PTB-Seminar on VUV and EUV Metrology 2025

13 Oct 2025, 08:00 → 15 Oct 2025, 15:30 Europe/Berlin

Lecture Hall in Hermann-von-Helmholtz Building (PTB Berlin)

Alexander Gottwald (PTB), Michael Kolbe (Physikalisch-Technische Bundesanstalt (PTB), Berlin), Rebecca Hrascanec (7.1 Radiometrie mit Synchrotronstrahlung), Vivien Tritschler (7.2)

Scope

This year's seminar with the scientific sessions on October 14 -15 is the eighth in a series launched in 2011. It is a forum for interdisciplinary exchange between basic and technology-oriented researchers and industrial users. The topics cover the latest results from industrial applications of EUV radiation for lithography and measurement technology to developments for space-based VUV and EUV spectroscopy and the investigation of nanostructured surfaces. Following our seminar, PTB will host the workshop on "Quantification of Uncertainties in Nano-Metrology" (QUNOM) on October 16^th and 17^th, 2025.

Beforehand, on the afternoon of 13 October, we will be offering the opportunity to visit our Willy-Wien-Laboratory at the Metrology Light Source in Berlin-Adlershof. Registration with further data for the radiation protection office by Sep 30, 2025, necessary.

Previous seminars: 2011, 2013, 2015, 2017, 2019 , 2021, 2023.

Questions about further information may be addressed to: euv2025@ptb.de

Venue

The meeting will take place in the Helmholtz-Building of the Berlin-Charlottenburg campus site of PTB (Abbestr. 2-12, 10587 Berlin). The historical PTB campus is located within walking distance from Kurfürstendamm and Bahnhof Zoologischer Garten, right in the heart of Berlin's 'City West'.

Sponsors

The event is thanksfully supported by

         

EUV2025_Programm.pdf

Monday 13 October

16:00 → 18:30 Welcome and visit of the Metrology Light Source

Tuesday 14 October

08:30 → 09:30 Registration & Poster Set-up & Coffee

09:30 → 10:40 Opening & Keynote 1

09:30 Welcome, Opening and Overview

Speakers: Alexander Gottwald (PTB), Michael Kolbe (Physikalisch-Technische Bundesanstalt (PTB), Berlin)

10:00 Present and Future of EUV lithography

EUV scanners have been adopted worldwide for High-Volume Manufacturing (HVM) of 10-20 nm lithographic structures. Last year, the next generation of EUV scanners has started shipping, extending the resolution to feature sizes of below 10 nm.
Work has started on extending the resolution limits further by increasing the NA – and possibly even further by reducing the wavelength below 13.5 nm.

In parallel, the EUV source power has increased steadily, and this year the milestone of 1000 W has been achieved. While this enables high throughput and improves efficiency, both in terms of cost and in energy per wafer, this brings challenges to materials, manufacturing methods and cleaning.
Also in imaging, the increasing source powers require explicit consideration of heating and contrast loss due to spurious out-of-band wavelengths.

This presentation will give a comprehensive overview of the possibilities and challenges of EUV imaging, and the future directions being considered.
Also some of the physical and chemical interactions between the EUV photons and EUV-induced plasma with construction materials will be addressed, and the off-line testing requirements driven by those interactions.

Speaker: Mark van de Kerkhof (ASML)

Key1_MvdK _ EUV Lithography Present And Future _ handout.pdf

PTB_2025_ABSTRACT_MvdK.pdf

10:40 → 11:40 Session 1

10:40 EUV-induced oxidation of thin CrOx films on SiNk free-standing windows

For advancements in EUV lithography, the behaviour of thin film materials during EUV exposure in various background gas conditions is key to find the optimal material properties. Therefore, we investigated the EUV- and radical-driven degradation mechanisms of chromium oxide ($CrO_x$) cap layers (3-5 nm) on $SiN_k$ free-standing windows in a vacuum, hydrogen and water background. In this manner, the effects of EUV photons, and hydrogen and oxygen radicals can be decoupled. Additionally, the samples were thermally treated before EUV exposure to remove thermal effects due to the high power EUV beam.

During EUV exposure at the BESSY II synchrotron, the addition or removal of atoms is monitored using in-situ EUV transmission (EUVT) measurements. The EUVT was stable in vacuum and hydrogen background conditions, indicating that the $CrO_x$ cap layer is stable in high temperature and EUV (Figure 1). However, in a water background the EUVT decreased, which is typically explained by the oxygen absorption resulting in an increase in EUV absorption (EUVA). To calculate the EUVA, the EUVT and EUV reflection (EUVR) were measured ex-situ vs wavelength. Contrary to expectations, the EUVA did not increase, instead, the EUVR increased.

To investigate what processes occurred, the depth-dependent chemical composition was analysed with angle-resolved X-ray photoelectron spectroscopy (AR-XPS). First, the effect of the thermal treatment was investigated, which showed that the $CrO_x$ cap layer intermixed with the underlying $SiN_k$ layer during the thermal treatment. Therefore, the cap layer is not purely $CrO_x$ but an intermixed layer of $CrSi_xO_yN_z$.

AR-XPS analyses of the EUV exposed samples revealed that a $SiNO_2$ peak was formed in EUV + water but not in EUV + hydrogen. This $SiNO_2$ was formed in the intermixed cap layer. The oxygen probably originates from the water, but the EUVA did not increase because the Si and N content at the surface decreased. This probably desorbed as $SiH_x$ and $NH_x$ or $NO_x$ (Figure 2). So, explaining the EUVT decrease is more complex than only oxygen addition to the cap layer.

To test this hypothesis, the cap layer composition will be modelled to explain the EUVR increase. Additionally, offline oxygen radical exposures will test if these radicals also form $SiNO_2$ without EUV.

Speaker: Duncan Ramsamoedj (University of Twente)

EUV2025_abstract_CrOx_DR_XUV.pdf

11:00 Latent images in EUV photoresists measured with a lab-scale EUV scatterometry setup

The authors present their recent work on the identification and characterization of latent images in EUV photoresists using a small-scale EUV reflectometry and scatterometry setup at RWTH Aachen University. Optical metrology methods like scatterometry can identify latent images, if the photochemical changes in the resist lead to an optically detectable change of the material or a variation in the photoresist thickness [1,2]. In previous studies it was shown that these detectable changes might especially occur after a post exposure bake where the resist tends to shrink differently for exposed and unexposed areas [2].
In this experimental study, samples with the latent image of a line grating in an EUV photoresist are prepared by electron beam lithography and measured with EUV reflectometry and scatterometry. The samples are treated with a post exposure bake and one of the samples is developed to serve as a reference by providing the actual resist structure after development. Furthermore, two unstructured samples are prepared: one unexposed, the other exposed over the full surface at nominal dose-to-clear of the photoresist. These two samples are measured by EUV reflectometry to characterize the optical constants of the photoresist at the EUV spectral range which allows to identify if the chemical changes in the resist are detectable.
Additionally, in a preliminary study the influence of an EUV dose induced by the measurement itself is evaluated. It is shown that the additional EUV dose on the tested photoresists after the post exposure bake has a reduced influence on the solubility, making EUV scatterometry a feasible inspection method for latent images.

[1] Q. Zhang, K. Andrle, W. Chao, Z. Peng, W. Holcomb, R. Miyakawa, D. Kumar, A. Hexemer, P. Naulleau, B. La Fontaine, R. Ruiz and C. Wang, 2024, JM3 23(04), 044003.
[2] S. Schröder, L. Bahrenberg, B. Lüttgenau, S. Glabisch, S. Brose, S. Danylyuk, J. Stollenwerk, P. Loosen and C. Holly, 2022, JM3 21(02), 021208.

Speaker: Sophia Schröder (RWTH Aachen University - Chair for Technology of Optical Systems TOS)

EUV2025_abstract_Schröder.pdf

11:20 Quantitative characterization of angstrom-scale roughness via diffuse scatter at near-grazing geometry using a table-top 92eV HHG source

Extreme ultraviolet (EUV) and soft X-ray radiation can be efficiently utilized for non-destructive characterization of nanostructures with nanometer accuracy. Conventional scatterometry enables structural characterization of periodic structures [1] while diffuse scatter measurements allow to characterize surface and interface roughness [2]. In this work we demonstrate results of the first time (to the best of our knowledge) angstrom-scale surface roughness characterization from diffuse scatter with a table-top EUV high harmonics generation (HHG) source.
Roughness characterization was performed on a Ru thin film generated by ion beam deposition (IBD). The schematic of the experimental setup is shown in Fig. 1. Direct registration of diffusely scattered 92eV light was realized by utilizing synthetic aperture and high-dynamic-range detection coupled with noise suppression techniques.
The experimentally obtained spatial intensity distribution of scattered EUV light was fitted with 3 Gaussians in order to extract the intensity and width of high-frequency scatter (Fig.2). From these parameters roughness and correlation length values were derived.
We present promising results that demonstrate angstrom-scale precision and are in good agreement with AFM measurement (Table 1).
[1] L. M. Lohr, R. Ciesielski, S. Glabisch, et. al., 2023, Appl Opt 62(1), 117.
[2]. I. Busch and J. Stümpel, 2003, Appl Surf Sci 212–213, 201–203.

Speaker: Dr Vitaly Krasnov (imec)

EUV2025_abstract_Krasnov.pdf

11:40 → 12:00 Group Photo

12:00 → 13:00 Lunch

13:00 → 14:30 Postersession

13:00 Reminisce - Refurbishment of Mirrors to Increase Sustainability at Light Sources - Introduction of a European collaboration project

The degradation and Carbon contamination of X-ray optics is a well-known problem in X-ray beamlines that has been detrimental to beamline performance for several decades. This is particularly problematic, as such optics are usually highly specialized and coated with various materials. Consequences of such contamination include loss of flux, distortion of the wavefront, and reduction in focusing power and resolution.
With new and upgraded light sources having much higher photon intensity and repetition rates that are built in several places in the world, more rapid accumulation of contamination and damage will occur. This currently places X-ray optics amongst the most expensive consumables at Synchrotrons and XFELs, in terms of cost and beamline downtime. The cost per optic is typically tens of thousands of Euro and can exceed 100k Euro. With that and production times up to more than 1 year, the option of refurbishing and cleaning X-ray mirrors to overcome this problem is substantial.
This has been discussed within the community for a long time and on several occasions in metrology and X-ray optics meetings, emphasizing the need to find suitable methods. Numerous trials, independently conducted in recent years by European experts in X-ray optics, have been made. Various treatments to remove organic residues from the samples have been tried, including: adding a partial pressure of oxygen into the vacuum vessel; illumination with UV light or plasma treatment; chemical stripping of contaminated optical coatings; or repolishing the substrate. Research has progressed slowly, on an ad-hoc basis, with minimal funding. Due to a lack of time and samples, extending those single tests to systematic studies is difficult. First experiences showed that cleaning techniques that work for one mirror aren’t effective for a nearby optic, pointing to hidden complexities, such as temperature, X-ray flux and wavelength spectra, and proximity to nearby sources of carbon (such as motors or cables). These issues clearly show that coordinated action is now required.
For that purpose, a workshop dedicated to “Cleaning and Refurbishment of Optics” was held at Trieste in February 2025. It was attended by many experts representing most synchrotron and XFEL labs in Europe and the collaboration project REMINISCE (Refurbishment of Mirrors to Increase Sustainability at Light Sources) has been. In a large and coordinated group effort of European experts from various relevant scientific disciplines, REMINISCE aims to:
- Determine essential experimental variables for dynamic growth and in- and ex-situ removal of carbon contamination on X-ray optics.
- Develop reliable methods to clean and refurbish X-ray optics to a pristine condition for optimal beamline performance, underpinned by surface science and optical metrology feedback.
- Develop processes compatible with full-size, beamline X-ray optics (e.g up to one metre in length).
- Share Reproducible technical recipes and community-wide recommendations
The project and first results of preliminary studies will be presented here.

Speaker: Silja Schmidtchen (European X-Ray Free Electron Laser Facility)

PTB-EUV_2025_Abstract_Schmidtchen.pdf

13:06 New online polarization diagnostics for 3rd harmonic afterburner radiation

Measurements of the photoionization of rare gases have been an essential method for photon diagnostics at FLASH since the start of operation in 2005. Particularly at FLASH2, photoelectron TOF spectroscopy has been used for non-destructive monitoring of the FEL wavelength. With the online-photo-ionization spectrometer OPIS – a 4-channel eTOF spectrometer device – information about beam properties can be provided to the FLASH users as well as machine operators. In particular for the special two-color mode of FLASH2, we could demonstrate its ability for efficient monitoring of both wavelengths.
Recently, an APPLE-III type undulator has been added to the radiator beamline in FLASH2 to enable variable polarization of the FELs 3rd harmonic. With an afterburner scheme FLASH2 can now generate increased output power in a high photon energy range up to 860 eV with selectable polarization between the two linear and two circular modes. Here again photoelectron spectroscopy has been applied for polarization diagnostics with a 16-channel eTOF spectrometer instrument by measuring the angular distribution of photoemission. Results from the experimental studies of the afterburner undulator commissioning will be presented.

Speaker: Markus Braune

13:12 In-situ Cryogenic Cleaning of Tin-Contaminated EUV Optics

Norbert Böwering(1) and Christian Meier(1)
Bowering@physik.uni-bielefeld.de
(1) Bielefeld University, Universitätsstraße, 33615 Bielefeld, Germany

Considerable tin-splash contamination can occur on multilayer-coated optics when used with tin-based plasma light sources in EUV lithography applications. Generally, thick tin deposits cannot be removed sufficiently fast by plasma etching; this requires the development of alternative cleaning techniques. We have investigated cryogenic methods for substrate cleaning.
For high-purity tin drops of ~3 mm diameter, deposited onto various samples, we have studied in-situ cleaning concepts based on the initiation of tin pest leading to a phase transformation during cooling followed by considerable volume expansion and embrittlement of the tin splash. As substrates, both unstructured and structured silicon-wafer and multilayer-coated mirror samples with up to 6 inches in diameter were examined, as well as a thick solid piece of silicon carbide. The temperature-dependent sticking behavior of tin splashes deposited in a vacuum chamber was analyzed on multilayer-coated samples with different cap layers. During subsequent substrate cooling to temperatures of -30 °C and below, initially adhesive deposits were fully converted in-situ to removable brittle gray tin in less than 24 hours. Typically observed growth rates of the α-Sn phase were 10-15 µm/min.
After removal of the detached tin pieces on a Mo/Si-coated sample, analysis of EUV multilayer reflectivity at PTB Berlin showed a reduction by only 0.5 % at a wavelength of 13.5 nm. A ~10 mm diameter splat on a Mo/Si-coated sample with 3 nm thick ZrO2 cap layer was converted to gray tin in less than 9 hours. The tin-splash transformation on a grating-structured Si-wafer sample was also studied with similar conversion results [1]. In addition to phase transformation, other alternative cryogenic cleaning methods, using strong cooling to temperatures of -120 °C and below, both in-situ and ex-situ, and also ex-situ cleaning by exposure to a high-pressure jet of CO2-snow flake aerosols, were analyzed.

[1] N. Böwering, Ch. Meier, J. Vac. Sci. Technol. B 38, 2020, 062602.

Speaker: Dr Norbert Böwering (Fakultät für Physik, Universität Bielefeld)

EUV2025_abstract_Böwering_Poster.pdf

13:18 Ultra-compact inline transmission grating spectrograph for extreme ultraviolet wavelengths

For the measurement of the spectral distribution in a broadband wavelength range, spectrographs are typically used. Depending on the target application, the spectrograph can be designed for reflection or transmission mode. Although reflective spectrographs generally offer a higher resolution due to the possibility of applying aberration corrections and focusing options, the main advantages of a transmission grating spectrograph are its compactness and the possibility of inline positioning while still offering sub-Ångström spectral resolution if designed carefully.
In this contribution the design, realization, and characterization of an EUV transmission grating spectrograph (EUV-TGS) are presented. The main module of the realized EUV-TGS comprises a spectral and spatial filter and a high-resolution transmission grating with a period of 80 nm. All components were fabricated in-house using multi-step cleanroom processes. For high resolution spectral monitoring in the target wavelength range of 10 nm to 20 nm, a two-dimensional detector is positioned in proximity to the main module. The overall dimensions of the EUV-TGS have been minimized to 25 x 25 x 40 mm³, allowing for simplified integration into existing beamlines and experimental setups. The realized prototype demonstrates a spectral resolution of 0.08 nm, perfectly suited for the detailed investigation of EUV radiation sources.
In our use case, the EUV-TGS prototype is integrated in an interference lithography setup for monitoring tasks, demonstrating that this ultra-compact spectrograph can easily be introduced into experimental setups that will benefit from an inline spectroscopic diagnostic option.

Speaker: Sascha Brose (RWTH Aachen University - Chair for Technology of Optical Systems TOS)

EUV2025_abstract_Brose.pdf

13:24 Stand-alone extreme ultraviolet (EUV) metrological system for optical characterization of lithography materials

With the rapid growth of data-intensive applications such as autonomous driving and artificial intelligence, the performance requirements of semiconductor devices have significantly escalated. In parallel, the critical dimensions of circuit patterns have been reduced to the nanoscale, necessitating the adoption of extreme ultraviolet (EUV) lithography to overcome the resolution limits of conventional ArF-based photolithography [1]. As line widths shrink below the resolution limits of traditional methods, the development of key EUV-compatible materials—such as photoresists and pellicles—has become imperative. Accurate characterization of these materials at the EUV wavelength of 13.5 nm is essential; however, most EUV metrology facilities rely on synchrotron radiation sources, which are limited in availability and accessibility. This underscores the need for a compact, portable EUV measurement system capable of supporting industrial-scale evaluation demands.
In this study, we present the development of a miniaturized EUV characterization platform employing a Z-pinch plasma EUV source as shown in Fig. 1. The system comprises an EUV light source, an optics chamber with a high-reflectivity optical assembly, and a sample chamber for sample evaluation. Discharge parameters were optimized to maximize emission at 13.5 nm, achieving an optical intensity of 3.5 W/cm². Using this setup, we performed pattern transfer verification on a metal-oxide resist (MOR)-based EUV photoresist sample with a line/space (L/S) structure, enabling quantitative assessment of the resist's sensitivity and contrast. Furthermore, by measuring EUV power variations, we validated the system’s capability to evaluate EUV transmission properties of pellicle candidates.
The developed system offers a promising solution for the comprehensive optical evaluation of EUV-specific materials and is expected to facilitate rapid material screening and iterative development in EUV lithography processes.

Speaker: Wooram Kim (Korea Research Institute of Standards and Science (KRISS))

EUV 2025_W.Kim(KRISS)_Stand-alone extreme ultraviolet (EUV) metrological system for optical characterization of lithography materials.pdf

13:30 Influence of experimental conditions on EUV / X-ray fluorescence yields investigated with the WDSX-300

The WDSX-300 spectrometer, developed at the Institute for Applied Photonics (IAP) in cooperation with Nano Optics Berlin (NOB), has been designed for wavelength dispersive X-ray fluorescence analysis with a scanning electron microscope (SEM-WDX) in the EUV and soft X-ray range. With the currently installed setup, an energy range from 30 eV to 400 eV can be covered, including K fluorescence lines of elements like Lithium [1] or Boron. While working in the EUV range, experimental conditions are important: the sample damage from the X-ray excitation and the short propagation range of the X-ray fluorescence in the sample must be considered. Accordingly, the influence of the incidence angle, dose and kinetic energy of the electron beam on X-ray fluorescence (intensity and line shape) has been investigated.
X-ray fluorescence is produced through excitation by an electron beam in a JEOL 6400 SEM (beam focus size 4 µm – 20 µm, current ~ 200 nA). The emitted X-rays are then analysed in the WDSX-300 [1]: three hybrid reflection zone plates (h-RZPs) focus X-rays from the sample to straight lines on a CCD camera (greateyes GE 2048 512 BI UV1, 2048 x 515 pixels at 13.5 µm). High sensitivity in the extreme ultraviolet (EUV) / soft X-ray range is achieved by a wide sagittal acceptance, high diffraction efficiency and the focussing effect of the RZPs, while the spherically curved substrate [2-3] (radius 380 cm) provides the necessary spectral range. The compact layout of the WDSX-300 (309 × 156 × 165 mm³) makes it suitable for tabletop spectrometers, laboratory setups and an add-on to beamlines at large-scale facilities.
[1] K. Hassebi, N. Rividi, M. Fialin, A. Verlaguet, G. Godard, J. Probst, H. Löchel, T. Krist, C. Braig, C. Seifert, R. Benbalagh, R. Vacheresse, V. Ilakovac, K. Le Guen, and P. Jonnard, 2024, X-ray spectrometry 54(2), 76-85.
[2] J. Probst, C. Braig, and A. Erko, 2020, Appl. Sci. 10(20), 7210.
[3] C. Braig, J. Probst, E. Langlotz, I. Rahneberg, M. Kühnel, A. Erko, T. Krist, and C. Seifert, 2019, Proc. SPIE 11109, 111090U.

Speaker: Valentin Stoytschew (IAP-Adlershof e.V.)

Abstract_Stoytschew.pdf

13:36 Enhanced thin film characterization through combined S- and P-polarized EUV reflectometry

This work investigates the potential benefits of employing combined s- and p-polarized extreme ultraviolet (EUV) reflectometry for the accurate and efficient determination of thin film optical constants and geometrical parameters. EUV reflectometry (EUVR) is a powerful non-destructive technique for characterizing thin films, which is crucial for advancements in fields such as lithography and nanotechnology.
Our study compares three distinct data analysis approaches: utilizing s-polarized, p-polarized, and a simultaneous combination of both s- and p-polarized EUV data. Through iterative calculations, we observe that while s-polarized data can yield reliable results, convergence speed is often an issue. Analysis of p-polarized data alone appears to offer increased sensitivities and, in some instances, can accelerate the convergence.
The most promising results are achieved when combine s- and p-polarized EUVR data simultaneously. This combined approach appears to lead to a faster convergence rate including reduced uncertainities.
Our findings suggest that the simultaneous utilization of s- and p-polarized EUVR data could offer an effective methodology for rapid, robust, and accurate thin film characterization.

Speaker: Samira Naghdi (Physikalisch-Technische Bundesanstalt (PTB) Berlin)

SNaghdi_EUV_VUV_Seminar.pdf

13:42 Investigation of the Optical Constants of Amorphous SiO₂ and Y-Cut Quartz from the Extreme to the Vacuum Ultraviolet Spectral Region

Fused silica (amorphous SiO₂) and quartz crystal both feature ultra-low thermal expansion and exceptional transparency from the visible to the deep-ultraviolet. Quartz additionally offers strong piezoelectricity and pronounced birefringence, making both materials cornerstones of modern optics. Yet their optical properties in the extreme- and vacuum-ultraviolet (35–140 nm) remain largely unexplored because metrology in this spectral region is highly demanding; consequently, reliable optical constants of these two SiO₂ polymorphs are still scarce.
To fill this gap, we carried out the angle-resolved reflectometry of thermally grown amorphous SiO₂ and Y-cut α-quartz from 36 nm to 140 nm at a synchrotron beamline. A transfer-matrix model, solved with Markov-chain Monte Carlo sampling, yielded n and k for the ordinary and extraordinary crystal axes together with their relative uncertainties. Two sharp resonances were resolved in quartz on each optic axis whereas amorphous SiO₂ displays only a single, broader resonance matching the feature on quartz’s ordinary axis. The crystalline phase shows noticeably higher absorption at this wavelength, consistent with its greater oscillator strength. Although the materials are bulk-opaque below ≈130 nm, their complex refractive indices still govern reflection, scattering, and absorption in emerging EUV/VUV technologies.

Speaker: Najmeh Abbasirad (PTB)

Abstract_Abbasirad.pdf

13:48 Simulated XUV spectroscopy in the laboratory with a polycapillary half lens and reflection zone plates

We design a wavelength dispersive spectrometer for the extreme ultraviolet, based on a collimating polycapillary lens (PCL) and an array of reflection zone plates (RZPs). The two-channel instrument is optimized for narrow-band fluorescence or inverse photoelectron spectroscopy around 15 eV and 36 eV, respectively. The halved PCL is composed of $\sim 10^{6}$ tapered borosilicate glass tubes with a mean diameter of 5 µm. The device widens the effective, accepted solid angle for photons from the micron-sized source by about one order of magnitude and collimates the incident radiation to a quasi-parallel beam of zero divergence, which is diffracted into the $(+1)^{\textrm{st}}$ order by one of the RZPs. These holographic 2-D varied-line space gratings are fabricated with a Carbon-coated, laminar profile on a common, planar Si substrate. The spectra, recorded by an, e.g. CCD camera, feature a relatively high resolving power $E/\Delta E\approx 70\pm 10$, as shown in Fig. 1. With its short optical path length of less than 0.9 m, the spectrometer fits in typical laboratories. The spectrometer operates in the parallel, flat field mode, thus no moving parts are required.

Speaker: Jürgen Probst (NOB Nano Optics Berlin GmbH)

euv-2025-abstract.pdf

13:54 X-ray standing wave coherence length in grazing exit geometry

Angle-resolved X-ray fluorescence is a powerful tool for elemental depth distribution characterization of thin films. This technique is based on the formation of an X-ray standing wave (XSW), which is strictly dependent on the coherence length of the propagating wave. In grazing incidence geometry finite coherence length leads to a wave modulation which reduces interference fringes contrast [1]. Coherence length in modern lab-based X-ray sources is mostly influenced by transverse coherence length which is given by an angular divergence.
In grazing emission geometry a fluorescent specimen radiates spherical waves in all directions. In the far field, the wave can be considered as a plane wave and XSW is formed due to interference effects in the structure. As such, the signal is smeared only by the detector strip opening angle. We assume that the incident wavefield transverse coherence length doesn’t take part in XSW formation.
To check this assumption, we investigate the effect of transverse coherence length in both grazing incidence and grazing emission geometries by varying the beam footprint on a [W/B4C/Si/B4C]x50 multilayer sample. We show that, while in grazing incidence the experimental features “smear out” with an increasing footprint, for grazing emission the smearing is less pronounced due to the independence of XSW formation on transverse coherence length.
These results show proof of the principle that a natural limitation for the measurable layer thickness stated in [1] is not applicable in grazing emission fluorescence geometry, and may help in further optimization of XSW measurements in GE geometry.

[1] D. Ingerle, 2017, Ph.D. thesis, University of Vienna, p. 30

Speaker: Timur Terentev (University of Twente)

EUV2025_abstract_TerentevT.pdf

14:00 Exploring the soft X-ray energy range for next generation nanostructure metrology

The continuously shrinking dimensions of the features in the semiconductor industry as well as their increasing complexity require innovative metrology solutions. Non-destructive methods with high throughput that are able to assess complex 3D structures are of major importance. Measurement techniques based on light-structure interaction allow fast and non-destructive inspection of structured areas and are already widely used from the infrared to the hard X-ray spectral range.
Soft X-rays are suitable for comprehensive and high-resolution metrology of nanostructured surfaces and thin layers. They are suitable for probing buried layers, and surface contamination and imperfections. In comparison to X-rays, higher angles of incidence are also possible without compromising the surface sensitivity. Therefore, this energy range is being systematically explored for the evaluation of nanostructured surfaces. However, the use of soft X-rays for metrology application is challenged by the limitation of optical data of the materials. Often, different sources report inconsistencies for many technologically relevant materials and mostly, the available optical data are given without calculated uncertainties. Here, we report on the on-going sensitivity studies in the soft X-ray energy range, where we focus on different layer thicknesses and evaluate, as well, fundamental parameters. Furthermore, the combination of scatterometry with fluorescence, in a so-called hybrid method, is also under exploration for the characterization of nanostructured surfaces.
The project (grant agreement number 101096772) is supported by Chips Joint Undertaking and its members, including the top-up funding of Belgium and the Netherlands.

Speaker: Dr Analía Fernández Herrero (PTB)

EUV2025_abstract.pdf

14:06 Scatterometry Applications: Addressing Model Inaccuracies

Scatterometry provides a fast and indirect method for nano-optical shape reconstruction from measured light intensities. The shape parameters are determined by solving an inverse problem, that is based on the forward model. Bayesian inversion is a powerful tool for solving inverse problems but limited by the complexity of the forward model. As nanotechnology advances and structures become smaller, a higher resolution of the forward model becomes necessary. For such computationally expensive problems, the forward model usually has to be simplified. The inaccuracy of the forward model, the so-called model error, is often ignored in the Bayesian setting and thus causes an additional error in the posterior distribution approximation. Including the model error in the Bayesian setting allows us to use a simplified forward model for faster computation while still obtaining a posterior distribution close to the exact one.

In the presented approach, an arbitrary (e.g. Gaussian) reference density is transported to the desired Bayesian posterior distribution, which is then in turn given as the push-forward under the transport. In the presented approach, the model error is included as a stochastic variable distributed according to the push forward of the reference density under the model error function. Hence, samples from the reference density are transported to the posterior approximation and from there under the model error function to the model error distribution. This approach provides a more flexible description of the model error and allows us to generate posterior samples efficiently. Furthermore, our approach is based on the theory of invertible maps, which allows us to employ arbitrary machine learning architectures as the invertibility of the transport is guaranteed through an analytical construction. For the applications presented here, we focus on simply connected feed-forward networks, which in this context is commonly known as invertible neural networks. We demonstrate the effectiveness of our approach by providing an example of a nano-optical shape reconstruction task that is typically encountered in EUV scatterometry.

Speaker: Maren Casfor (Physikalisch-Technische Bundesanstalt)

EUV2025_abstract_MarenCasfor.pdf

14:12 Calibration of a broadband reflective spectrometer for high-resolution spectral characterization of radiation sources

The development of compact radiation sources has enabled a multitude of lab-size applications, especially in the field of metrology [1]. A broadband spectral characterization of the source emission is of utmost importance for the investigation of photon-induced processes and metrology [2].
In this study, the authors present a unique setup and corresponding measurement results for the high-resolution broadband spectral characterization of radiation sources covering the extensive wavelength range from 5 nm to 1000 nm [3]. The achievable resolution is 0.02 nm in the short wavelength range and the resolving power ($\lambda/\Delta\lambda$) exceeds 500 for the full range.
For the vacuum wavelength range (5 nm to 200 nm) the setup employs several flat-field diffraction gratings with varying line density as dispersive elements. The wavelength range above 200 nm is measured with two Czerny-Turner spectrometers. Higher diffraction orders are filtered out by a selection of spectral thin film filters, necessitating the collection of multiple partial spectra. The main components of the spectrometer and the thin film filters have undergone rigorous characterization at the Physikalisch-Technische Bundesanstalt (PTB, Berlin). The results from these characterizations are then used to correct the measured partial spectra, resulting in relatively calibrated partial spectra. These are combined to obtain the full spectrum without any higher diffraction order contributions, which is then absolutely calibrated through a measurement of the absolute intensity in a small interval.
The contribution will cover the overall design, the wavelength calibration and the relative as well as the absolute intensity calibration of the measured spectra.

[1] S. Schröder, L. Bahrenberg, N. K. Eryilmaz, S. Glabisch, S. Danylyuk, S. Brose, J. Stollenwerk, P. Loosen, (2020), Proc. SPIE 11517, 115170S
[2] Z. Bouza, J. Byers, J. Scheers, R. Schupp, Y. Mostafa, L. Behnke, Z. Mazzotta, J. Sheil, W.M.G. Ubachs, R. Hoekstra, M. Bayraktar, O. Versolato, (2021), AIP Advances 11, (12), 125003: 1-9
[3] I. Gisch, S. Schröder, S. Glabisch, S. Brose, S. Danylyuk, J. Stollenwerk, C. Holly, (2024), Proc. SPIE 13215, 1321515

Speaker: Sophia Schröder (RWTH Aachen University - Chair for Technology of Optical Systems TOS)

EUV2025_abstract_Gisch.pdf

14:18 Using Monte Carlo-based Uncertainty Quantification for Free-Form XRR

X-Ray Reflectivity (XRR) is a well-established metrology method for multilayer characterization. With XRR, we determine the structural parameters describing the optical structure of a given sample (i.e., layer thickness, intermixing/roughness and optical density); we do so by solving the related inverse problem. To do so, one uses a model which calculates the reflectivity given a set of parameters. Depending on the chosen model, different features in the optical structure can be simulated and fitted. Normally, one would use a model-dependent approach, which assumes a pre-determined interface shape. In our work however, we use the Free-Form model [1]. This model gives us the ability to fit structures without a pre-assumed layer structure and interface shape; this is done by splicing up the multilayer structure into small lamellae of width d and individual refractive index. Solving this inverse problem is then done by fitting the correct refractive index for each considered lamella. We now aim at determining the estimated error in the fitted refractive indices. We use a Monte Carlo Markov Chain uncertainty quantification routine and investigate regularisation methods to repress multimodalities.

[1] S. N. Yakunin, I. A. Makhotkin, K. V. Nikolaev, R. W. E. van de Kruijs, M. A. Chuev, and F. Bijkerk. Combined EUV reflectance and X-ray reflectivity data analysis of periodic multilayer structures. Optics Express, 22(17):20076–20086, August 2014. ISSN 1094-4087. doi: 10.1364/OE.22.020076.

Speaker: Hendrik Willem Lokhorst (University of Twente)

abstract for EUVR2025.docx

14:24 On the way to automatized optics adjustment at FEL sources

In recent years, the Institut für Nanophotonik Göttingen e.V. (formerly Laser-Laboratorium Göttingen) has successfully developed several Hartmann wavefront sensors in collaboration with DESY. These sensors have been designed for the precise focus characterization and optics alignment of the FEL FLASH in the soft X-ray spectral range, with a wavelength of approximately 5 - 40 nm. Even when using those wavefront sensors, the optimal adjustment of optics is very time-consuming and can only be successfully carried out by experienced beamline scientists. This is especially true for the often used so-called Kirkpatrick-Baez (KB) optics with up to 14 strongly coupled degrees of freedom. Furthermore, the optics adjustment process must be repeated frequently due to changing beam requirements of the users and occasionally varying beam characteristics of the SASE FELs during user experiments. Consequently, there is a high level of interest in the automation of fine-tuning of the focusing optics using rapid "machine learning" algorithms. This is the scope of the BMBF project FELFocus.
We will present first results of automated focus alignments using the KB-optics system at FL23/FLASH2. In order to test the automation, the focus of the KB optics system was optimized by bending the two KB mirrors with initially 4 actuators, whereby the wavefront sensor measures the wavefront and the intensity distribution in real-time at the location of the sensor. Using Fresnel-Kirchhoff integration of these data, the focus size and the intensity distribution at the focus position are calculated. Both the wavefront shape (Zernike coefficients) and the computed focal characteristics are used for an automated control of the actuators of the motorized KB optics. In addition to conventional minimization also advanced machine learning methods based on multi-objective optimization were employed. The latter can significantly speed up the automated focus alignment.

Speaker: Klaus Mann (Ifnano)

KlausMann_Abstract_011025.pdf

14:30 → 15:40 Session 2

14:30 Metrology Light Source: Current Achievements and Future Horizons

The Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, operates the 629 MeV electron storage ring Metrology Light Source (MLS) in Berlin-Adlershof in close collaboration with Helmholtz-Zentrum Berlin (HZB). With more than 8000 operating hours per year, the MLS supports a broad range of metrology applications. In dedicated shifts, it also serves as a unique testbed for innovative concepts in the generation of coherent synchrotron radiation. In particular, proof-of-principle experiments on steady-state microbunching (SSMB) are underway, a promising approach to provide kilowatt-level average power radiation in the extreme ultraviolet regime at future light sources. This talk will present recent developments and outline future perspectives of the MLS.

Speaker: Carsten Mai

15:00 Holography is growing along with the requirements of new beam line gratings

Zeiss is a leading manufacturer of high-quality holographic diffraction gratings, with an emphasis on beamlines in synchrotrons and linear accelerators as well. These gratings exhibit exceptional properties, including low stray light, high diffraction efficiency, and the absence of regular addressing errors in grating line distribution. Additionally, Zeiss has the capability to produce both laminar and blazed gratings, with the option to fabricate them on silicon substrates. Plus, Zeiss’s metrology enables acquisition of VLS data already during grating manufacturing, thus providing the opportunity to correct for line distribution errors prior to the irreversible final step – the etching of the grating profile. A notable advantage of Zeiss's technology is the ability to manufacture large-scale diffraction gratings, ranging from 300 mm to 500 mm in size, which are crucial for advanced beamline applications. In this presentation, we will delve into the underlying technology that enables these innovations and present several exemplary gratings from recent projects.

Speaker: Mr Matthias Burkhardt (ZEISS Microoptics, Carl Zeiss Jena GmbH)

SES2_Burkhardt_CZ.pdf

15:20 The BEAR beamline at Elettra 2.0

The beamline BEAR was open to users from 2003 till July 2025, and it is expected to reopen in 2028 after the upgrade to Elettra 2.0. BEAR is a IOM-CNR bending magnet beamline, working in the spectral region 2.8-1600 eV (443-0.775 nm) [1].
Elettra is the Italian synchrotron radiation facility and it is currently upgrading to Elettra 2.0, a new light source with a significant enhancement of the brilliance and the coherence of the radiation: the critical energy will increase from 3.2 to 5.55 keV, the electron energy from 2.0 to 2.4 GeV, the ring current from 300 (160 in case of 2.4 GeV) to 400 mA.
The new beamline BEAR, which will be moved from exit 8.1 to the dipole exit 3.1, will take advantage of the improved characteristics of the light source: the intensity will increase of a factor 2.5, and the linear power density will increase from present 13.5 W/mrad to 34 W/mrad. Moreover, the electron beam size will reduce from 113x33 μm2 to 9.5x8.5 μm2 (HxV), and consequently the light source will reduce in particular in the horizontal plane. The improvement in brilliance and coherence will allow in particular to enhance the signal to noise ratio, reduce the acquisition time, and enable the analysis of samples at micrometer scale.
The beamline layout, based on the Plane Mirror Plane Grating (PMPG) configuration, with a very high flux, important energy resolution and a small spot size [2], will be maintained, but also adapted to exploit the properties of the new light source. The main improvement on the monochromator will consist in the enhancement of the UV-EUV spectral range by replacing the low energy grating of the NIM (Normal Incidence Mirror) channel with a new one optimized in the FUV spectral range and by adding a second grating in the grazing incidence channel optimized for the EUV range.
The existing end station, conceived mainly for surface science, will be flanked by a second scattering chamber, dedicated to the calibration activity, in particular to the characterization of larger optical elements and instrumentation. For this purpose, the refocusing section must be redesigned, mounting, beside the actual ellipsoidal mirror, a new toroidal mirror with a much longer focal distance and a lower divergence of the beam, preserving the order of magnitude of the size spot, thanks to the lower emittance of Elettra2.0.
All the optics of the beamline will be cleaned with ultraviolet/ozone treatment to reduce the surface carbon contamination due to more 20 years of synchrotron life, and enhance the beamline transmission in particular in the spectral region of C K edge at 4.35 nm.
[1] S. Nannarone et al., 2004, AIP Conf. SYNCHROTRON RADIATION INSTRUMENTATION 705, 450-453.
[2] G. Naletto, M.G. Pelizzo, G. Tondello, S. Nannarone, A. Giglia, 2001, Proc. SPIE 4145, 105-113.

Speaker: angelo giglia (CNR, ISTITUTO OFFICINA DEI MATERIALI)

The BEAR beamline at Elettra 2.0.pdf

15:40 → 16:20 Coffee Break

16:20 → 17:50 Session 3

16:20 High speed optical inspection of wafers

The broadband plasma patterned wafer defect inspection systems enable wafer-level discovery of yield-critical defects and inline monitoring for advanced logic and leading-edge memory design nodes. We will address the application of optical wafer inspection and its image capture methodology, including difference image generation, sub-resolution sensitivity and speed. We will introduce the broadband plasma inspection system and its role in chip manufacturing and provide some details on how our core technologies – broadband plasma light source, optics, sensors, algos – enable detection of small defects at speeds required for production as well. We will touch on challenges associated with differentiating between critical defects and irrelevant ones, maintaining defect sensitivity as pattern feature and respective defect sizes continue to decrease, and scaling of wafer noise originated from three-dimensional pattern irregularities like surface and line edge roughness. We will discuss the dependence of defect contrast on film stack geometry and materials, the need of optical constants notion for modeling of the defect sensitivity. The advantages of the use of broadband light for maximizing defect signal and for reducing wafer noise will be addressed as well.

Speaker: Dr Larissa Juschkin (KLA Corporation)

abstract_juschkin_PTB25.pdf

16:50 Updated Optical Data for Transition Metals in the EUV Range

Transition metals are key constituents of EUV optical elements. Most transition metals have high melting points and are compatible with Ion Beam Deposition (IBD) alongside magnetron sputtering techniques. These techniques allow coating dense, very smooth and pure thin films. Metals are used as main coatings and also as secondary layers such as capping layers and interdiffusion barriers. The optical response of metallic films can be tuned by alloying, and the chemical stability can be improved with nitridation and other methods. Most metals are also suitable for Si and Si-based substrates. Those advantages made transition metals essential for soft X-ray and EUV optical elements.

However, the available optical data on transition metals in the EUV are rather inconsistent for different sources [1-3]. When designing optical elements, differences in the available data for the same material complicate materials selection. Inconsistencies are even observed for the oxidation resistant and environmentally stable Platinum-Group Metals, and that comes at a surprise. Presumably, that is due to the dispersion profiles of the overlapping N- and O-shells of these elements, which cannot be easily assessed by theoretical calculations. Such fine-structure in the dispersion profiles cannot be properly captured by older experimental data for discrete energies. This investigation therefore requires continuously tunable radiation sources, such as storage rings.

In our contribution, for the EUV spectral range, we present the determination of optical constants from eleven transition metals: Cr, Co, Ni, Nb, Rh, Pd, W, Os, Ir, Pt, and Au in addition to ZrO2. Synchrotron reflectometry was used on thin films with thicknesses ∽30 nm to ∽50 nm. Markov chain Monte Carlo (MCMC)-based Bayesian inferences were used to work out the relevant reflectometry inverse-problems. We present our results in comparison with external literature.

[1] B. Henke, E. Gullikson, and J. Davis, 1993, Atom. Data Nuc. Data Tab. 54(2), 181-342
[2] D. Windt, 1998, Comp. Phys. 12(4), 360-370.
[3] E. Palik (Ed.), 1985, Handbook of Optical Constants of Solids, Academic.

Speaker: Qais Saadeh (Physikalisch-Technische Bundesanstalt (PTB))

EUV2025_abstract_template_QS_03072025_FS_08072025.pdf

17:10 Dimensional and analytical metrology of semiconductor nanostructures employing X-ray fluorescence techniques

The rapid advancements in nanoelectronics have led to a profound increase in the complexity of three-dimensional nanostructures utilized in cutting-edge transistor architectures. As this complexity grows, the demand for precise metrology becomes paramount to ensure successful fabrication. X-ray fluorescence techniques, when employed in specific operational modes, emerge as a valuable tool, offering quantitative insights into semiconductor applications. Moreover, these techniques can be seamlessly integrated with metrology pads of typical sizes featuring homogeneous structures.

The implementation of reference-free XRF quantification schemes, alongside physically calibrated instrumentation and excitation spot sizes in the low micrometer [1] or even nanometer range [2], enables the quantitative determination of lateral elemental composition. Notably, this method surpasses electrons in terms of achievable information depth and can thus provide information about sub-surface features of nanoobjects, all achieved without the need for destructive sample preparation.

The application of Grazing Incidence [3] or Grazing Exit X-ray Fluorescence analysis [4] enhances sensitivity to sub-nanometer levels, allowing for precise measurement of dimensional properties in nanostructures. Critical parameters such as line widths or heights can be accurately determined through these techniques. Importantly, both approaches are non-destructive, offering statistically more relevant information compared to transmission electron microscopy, as they average over several nominally identical nanostructures.

In summary, we will show how employing advanced X-ray fluorescence methods can support navigating the intricate landscape of nanoelectronics metrology.

[1] P. Hönicke et al., Nanotechnology, under revision
[2] A. Wählisch et al., Small (2023) 19, 2204943
[3] P. Hönicke et al., Nanotechnology (2020) 31, 505709
[4] P. Hönicke et al., Small (2022) 18, 2105776

Speaker: Dr Philipp Hönicke

17:30 VUV-Ellipsometry on graphene on Ge(111)

Extending ellipsometry to the vacuum-ultraviolet spectral region, i.e., photon energies larger than 6.5 eV, is not only demanding in terms of the instrumentation but also with regard to the mathematical modelling of the measured data in order to obtain a physical interpretation in terms of the dielectric function or the optical constants of a material. In particular the extreme surface sensitivity of the measurement at these energies makes this a valuable tool for investigation of surfaces and thin film materials.

In this contribution we present results on experimental investigations of the optical properties of bulk semiconductors, Si, Ge and SiC, and thin solid films (graphene and h-BN) grown on them.
The ellipsometric angles of these material systems have been measured at different incidence angles and photon energies ranging from 1.6 eV up to 25 eV, in order to characterise the full electronic structure.
In a subsequent step, we extracted the optical constants and the dielectric function of the materials using a fitting method and a methodology which is compliant with international metrological practice.

Speaker: Mattia Mulazzi (PTB, Abteilung 7.1)

18:00 → 21:00 Get Together Receiption and Buffet Dinner

Wednesday 15 October

08:30 → 09:00 Registration 2nd day

09:00 → 09:40 Keynote 2

09:00 Optics for EUV-Lithography, now and then

EUV-Optics with NA=0.33 are fully industrialized and are a key ingredient to the state-of-the-art semiconductor devices. We will shortly comment on the relevant performance indices.
We will also describe the outstanding performance of the high-NA optics with NA=0.55 currently being introduced into the market. Finally, we will comment on the challenges that would come with either higher NA (hyper-NA) or shorter wavelengths (beyond EUV) with a special focus on metrological challenges like the need for precise optical constants, roughness metrology, and reflectometry.

Speaker: Hartmut Enkisch (Carl Zeiss SMT GmbH)

EUV2025_abstract_SMT_HEnkisch.pdf

09:40 → 10:20 Session 4

09:40 Development of a Compact Capillary Discharge for Actinic EUV Metrology Applications

Norbert Böwering(1) and Christian Meier(1)
Bowering@physik.uni-bielefeld.de
(1) Bielefeld University, Universitätsstraße, 33615 Bielefeld, Germany

Conventional EUV lasers in pinched capillary-discharge configurations for neon-like argon ions at 46.9 nm have mostly employed fairly inefficient excitation schemes using very high supply voltages (≥100 kV) and very large stored energies (several 100 J). This resulted in quite low pulse conversion efficiencies of ~10-7, or even less. In view of potential actinic metrology applications for the nitrogen recombination transition at the shorter wavelength of 13.4 nm, we discuss the development of a novel, highly compact capillary-discharge design with hollow-cathode trigger, automatic pre-ionization and integrated excitation-pulse compression.
We examine a low-inductance configuration with a single switch in an LC-inversion discharge scheme using ceramic capacitors of currently 16 nF total capacitance that is operated at peak currents of several kA using bipolar supply voltages below 20 kV at pressures of a few Pa, corresponding to the left side of the Paschen curve. High-voltage probes and a Rogowski coil are attached for waveform analysis. Based on detailed computer modeling of the plasma evolution in discharges of capillaries with 3 mm inner diameter, the failure to observe the predicted lasing action on the Balmer-α line of N6+ at 13.38 nm in the attempts of several other research groups was previously attributed to the detrimental influence of ablated capillary material [1].
With a hollow-cathode electrode and end-on differential pumping on the opposite capillary side in our configuration, we have used a translucent capillary with very smooth inner surface and fairly large inner diameter (¼ inch) to minimize wall ablation. This configuration also permits side-on observation of pinched visible light emissions. We report on initial tests with recorded voltage waveforms in different switching schemes and few-ns light emission periods in single-pulse operation with dry air.

[1] M. Vrbova, P. Vrba, A. Jancarek, M. Nevrkla, N. A. Bobrova, P.V. Sasorov, Phys. Plasmas 26, 2019, 083108.

Speaker: Dr Norbert Böwering (Fakultät für Physik, Universität Bielefeld)

EUV2025_abstract_Böwering_Oral.pdf

10:00 High repetition rate, high average power XUV sources based on High Harmonic Generation

High harmonic generation (HHG) sources [1] provide fully coherent laser-like radiation in the Extreme-Ultraviolet (EUV/XUV) and soft X-ray spectral range with femto- or attosecond pulse duration. These sources are ideal metrology tools in scientific or industrial applications: The short wavelength allows very high spatial resolution in scattering or imaging of nanostructures created by state-of-the-art microchip EUV lithography. Their spectral bandwidth and ultrashort pulse durations are very useful in ultrafast spectroscopy to investigate the fast reactions occurring in solar cells, catalysis or to observe directly the carrier dynamics in superconductors or next generation semiconductors. For all these applications, HHG sources with high repetition rates and high average powers are required to minimize the acquisition times, leading to a fast turnaround in sample characterization. At the same time, a high robustness and stability of the source is needed for low-maintenance operation with minimal downtimes. Based on robust high-power femtosecond Ytterbium (Yb) laser systems together with the multipass cell pulse post compression technology [2], Class 5 Photonics is developing the Moonlander high harmonic generation source to fulfill these requirements.
In our contribution, we will present the current status of our HHG source driven by the Class 5 Black Dwarf laser, providing 17 W average power at 100 kHz repetition rate and a pulse duration of 29 fs. We measured an XUV output power of 1.4 μW for argon target gas and 2.8 μW for krypton target gas within the aluminum filter window (22-75 eV). The measured key parameters are shown in Figure 1. We will also show first results with a driver at an average power of 50 W at much high repetition rate of 750 kHz. Our next steps are the demonstration of stable operation of our HHG source for driver lasers at 200 W and 500 W average power within the joint research project MEGA-EUV with DESY, University of Hamburg and Amphos. We will discuss the technical challenges and prospects in the power scaling of these XUV sources.
[1] X. F. Li et al., Phys. Rev. A 39, 5751–5761 (1989)
[2] A.Viotti et al., Optica 9, 197-216 (2022)

Speaker: Bastian Manschwetus (Class 5 Photonics)

PTB EUV Workshop 2025 HHG paper.pdf

10:20 → 10:50 Coffee break

10:50 → 12:30 Session 5

10:50 Development and Characterization of Wide-Bandgap Photodetectors Tailored for the EUV-VUV Solar Radiometer SoSpIM

n 2009, the ESA microsatellite PROBA2 was launched, carrying a radiometer in its scientific payload designed to monitor solar emissions across four broadband channels in the UV-VUV spectrum. This instrument, named LYRA, was developed under the lead of the Royal Observatory of Belgium and featured pioneering wide-bandgap photodetectors made from diamond, which were particularly well-suited for observing high-energy photons. Notably, this represented the first use of such detectors in space. Prior to launch, the filters and detectors underwent rigorous characterization at the PTB laboratory, and their performance was continuously monitored throughout the still on-going mission via regular in-flight calibration campaigns.

Building on this extensive experience, a new solar radiometer is currently under development. Named SoSpIM, the instrument is scheduled for launch in 2028 aboard JAXA’s Solar-C mission, with the PMOD/WRC institute in Davos, Switzerland, serving as the Principal Investigator institute. The Royal Observatory of Belgium is responsible for selecting, providing, and characterizing the optical components, with much of the characterization work again carried out at PTB. As part of this effort, a new generation of AlN photodetectors has been developed. These detectors feature an adapted design to address some of the limitations identified in the LYRA detectors.

This presentation will provide an overview of the SoSpIM instrument and share results from the ground-based characterization campaigns of its optical components. A comparative analysis of their performance against the LYRA detectors will also be discussed.

Speaker: Marie Dominique (Royal Observatory of Belgium)

Dominique_abstract.pdf

11:20 Observing the sun at short wavelengths – Optical coatings for the solar telescopes MUSE and EUVST

Astronomical spectroscopy is an indispensable tool for probing the physical and chemical
properties of celestial objects. When moving from the infrared and the detection of molecular
vibrations and rotations towards the visible spectral range and the observation of characteristic
gas absorption lines, the increased energy at even shorter wavelengths in the extreme ultraviolet
(EUV) spectral range allows for the detection of emission lines from highly ionized atoms. This
is ideal when observing the sun, as these highly ionized states only occur, if sufficiently high
temperatures are reached, making EUV-spectroscopy a perfect temperature probe that can
handle millions of degrees Kelvin.
The talk will focus on the two solar telescopes: MUSE (Multi-slit solar explorer) and EUVST
(EUV High-throughput Spectroscopic Telescope) that are planned for launch in 2027 and 2029,
respectively. The scientific aim of both solar missions is to gain a deep understanding of the
physical processes that drive the heating of the sun’s hot atmosphere as well as the physical
processes involved in space weather events like flares and coronal mass ejections, which have
a direct impact on our modern infrastructure (e.g. satellites, power grids, GPS accuracy, etc.).
Even though both satellites will share in part the same spectroscopic information, the approach
is different and thus highly complementary. This leads to fundamentally different requirements
on the optical coatings for the mirrors and gratings: EUVST is designed as a single-slit
spectrograph covering a very broad temperature range of emission lines, which requires a
broadband aperiodic EUV multilayer design. MUSE on the other hand is designed as a multislit spectrograph with a large field of view at a cadence of just 12 seconds to, for the first time,
fully capture the rapid temporal evolution of large-scale coronal events under the slit(s) of a
spectrograph. This requires multi-channel, extreme narrowband coatings to avoid spectral
crosstalk.
At optiX fab, we successfully coated the flight hardware for both solar telescopes this year.
Besides the challenging spectral reflectance requirements, the talk will address mechanical and
coating stress properties that needed to be tailored to handle the lightweight structure of the
substrates to support the revolutionary imaging targets of both telescopes with respect to
resolution and field of view as well as the high temporal resolution.

Speaker: Dr Marcus Trost (optiX fab.)

Observing the sun at short wavelengths_PTB.pdf

11:50 Very thin layers for state-of-the-art x-ray mirrors

State-of-the-art x-ray beamlines are equipped with x-ray mirrors according to their applications such as surface and materials science, chemical dynamics, photo emission spectroscopy, protein crystallography, x-ray microscopy [1]. In the case of free-electron lasers (FEL) and synchrotron radiation sources, total-reflection mirrors are an important optical element to guide and shape the beam. Such a mirror is often a single layer on a silicon substrate. The layer material is selected to its x-ray optical properties such as refractive index and micro-roughness. For the preparation of this layer material, it is important to achieve nearly bulk density, then the critical angle is perfect for its work. The typical layer thickness is in the range from 30 nm to 100 nm. At our Helmholtz-Zentrum Hereon, the most coated thin-film materials are B4C, C, Ni, Pt, Ru, W in the last decades [2-4]. Two different magnetron sputtering facilities enable us to apply a wide range of different coatings with high precision and reproducibility up to a length of 1.5 m. Nowadays the interest in double and triple layer material systems has increased, because they can improve the overall mirror performance in terms of x-ray reflectivity and possibly extend its lifetime [5]. A bonding layer can support film adherence and stability. Furthermore, it can be used for chemical removal after a long operation time. After this, the substrate can most likely be re-coated. A top layer can prevent oxidation or any chemical reaction of the underlying layer. The different layer properties in this stack can also be used in different photon energy ranges. In sum the film thickness of the different layers is reduced that means some nanometers should be grown without any imperfections. We investigated some thin film materials in the range of some nanometers by means of x-ray reflectivity using our new diffractometer. With a special x-y stage it is now possible to measure a larger area of the mirrors. Furthermore, the micro-roughness of these layer systems has been measured by means of with light interferometry (WLI) and AFM to evaluate the impact of more interfaces in the layer stack.

[1] D. T. Attwood, Soft x-rays and extreme ultraviolet radiation: principles and applications, 1999, Cambridge University Press, 127.
[2] M. Störmer, F. Siewert, J. Buchheim, A. Pilz, M. Kuhlmann, E. Ploenjes, K. Tiedtke 2014, Proc. SPIE 9207, 92070H.
[3] M. Störmer, H. Gabrisch, C. Horstmann, U. Heidorn, F. Hertlein, J. Wiesmann, F. Siewert, A. Rack, 2016, Rev. Sci. Instruments 87, 051804.
[4] M. Störmer, F. Siewert, C. Horstmann, J. Buchheim, G. Gwalt, 2018, J. Synchrotron Rad. 25, 116.
[5] S. Schmidtchen, F. Siewert, R. Barrett, S. G. Alcock, D. Dennetiere, K. Bagschik, M. Altissimo, U. Wagner, M. Störmer, A. Fernandez Herrero, I. Sics, L. Pickworth, 2025, Proc. SPIE 13533, 1353306.

Speaker: Michael Störmer (Helmholtz-Zentrum Hereon)

EUV2025_abstract_stormer.pdf

12:10 Agnostic compensation of periodic errors in interferometric position sensing

Interferometric position sensors play a vital role in semiconductor fabrication, nanotechnology, and advanced scientific applications requiring sub-nanometer accuracy in position monitoring and control. The core operating principle of these sensors relies on the quadrature detection scheme, which enables the extraction of the interferometric phase from two phase-shifted sinusoidal signals. However, in practice, imperfections such as sensor offsets or gain imbalances often distort the resulting quadrature signals, resulting in systematic periodic errors in position sensing. While Heydemann’s widely used compensation method addresses these distortions, it assumes the Lissajous figure formed by the quadrature signals is elliptic—a condition not always met in practice, particularly for interferometer designs, such as Fabry-Perot or those generating quadrature signals via laser current modulation.

Figure 1: a) A distorted Lissajous figure (inset) leads to periodic measurement errors. However, if the target motion is tracked over multiple fringes and only phase values differing by $2\pi$ are used, the correct trajectory can be estimated with high accuracy. b) A comparison between the experimentally observed phase angle distribution $\rho_{\rm exp}(\phi)$ and the ideal distribution $\rho_{\rm id}(\phi)$ enables the derivation of a compensation lookup table (LUT). Specifically, this involves calculating the cumulative sums of both distributions.

In this work, we present a novel compensation algorithm that does not rely on the assumption of an elliptic Lissajous figure. Instead, it requires only that measurement errors be periodic, the target motion follow a smooth trajectory consistent with physical laws, and that the data be either timestamped or uniformly sampled. The method comprises two distinct phases: a learning phase and a compensation phase. During the learning phase, which typically comprises 5 to 10 fringes, the algorithm first estimates the motion of the target in a manner that is unaffected by periodic errors. This information, along with the distribution of measured phase angles, enables the algorithm to infer a compensation function which effectively corrects for periodic errors. Our novel approach enables robust compensation across a broader range of interferometric sensor applications.

[1] P.L.M. Heydemann, 1981, Applied Optics 20(19), 3382.

Speaker: Christian Schwemmer (deutsch)

EUV2025_Schwemmer.pdf

12:30 → 13:50 Lunch & Poster & Sponsor presentation

13:50 → 15:10 Session 6

13:50 High-Resolution, High-Efficiency Spectrometers in the EUV and Soft X-ray Range

Reflection zone plates (RZPs), an innovative class of 2-D varied line space gratings that combine dispersion, one- or two-dimensional focusing and reflection in one element [1], are good candidates to provide optimized efficiency at high energy resolution. Conventional RZPs on planar substrates suffer from a narrow energy range in parallel spectra registration, limiting the applications of this type of optics to monochromatization. Recent developments in theory, technology, and metrology of RZPs make it possible to fabricate off-axis, parallel-line-projecting RZPs [1] on spherical substrates with a small radius of curvature down to 2 m, extending the range wherein high-resolution flat field spectra can be measured in parallel on a single, two-dimensional pixel detector within an interval of about ±25 % around the design energy [2]. With customized RZPs, additional aberration correction of the substrate or other optics in the system becomes feasible [3].
In our contribution, we present different usage examples of RZPs, combining their simplicity and efficiency in spectrometers and monochromators. We present results obtained with novel parallel wavelength dispersive X-ray spectrometers, developed at NOB Nano Optics Berlin GmbH in cooperation with the Institute of Applied Photonics e. V., designed for the analysis of fine structures in the energy states of the bonding electrons in compounds of ultra-light elements like Li or B. The lab-based spectrometer “WDSX-300” has an energy resolution of 0.3 eV at the Al L$_{2,3}$-edge with an overall efficiency of the optics up to 30 % in the energy range of (35 – 130) eV [4]. Thanks to its versatile construction and compact setup with an overall optical path length of 0.3 m, it can be mounted on any commercially available scanning electron microscope or electron probe micro-analyzer. As an option, the spectrometer can be equipped with RZP optics for the energy range up to 1200 eV.
A challenge with all wavelength dispersive X-ray spectrometers is the presence of higher diffraction orders in the spectra. For example, the fifth order of C K$_α$ appears at 55.4 eV, very close to the first order of Li K$_α$ at 54.3 eV. We have improved our optical setup to reduce the higher order contributions in our spectra, including the most abundant carbon and oxygen contributions. We are aiming for a reduction by a factor of $10^4$, as predicted by simulations, and hope to confirm this expectation in upcoming experiments.
[1] C. Braig, H. Löchel, R. Mitzner et al., 2014, Opt. Express 22, 12583 – 12602.
[2] J. Probst, C. Braig, and A. Erko, 2020, Appl. Sci. 10, 7210.
[3] J. Probst, C. Braig, E. Langlotz et al., 2020, Appl. Opt. 59, 2580 – 2590.
[4] K. Hassebi, N. Rividi, M. Fialin et al., 2025, X-Ray Spectrom. 54(2), 76 – 85.

Speaker: Jürgen Probst (NOB Nano Optics Berlin GmbH)

EUV2025_abstract_Probst.pdf

14:10 A lab-scale EUV high intensity exposure setup for small-spot exposures and angular resolved photoelectron spectroscopy

The extreme ultraviolet high intensity exposure (EUV-HIEX) setup is a compact tool for irradiation of samples with high EUV doses designed to achieve maximum intensity within a small spot size in the sample plane. Applications can be found in the field of accelerated lifetime studies where the EUV-material interaction is investigated. Further applications are EUV induced outgassing studies and material modifications within adjustable gas atmospheres covering industrial to fundamental research tasks.
For the realization of the experimental setup the broadband EUV radiation is emitted by a discharge-produced plasma EUV source. The radiation is deflected using a multilayer (ML) mirror consisting of alternating Mo/Si layers optimized for an incidence angle of 45 degrees. The initial broadband emission is narrowed down by the ML mirror to in-band EUV at a main wavelength of 13.5 nm. By the implementation of a two shell Wolter collector, using external total reflection under grazing incidence angle, the emitted radiation is collected within a focal point focused into sample plane. The spatial distribution in the proximity of the sample plane is tunable, offering tunability ranging to from a small-spot (≈ 80 μm FWHM) to a top-hat distribution (≈ 200 μm FWHM). In addition, a SiN/Zr thin film system serves as an out-of-band filter with a high transmission for wavelengths between 5 nm and 20 nm.
The recent enhancements to the EUV-HIEX include the implementation of a monochromator to prepare the beam for ARPES measurements, ensuring a narrowband photon energy at 91.85 eV. In extension to this, an ARPES system is being implemented which, in addition to coupling monochromatic EUV radiation, also provides excitation in the UV at approximately 21 eV and in soft X-ray at approximately 1,500 eV. The refined setup not only enables rigorous testing of industrially relevant EUV components but also provides a robust analyzation platform for advancing fundamental research in material science.

Speaker: Linus Nagel (RWTH)

EUV2025_abstract_Nagel.pdf

14:30 CD SEM measurements on EUV photomasks

The continuous scaling of semiconductor devices has driven the adoption of Extreme Ultraviolet (EUV) lithography in advanced nodes. The precision and accuracy of Critical Dimension (CD) measurements on EUV masks become increasingly critical.
A particular challenge in CD measurement on EUV masks is to minimize the influence of the linearity of the CD-SEM on the measurement results. Especially at very small feature sizes, even slight non-linearities in the measurement chain can lead to significant deviations. To ensure high measurement accuracy and reproducibility, both the CD-SEM scan parameters (e.g., beam current, acceleration voltage, pixel size, and averaging of scans) and the image evaluation parameters (such as threshold settings, edge detection, and filtering algorithms) have been specifically optimized. These measures reduce systematic sources of error and enable more precise capture of the actual feature dimensions. To calibrate and validate CD measurements, the Advanced Mask Technology Center (AMTC) utilizes an EUV photomask calibration standard provided by the Physikalisch-Technische Bundesanstalt (PTB). This calibration standard contains CD feature line/space patterns with minimum CD sizes down to 47.4 nm. However, this limit no longer encompasses the smallest features present on current EUV masks, which can be below 30 nm.
Another growing challenge is the increasing presence of curvilinear structures in modern mask designs. Traditional CD measurement methods, which typically rely on linear edge-to-edge distances, are no longer sufficient for accurately characterizing these complex shapes. For such non-rectilinear features, classical CD metrics lose relevance, and instead, shape-based metrology must be applied. One effective approach is the use of Edge Placement Error (EPE) measurements, which compare the actual feature contours on the mask to the intended design. Further improvements in the accuracy of EPE-based metrology have been achieved through refinement of CD SEM scan parameters. These enhancements enable reliable quantification of both global and local deviations.

Speaker: Torben Heins

EUV2025_CD_SEM_measurements_on_EUV_photomasks.pdf

14:50 High throughput AFM metrology for high-NA EUV lithography

The semiconductor industry has continued its push on scaling with the use of multi-patterning while in parallel introducing 3D transistor architectures and advanced packaging to achieve higher transistor density and performance gains. High-NA EUV lithography machines are expected to provide relief from multi-patterning translating to fewer process steps and increased yield. However, the use of higher numerical aperture has a direct impact with reduced depth of focus among others, requiring process optimization especially for use in high-volume manufacturing (HVM), not limited, but critically on the focus and exposure parameters of the lithography tool [1]. With maturity, the feature sizes are also expected to reduce further especially at the interconnect and contact-hole layers directly above the transistor devices.
CD-SEM has been the industry workhorse, especially in HVM. However, with further scaling, CD-SEM suffers from low-SNR and photo-resist shrinkage, and significant modelling and complementary techniques are required to capture the CDs accurately [2]. While AFM is well-suited to provide fully non-destructive measurements on photo-resist wafers either after exposure or development with the required resolution, they are generally considered slow and are not in large use in HVM environments. In this work, we present our results obtained by using a high-throughput fully automated AFM based metrology tool for wafers processed using high-NA EUV lithography tool at IMEC [3].
The wafer is processed using a focus exposure matrix and consists of contact holes of pitch 40, 36 and 32nm. Shots are measured on the wafer along the focus and exposure change lines, where the first set of measurements targets capture of resist thickness and resist loss information, showing that the former is more stable over dose variation than focus, with the latter having a linear trend with dose variation and being stable over focus. The second set of measurements targets the contact holes of different pitches to capture variation in hole depth, critical dimensions (CD) at various height. The results indicate an increase in both depth and CD with increase in dose, while changes in focus are more closely linked to defectivity. These results show the coming of age of high-throughput AFM for the upcoming semiconductor nodes where high-NA EUV lithography is expected to be used in HVM.

Speaker: Po Cheng Wu

[Abstract for 328. PTB VUV EUV] High throughput AFM metrology for high-NA EUV lithography.pdf

15:10 → 15:30 Wrap-up and Closure