Meiyi Wu

Accurate and efficient tuning of small-radius-of-curvature (RoC) bendable mirrors remains challenging under variable beamline conditions, while conventional analytical tuning is often inadequate. To address this problem, this study presents the design, modeling, and multi-method optimization of a small-RoC bendable mirror system developed as a preparatory study for the Shenzhen Superconducting Soft X-ray Free-Electron Laser. The system adopts a four-point bending structure with an asymmetric support layout, and integrated force sensors are used for direct real-time force monitoring. A physically grounded analytical route based on classical bending theory is first used to assess the basic deformation capability of the system. A data-driven route is then developed by combining a transfer learning neural network (TLNN) with a differential evolution (DE) algorithm. The TLNN surrogate model is trained on simulated force–profile pairs generated by finite element analysis and refined using matrix-style measured force-profile pairs. The results show that, within the investigated center RoC range of 450–700 m, the differences in fitted image distance and center RoC are maintained within 5 mm and 0.55 m, respectively. The root-mean-square height prediction errors are at the nanometre level. Furthermore, the TLNN–DE framework is able to recover the end-force configuration corresponding to a prescribed target surface profile. The deviation between the recovered and measured end forces is kept within 0.5 N. These results demonstrate that the proposed framework provides an effective and transferable route for data-driven optimization of bendable mirrors and other active or adaptive optical systems in free-electron laser and synchrotron radiation facility beamlines.

Développement de la méthodologie des ondes stationnaires pour sonder les processus physico-chimiques aux interfaces des multicouches périodiques: La qualité des interfaces dans les multicouches périodiques est essentielle au développement de miroirs réfléchissant efficacement dans les domaines des rayons X et extrême ultraviolet (X-EUV). De manière générale, la structure des interfaces dépend des possibles interdiffusion et processus chimiques aux interfaces entre couches. L'idée principale de cette thèse est d'appliquer la technique des ondes stationnaires dans le domaine X à la caractérisation de matériaux, principalement mais non exclusivement aux multicouches périodiques. Cette méthode est basée sur l'interférence de deux faisceaux de rayons X cohérents. L'interférence constructive sur un plan anti-nodal amplifie le champ électrique tandis que l'interférence destructive minimise ce dernier sur un plan nodal. Cette technique des ondes stationnaires dans le domaine X permet l'excitation (photoémission, fluorescence, ...) d'endroits spécifiques dans un empilement périodique de matériaux. De cette manière, les spectres expérimentaux ainsi obtenus sont principalement les spectres caractéristiques des atomes situés sur un plan anti-nodal. Combinée avec d'autres techniques expérimentales telles que la spectroscopie d'émission X (XES) ou la spectroscopie de photoélectrons dans le domaine X (XPS), une information sélective en profondeur, avec une sensibilité sub-nanométrique, peut être obtenue.

Gravity-induced surface deviations in bendable mirrors degrade the performance of Kirkpatrick–Baez focusing systems at synchrotron and free-electron-laser beamlines. Conventional gravity compensation models ignore the inactive end length introduced by end clamping in four-roll benders. This omission leads to a systematic mismatch between predicted and measured surface shapes. This study develops an analytical model with end-loss correction within the Euler–Bernoulli beam framework for bendable mirrors. The model provides closed-form expressions for the required moments, the symmetric support forces, and the associated optimal dimensionless parameters. Finite element analysis verifies the predictions across representative geometries and load cases. With end loss included, the root-mean-square slope deviation decreases by about one order of magnitude to below 20 nrad, which meets the requirements for diffraction-limited focusing. This work establishes a robust framework for high-precision bendable mirror systems, with clear potential to reduce tuning cost and improve beamline optical performance.

The damage threshold of an Au-coated flat mirror, one of the reflective optics installed on the FEL-2 beamline of the Dalian Coherent Light Source, China, upon far-UV free-electron laser irradiation is evaluated. The surface of the coating is characterized by profilometer and optical microscope. A theoretical approach of the phenomenon is also presented, by application of conventional single-pulse damage threshold calculations, a one-dimensional thermal diffusion model, as well as finite-element analysis with ANSYS.

BackgroundA plausible approach for mitigating the mask 3-D (M3D) effects observed in extreme ultraviolet (EUV) lithography is to replace the existing mask absorber with alternative materials. Absorbers with a high EUV extinction coefficient k allow for lower best focus variation (BFV) through pitch and reduced telecentricity errors (TCEs).AimWe aim to evaluate Ta-Co alloys as potential high-k mask absorbers from material suitability and imaging standpoints.ApproachWe study the film morphology, surface composition, and stability of Ta-Co alloys in mask cleaning solutions and a hydrogen environment as present in the EUV scanner to assess the material suitability from an experimental aspect. Optical constants for three selected compositions, viz., TaCo, Ta2Co, and TaCo3, were determined from EUV angle-dependent reflectivity measurements. Next, utilizing rigorous simulation software, the imaging performance of Ta-Co alloys is evaluated and compared with the reference absorber. The recommended absorber thickness for Ta-Co alloy absorbers is based upon normalized image log slope (NILS) enhancement, threshold to size, and balancing of diffraction order amplitudes. A 10 nm line and space pattern with a pitch of 20 nm and 14 nm square contact holes with a pitch of 28 nm are used for the simulation study using high numerical aperture 0.55 EUV lithography process settings. The primary imaging metrics for through pitch evaluation include NILS, TCE, and BFV.ResultsThe Ta-Co alloys exhibit a higher EUV extinction coefficient k compared with the currently used Ta-based absorber. TaCo and Ta2Co demonstrate smooth surfaces and are stable in a hydrogen environment and in mask-cleaning solutions.ConclusionTa-Co alloys allow for a reduction in M3D effects at a lower absorber thickness compared with a 60 nm Ta-based reference absorber.

An alternate mask absorber is a generally applicable approach as a mitigation strategy for Mask 3-D effects (M3D) observed in EUV lithography. It is also an efficient solution in a production-worthy environment compared to multilayer modification or Source Mask Optimization (SMO) techniques. Absorbers with a high EUV extinction coefficient k allow for lower Best Focus Variation (BFV) through pitch and reduced Telecentricity Errors (TCE). This study evaluates Ta-Co alloys as potential high-k mask absorbers. It includes an experimental study of film morphology, surface composition, and stability of Ta-Co alloys as well as a theoretical investigation of the imaging performance. The optical constants were determined from EUV angle-dependent reflectivity measurements for three selected compositions, viz. TaCo, Ta₂Co, and TaCo₃. The Ta-Co alloys exhibit a higher EUV extinction coefficient k compared to the currently used TaBN absorber. TaCo and Ta₂Co demonstrated smooth surfaces, were stable in a hydrogen environment, and in mask cleaning solutions. These qualified compositions of Ta-Co alloys were selected for aerial image simulations and compared with a TaBN absorber. The optimized absorber thickness for Ta-Co alloy absorbers is based upon NILS enhancement, Threshold to Size (TtS) and balancing of diffraction order amplitudes. A 10 nm alternate line-space pattern with a pitch of 20 nm and 14 nm square contact holes with a pitch of 28 nm were considered for the simulation study using High NA 0.55 EUV lithography process settings. The through pitch imaging performance was evaluated using NILS, TCE and BFV as metrics. Ta-Co alloys allow for a reduction in M3D effects at smaller absorber thickness compared to a 60 nm TaBN absorber.

Development of efficient absorber masks and highly reflective mirrors in the EUV spectral range is a key challenge for upcoming lithography techniques in semiconductor technology. There is an improved need to precisely know the optical constants of the materials at hand for specialized applications such as phase shifting absorber masks. A further field of application is the interpretation and accompanied modeling of scattering data of complex nanostructures. At PTB, we measured the spectral reflectance of thin film samples in the angular range from normal incidence to grazing incidence in the range of 10nm to 20nm using PTB’s lubrication-free Ellipso-Scatterometer at the soft x-ray beamline at the electron storage ring BESSY II. This allowed us to determine the optical constants of a variety of metals, semimetals and their alloys from model fits based on Fresnel’s equations for layered material stacks.

Novel mask absorber designs are catching the attention of the EUVL community due to their ability to mitigate mask 3D effects. Material selection is part of such an optimization. We propose several candidates as novel EUV lithography mask absorbers, namely TaTeN, Ru–Ta, and Pt–Mo alloys. The choice of these materials is based on their theoretical performance evaluated by EUV imaging simulation based on their complex refractive index N ( λ ) = n ( λ ) + ik ( λ ) , where the optical constants n and k relate to the phase velocity and the absorption of electromagnetic radiation with a wavelength λ, respectively. The materials are deposited as thin films on Si substrate with an additional Ru layer to mimic the cap of the multilayer mirror on the real mask. The experimental n and k values are determined by analyzing EUV reflectivity data obtained using a 13.5-nm synchrotron EUV radiation. The imaging simulation presented consists of calculating several imaging metrics including non-telecentricity, normalized image log-slope, and threshold-to-size for specific use cases using the novel absorber. It also compares the proposed materials with the reference TaBN absorber. TaTeN shows higher absorption than TaBN and refraction closer to 1, which improves phase matching for a high k absorber. The refractive index of Ru–Ta and Pt–Mo alloys exhibits a large difference from that of air and provides the required phase shift of attenuated phase shift masks. The characterizations of these materials target the requirements of an EUVL mask: durability for mask cleaning, mask lifetime, and etchability for mask patterning. The stability is first tested against several standard mask cleaning solutions by a beaker test for up to 24 h. The samples are also exposed to hydrogen plasma to imitate the working environment in an EUV scanner. Concerning material patterning, chemical reactive ion etch is applied for preliminary tests. A proper etch recipe is found for TaTeN with a good etch rate (about 60 nm / min) and good selectivity to the Ru underlayer (Ru etch is negligible).