Project number:  14HTSM05
Title: Exploring litho-induced mirror damage mitigation thresholds
Acronym: MiLiMiD

Executive organisational unit: XUV Optics, MESA+ Institute for Nanotechnology, University of Twente  
Programme management: Prof. Dr. F. Bijkerk (f.bijkerk@utwente.nl)

Project leaders: Dr. E. Louis (e.louis@utwente.nl), Dr. I.A. Makhotkin (i.makhotkin@utwente.nl).

Duration:  2014-2019

Researcher:  Igor Milov (i.milov@utwente.nl)] (PhD defense 09.09.2020), Dr. I.A. Makhotkin.

Industrial partner:  Carl Zeiss SMT

Objectives:

In the field of rapidly developing high-power X-ray light sources such as 4th generation synchrotrons and free electron lasers (FEL), one has to supply optical elements for such facilities that will survive the exposure to high doses of radiation. The MiLiMiD project aimed to understand various physical mechanism responsible for damage of materials exposed to high fluence ultrashort laser pulses. A detailed understanding should open a possibility to design optics sufficiently resistant to laser-induced damage.

Results:

To reach that goal, the project was separated into experimental and theoretical parts. Both parts aimed to develop a computational tool to model damage phenomena caused by ultrashort laser pulses with various wavelengths (from optical to hard X-rays) and various exposure regimes (single pulse and multi-pulse exposures).

Several damage experiments were performed at the Free electron Laser in Hamburg (FLASH) in the frame of a large international collaboration, with the goal to define damage thresholds of ruthenium and carbon thin films, silicon substrates and Mo/Si multilayers (all materials are relevant as optical elements for the XUV and X-ray range) at various experimental conditions. Experiments with an optical femtosecond laser were also performed.

Post-mortem analysis with optical microscopy, SEM, TEM and AFM revealed the details of damage morphologies, indicating that the process of ablation was responsible for the observed single-shot damage craters. In the multi-shot regime with fluence per pulse below the single-shot ablation threshold, but higher than the melting and cavitation thresholds, significant damage accumulation effects were observed. In the multi-shot regime with fluence per pulse more than 10 times lower than the ablation threshold only minor target surface modifications (oxidation) were detected, which resulted in an XUV surface reflectivity change of ~1%.

The simulations consisted of three parts: a Monte Carlo code to describe the photoabsorption and electron cascading processes, two-temperature hydrodynamics to describe temperature and pressure evolution in the regime of thermal non-equilibrium between electrons and lattice, and molecular dynamics with Monte Carlo electrons to describe the long timescale material evolution until complete recrystallization. The latter part enabled a one-to-one comparison with the experimental observations, explaining the details of the ablation process. A good agreement between simulations and the experiments validated the computational scheme and a complex parametrization of Ru.

Laser-induced ablation in Ru thin film simulated with large-scale molecular dynamics.

A wide range of incident photon energies (from optical to hard X-ray) was explored. A surprising similarity in Ru ablation observed for various light sources was explained based on detailed simulations of electron cascading after photoabsorption. The observed similarity in damage for a wide range of irradiation conditions can be exploited for other materials than the example of Ru. With that, one can possibly avoid numerous damage experiments at busy and expensive beam lines.

Follow up:

This project is continued as part of the X-TOOLS project, with a general focus on understanding fundamental properties of matter (metals, but also semiconductors) exposed to ultrashort light. As a particular example, we will study the kinetics of non-equilibrium cascading electrons in various materials. Such processes as photoabsorption, elastic and inelastic scattering of low-energy electrons and its emission from thin film materials will be investigated. 

Key publications:

1.       Milov, I., et al. "Two-level ablation and damage morphology of Ru films under femtosecond extreme UV irradiation." Applied Surface Science 528 (2020): 146952.

2.       Milov, I., et al. "Similarity in ruthenium damage induced by photons with different energies: from visible light to hard X-rays." Applied Surface Science 501 (2020): 143973.

3.       Makhotkin, I. A., et al. "Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single-shot ablation threshold." JOSA B 35.11 (2018): 2799-2805.

4.       Milov, I., et al. "Modeling of XUV-induced damage in Ru films: the role of model parameters." JOSA B 35.10 (2018): B43-B53.

5.       Milov, I., et al. "Mechanism of single-shot damage of Ru thin films irradiated by femtosecond extreme UV free-electron laser." Optics express 26.15 (2018): 19665-19685.

6.       Makhotkin, I.A., Sobierajski, R., Chalupský, J., Tiedtke, K., de Vries, G., Störmer, M., Scholze, F., Siewert, F., van de Kruijs, R.W.E., Milov, I. and Louis, E., 2018. Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold. Journal of synchrotron radiation, 25(1), pp.77-84.