelengths, enabling us to go from monochromatic images to color photos.’

To demonstrate the potential of their imaging technique, Engels and his colleagues conducted a series of measurements on a tin vapor created by a series of laser pulses. A first laser pulse hits a microdroplet of tin produced by a droplet generator. This droplet is propelled by the laser and expands into a thin sheet. The tin is then vaporized by aiming a nanosecond-duration additional laser pulse at the sheet.

The researchers took a series of photos at 600 different wavelengths, ranging from 230 to 400 nanometers, so mostly in the ultraviolet regime. For each wavelength, 50 different measurements were collected. Though all of these photos were taken separately, they were taken at the exact same time after the vapor had been formed. By combining all of these photos it was possible to deduce the temperature, composition, and state of matter in the vapor.

Surprising first findings
In the paper, Engels and his colleagues report on their first findings with this method. ‘Much to our surprise, the temperature of the vapor turned out to be relatively uniform. What’s more, inside the vapor, we managed to see individual, free tin atoms and atoms that were clustered together into nanoparticles equally well. The ratio between these free atoms and nanoparticles turns out to be quite uniform throughout the entire vapor.’
Having this new optical diagnostic method, enables the researchers to now take the next steps. Engels says. ‘We are investigating what happens to the vapor if you change the vapor generation process parameters. How does a change in laser power affect the composition of the vapor? Is it possible to create a vapor where the properties are not uniform across the entire material? And what if we change the thickness of the tin sheet that we use to create the vapor?’

Broadly applicable
The developed method is not only applicable to study tin vapors, Engels stresses. ‘It is a generic diagnostic method that can be used to study any vapor. As long as you are able to get your detector close enough to the vapor you are studying, you can use this shadowgraphy technique to probe its temperature and composition.’ And where in this first paper, the authors ‘merely’ present static images of the vapor composition, it is also possible to capture changes over time, he says. ‘Simply take one photo after the other and put them together, and you’ve got a nanosecond movie that depicts the vapor’s evolution.’

D. J. Engels, R. A. Meijer, H. K. Schubert, W. J. van der Zande, W. Ubachs, and O. O. Versolato, High-resolution spectroscopic imaging of atoms and nanoparticles in thin film vaporization, Appl. Phys. Lett. 123, Issue 25, 12-18 (2023).
doi: 10.1063/5.0173871