Using a mask to unveil the hidden properties of EUV light

Published on February 24, 2023
Category EUV Generation & Imaging

When used in microscopy applications, extreme ultraviolet (EUV) light can be used for nanoscale imaging that also includes spectral information about the object under study.
However, to fully benefit it is crucial to be able to characterize and control the EUV light used. Researchers of the EUV Generation and Imaging group at ARCNL have developed a method to do so for broadband multi-spectral EUV sources. They designed a diffraction mask for EUV light and developed a computational approach to retrieve information about the light source from the diffraction patterns it creates. Their results offer possibilities to improve and control EUV light sources, paving the way for widespread use of EUV microscopy for nanoscience. On February 20, 2023 they publish their work in Optica.

Imaging in nanoscience or biology can be challenging, because visible light does not provide the nanometer scale detail needed, whereas imaging methods that do, like electron microscopy, are sometimes unsuitable because they need cryogenic cooling and careful sample preparation. EUV microscopy can fill the resolution niche between conventional and electron microscopy. On top of that, it can be used for spectroscopy measurements revealing the material properties of a sample, because EUV light interacts strongly with matter.

“The EUV light that we typically use has a very broad spectrum”, says postdoc Mengqi Du. “This is beneficial for use in spectroscopy and microscopy, because the more wavelengths we have in a light beam, the more information we can retrieve from a sample. However, to use this multi-colored light beam requires detailed information about intensity, wavelength and phase distribution in the EUV light source, which is not easy to retrieve.”

Revealing the properties of a pulsed EUV light beam that contains a broad spectrum of wavelengths: ARCNL researchers designed a diffraction mask behind which they record the diffraction pattern of the light. By moving the mask in several directions and recording multiple diffraction patterns, they collect data from which their computational method reveals properties of the light, like the wavefront shape (schematically depicted as blue circles)

Diffraction mask
Since EUV radiation is absorbed by almost every material on earth, researchers cannot use conventional optics like lenses and mirrors, but they can use diffraction.
“If we send EUV light through a mask with slits and holes, the light will bend and create a diffraction pattern that we can measure with a camera” Du explains. “We use algorithms to retrieve information from the diffraction pattern. This way we can both reconstruct the diffraction mask and retrieve information about the beam itself. With experiments and data analysis we were able to optimize the design for the mask and reveal properties of our EUV light source that were yet unknown.”

Tabletop EUV microscopy
Properties like the intensity distribution and phase information are important for microscopy applications of EUV light. Du: “In the end, for microscopy applications we would want to focus light and select the part of the spectrum we are interested in. Our diffraction mask allows us to have better control over the light. Also, because EUV microscopy is all about diffraction patterns, we will always need algorithms to retrieve information from our measurements. We have developed and refined the algorithm in such a way that we can also use it to retrieve information from the diffraction patterns of unknown samples. This takes us another step closer to the actual use of EUV light in tabletop microscopy.”

Mengqi Du, Xiaomeng Liu, Antonios Pelekanidis, Fengling Zhang, Lars Loetgering, Patrick Konold, Christina L. Porter, Peter Smorenburg, Kjeld S. E. Eikema, and Stefan Witte, “High-resolution wavefront sensing and aberration analysis of multi-spectral extreme ultraviolet beams”, Optica 10, 255-263 (2023).