Even high-end optical components exhibit small amounts of imperfections, which can easily limit the performance of optical systems with respect to imaging contrast, optical throughput, imaging ghosts, and increased light scattering.Characterizing the scattering properties of optical components is thus an important step during the development of sophisticated optical systems as well as to identify and steadily improve materials as well as manufacturing and assembling steps.This is illustrated for different optical components as well as optical systems.Furthermore, different characterization concepts are discussed, which allow overcoming typical limits for angles resolved light scattering measurements, such as scattering very close to the specular beam directions (off specular scattering angles < 0.1°) or measurements in retro-reflection, which are important for gratings used in Littrow configuration or optical mirrors for laser-based communication.

In this contribution we will present different methods for analyzing straylight measurements in spectrometer gratings. For this purpose two different but very common types of gratings are investigated: a binary high resolution littrow grating and a silicon-crystel echelle reflection grating. We will present several measurements and simulations on such gratings. The focus lies in particular on the difference between grating ghosts and homogeneous scattering background. It is worked out, that the homogenous background must be evaluated by the well-established concept of ”angle resolved scattering”. Though, it is advantegeous to use the concept of ”angle resolve efficiency” for ghost analysis. Further, a simulation method is presented that allows to calculate straylight in diffraction gratings. The method is applied for ghost and background analysis and it is shown that not only the particular type of disturbance but also the grating geometry itself affects the straylight level and distribution.

Because of its high sensitivity to even small objects and the quick measurement principle, angle-resolved scattering (ARS) measurements exhibit a promising potential as a rapid analysis tool for bacterial cells at small sample sizes and very low numbers of cells. In this study, investigations on scattered light from various bacterial cell samples revealed applicability down to single cell levels, which is a huge benefit compared to conventional methods that depend on time-consuming cellular growth over several hours or even days. With the proposed setup and data analysis method, it is possible to detect scatter differences among cell types, together with the cell concentration.

Conceptually, high-precision manufacturing is a sequence of production and measurement steps, where both kinds of steps require to use non-deterministic models to represent production and measurement tolerances. This paper demonstrates how to effectively represent these manufacturing processes as Partially Observable Markov Decision Processes (POMDP) and derive an offline strategy with state-of-the-art Monte Carlo Tree Search (MCTS) approaches. In doing so, we face two challenges: a continuous observation space and explainability requirements from the side of the process engineers. As a result, we find that a tradeoff between the quantitative performance of the solution and its explainability is required. In a nutshell, the paper elucidates the entire process of explainable production planning: We design and validate a white-box simulation from expert knowledge, examine state-of-the-art POMDP solvers, and discuss our results from both the perspective of machine learning research and as an illustration for high-precision manufacturing practitioners.

The availability of high-resolution and high-throughput lithographic fabrication technologies, such as electron-beam lithography, based on variable shaped beam writing and character projection opens the way for the flexible use of various optical nano-structures for some of the most demanding applications. We discuss the technical features, advantages, and limitations of these pattering approaches and show how they can favorably be combined to realize optical nano-structures for applications, which are as diverse as gratings for ultra-short laser pulses or high-resolution spectrometers, computer generated holograms for asphere testing, various optical meta-structures (lenses and gratings), or UV-polarizers.

Absorption losses and laser-induced damage threshold (LIDT) are considered to be the major constraints for development of optical coatings for high-power laser optics. Such coatings require paramount properties, such as low losses due to optical absorption, high mechanical stability, and enhanced damage resistance, to withstand high-intensity laser pulses. In this work, heterostructures were developed by sub-nanometer thin films of SiO2 and HfO2 using the plasma-enhanced atomic layer deposition (PEALD) technique. Thin-film characterization techniques, such as spectroscopic ellipsometry, spectrophotometry, substrate curvature measurements, X-ray reflectivity, and Fourier transform infrared spectroscopy, were employed for extracting optical constants, residual stress, layer formation, and functional groups present in the heterostructures, respectively. These heterostructures demonstrate tunable refractive index, bandgap, and improved optical losses and LIDT properties. The films were incorporated into antireflection coatings (multilayer stacks and graded-index coatings) and the LIDT was determined at 355 nm wavelength by the R-on-1 method. Optical absorptions at the reported wavelengths were characterized using photothermal common-path interferometry and laser-induced deflection techniques.

We present the design, manufacturing and characterization results of a customized high-resolution echelle grating. The grating was manufactured at Fraunhofer IOF and delivered to the NIRPS (Near Infrared Planet Searcher Instrument) consortium. The technology workflow for the manufacturing of the echelle grating is relying on wet-chemical etching, applied to crystalline silicon substrates, which enables the creation of highly determined micro-facets and surfaces over macroscopic dimensions. The echelle’s grating period and plateau size within one period are established based on electron-beam lithography. A binary pattern in a hard mask material is performed by dry-reactive ion etching while transferring the pattern in the silicon substrate is achieved by wet-chemical etching with potassium hydroxide. The grating is designed to operate at a blaze angle of 76° in a wavelength band of 0.9μm – 1.8μm. A gold coating is applied to increase the diffraction efficiency to about 70%; verified at wavelengths of 1030nm and 1640nm, respectively. The overall grating size is 78mm x 284mm providing a WFE of less than 70nm (RMS) measured throughout the full aperture. In this article we present the manufacturing workflow and structural inspection results of the manufactured echelle grating, having a critical eye on the impact of sub-surface defects of the initial silicon crystal. Moreover, we present optical performance test results covering diffraction efficiency, PSF, WFE and spectral ghosts. It is concluded that the imaging properties of the manufactured grating are as good as those of a plane gold mirror reference. Additional presentation content can be accessed on the supplemental content page. Additional presentation content can be accessed on the supplemental content page.

The development of a small size prototype of a UV-grating for the CUBES instrument of ESO’s VLT is presented. It has a line density of 3600 l/mm and is manufactured on a fused-silica substrate using electron-beam lithography, reactive ion etching and atomic layer deposition. In the ideal case the grating has a pure lamellar profile with a groove width in the range of 100nm only. To achieve a high polarization independent diffraction efficiency the grating depth is required to be in the range of 700nm and the duty cycle needs to be met with an accuracy in the nm-range. To achieve this high aspect ratio with sufficient accuracy a trimming process based on a conformal overcoating by ALD is performed.

The optical performance of coatings critically depends on the surface roughness, defects, and particles on the substrate. These imperfections can be elegantly characterized by light scattering, which also enables the characterization of freeform surfaces.

Subsurface damage (SSD) in optical components is almost unavoidably caused by mechanical forces involved during grinding and polishing and can be a limiting factor, particularly for applications that require high laser powers. In this contribution, non-destructive characterization techniques are evaluated with respect to their capability to determine SSD in fused silica. For this, differently polished surfaces with different SSD levels have been prepared. An initial destructive analysis using etching in hydrofluoric acid in combination with white light interferometry revealed a high amount of SSD in one of the sample types compared to a very low amount of SSD in a second one. It is shown that nondestructive absorption as well as scattering measurements are sensitive towards SSD related differences in the samples. Finally, laser-induced damage tests proved a significant impact of SSD on the laser stability by determining a reduced damage threshold of 31 ± 3 J/cm² for the sample with high amount of SSD compared to 45 ± 5 J/cm² for the high-quality polished sample.

Aluminum thin films with thicknesses between approximately 10 and 60 nm have been deposited by evaporation and sputtering techniques. Layer characterization focused on reflectance, optical constants, and surface quality. Reflectance fits have been performed using a merger of three standard dispersion models, namely the Drude model, the Lorentzian oscillator model, and the beta-distributed oscillator model. A thickness dependence of the optical constants could be established in the investigated thickness range.

Light scattering induced by contamination and defects on optical components can quickly limit the component’s performance. Therefore, imperfection analysis and budgeting are mandatory - but also challenging tasks. On the other hand, imperfections can be elegantly characterized using efficient, robust and non-contact light scattering techniques. This will be demonstrated in this contribution for area covering measurement approaches using laboratory instruments with highest sensitivity as well as elaborated sensor systems that are best suited for extended freeform surfaces. Moreover, the measurement results are used to derive practical imperfection scattering data and models that serve as input to model and predict the imperfection induced scattering on optical system level.

Sub-aperture fabrication techniques such as diamond turning, ion beam figuring, and bonnet polishing are indispensable tools in today's optical fabrication chain. Each of these tools addresses different figure and roughness imperfections corresponding to a broad spatial frequency range. Their individual effects, however, cannot be regarded as completely independent from each other due to the concurrent formation of form and finish errors, particularly in the mid-spatial frequency (MSF) region. Deterministic Zernike polynomials and statistical power spectral density (PSD) functions are often used to represent form and finish errors, respectively. Typically, both types of surface errors are treated separately when their impact on optical performance is considered: (i) wave aberrations caused by figure errors and (ii) stray light resulting from surface roughness. To fill the gap between deterministic and statistical descriptions, a generalized surface description is of great importance for bringing versatility to the entire optical fabrication chain by enabling easy and quick exchange of surface topography data between three disciplines: optical design, manufacturing, and characterization. In this work, we present a surface description by stitching the amplitude and unwrapped phase spectra of several surface topography measurements at different magnifications. An alternative representation of surface errors at different regimes is proposed, allowing us to bridge the gap between figure and finish as well as to describe the well-known MSF errors.

Over the last few decades, remarkable progress has been made in the field of multilayer coatings for the EUV spectral region, mainly due to the demands of EUV lithography for the semiconductor industry. The progress is associated with a deep understanding of the multilayer film growth, application of advanced smoothing technologies for multilayer interfaces, as well as the development of modern deposition and characterization techniques. The projection optics precisely coated by Mo/Si mirrors with 70 % reflectivity is the heart of the performance of industrial EUVL steppers recently developed by ASML for high-volume chip production. While the EUVL at 13.5 nm has just launched into the market, the first high-performance LaN/B multilayer mirrors with reflectivity up to 61 % were developed for the next emerging lithography generation with a reduced wavelength of 6.6 nm. Recent technological progress in EUVL optics and radiation sources triggered the development of compact microscopes in the water window spectral range (2.4 – 4.4 nm). The optimization of the multilayer design and deposition process of Cr/V, Cr/Sc and Cr/C mirrors resulted in reflectivity of 14.5% at 2.42 nm, 27.0 % at 3.16 nm, and 15,6% at a wavelength of 4.42 nm, respectively. These promising and still not limiting results indicate a large potential for future application of multilayer coated optics in various application fields such as microscopy in the water window, plasma diagnostics, spectroscopy, and astrophysics. SUMMARY Induced mainly by the production of more powerful and energy-efficient electronic circuits with the aid of Extreme Ultraviolet Lithography (EUVL), operated at the wavelength of 13.5 nm, optics developments in recent years have pushed the boundaries for reflection coatings at short light wavelengths further away [1]. Since all materials and gases absorb EUV radiation, the reflective optics make use of the constructive interference of the light partially reflected from many interfaces within the coated multilayer stack to achieve high reflectivity of more than 70 %, while a single surface reflects less than 1 % of EUV light near-normal incidence. The high absorption also requires that the EUVL process be carried out in a vacuum environment. Interference coatings made from molybdenum (Mo) and silicon (Si) enable theoretical peak reflectance values of up to 75.4 % at 13.5 nm. A challenge, however, is to achieve such high reflectance values in practice and combine this with high functional stability in order to realize an efficient operation of an EUV wafer stepper system. The projection optics precisely coated by Mo/Si mirrors with 70 % reflectivity is the heart of the performance of EUV wafer steppers [2] recently developed by ASML for high volume chip production with printing feature sizes of 7 nm and beyond [3]. In 2022 ASML hopes to ship a next-generation EUV system with increased numerical apertures, which is targeted for a printing resolution down to 3 nm in 2023 [3]. This paper summarizes the current progress and the present knowledge collected at Fraunhofer IOF in the design, fabrication, and characterization of EUV multilayer coatings with excellent optical performance and functional stability. FIGURE 1: General view (left) and substrate loading (right) of sputtering system NESSY-3 designed for the precision coating of EUV optics with lateral thickness gradient. [4]

Monte Carlo Tree Search (MCTS) has shown its strength for a lot of deterministic and stochastic examples, but literature lacks reports of applications to real world industrial processes. Common reasons for this are that there is no efficient simulator of the process available or there exist problems in applying MCTS to the complex rules of the process. In this paper, we apply MCTS for optimizing a high-precision manufacturing process that has stochastic and partially observable outcomes. We make use of an expert-knowledge-based simulator and adapt the MCTS default policy to deal with the manufacturing process.

Summary Over the last few decades, remarkable progress has been made in the field of multilayer coatings for the EUV spectral region, mainly due to the demands of EUV lithography for the semiconductor industry. The progress is associated with a deep understanding of the multilayer film growth, application of advanced smoothing technologies for multilayer interfaces, as well as the development of modern deposition and characterization techniques. The projection optics precisely coated by Mo/Si mirrors with 70 % reflectivity is the heart of the performance of industrial EUVL steppers recently developed by ASML for high‐volume chip production. While the EUVL at 13.5 nm has just launched into the market, the first high‐performance LaN/B multilayer mirrors with reflectivity up to 61 % were developed for the next emerging lithography generation with a reduced wavelength of 6.6 nm. Recent technological progress in EUVL optics and radiation sources triggered the development of compact microscopes in the water window spectral range (2.4 – 4.4 nm). The optimization of the multilayer design and deposition process of Cr/V, Cr/Sc and Cr/C mirrors resulted in reflectivity of 14.5% at 2.42 nm, 27.0 % at 3.16 nm, and 15,6% at a wavelength of 4.42 nm, respectively. These promising and still not limiting results indicate a large potential for future application of multilayer coated optics in various application fields such as microscopy in the water window, plasma diagnostics, spectroscopy, and astrophysics.

The potential of titanium and copper seed layers to enhance the optical properties of aluminum films for ultra-violet (UV) applications is analyzed. The seed layers significantly influence the initial layer growth of aluminum films. For the titanium-seeded aluminum, a surface roughness of 0.34 nm was observed. UV spectral reflectance measurements showed an average higher reflectivity of 4.8% for wavelengths from 120 nm to 200 nm for the aluminum film grown on the titanium seed layer. Furthermore, the titanium-seeded aluminum coatings were stable at an elevated temperature of 225°C and showed no increase in surface roughness or pinholes.

The 2019 Optical Interference Coatings measurement problem comprised the determination of the total backscattering, forward scattering, reflectance, and transmittance spectra of a multilayer system.

Diffuse reflecting (white) and highly absorbing (black) fused silica based materials are presented, which combine volume modified substrates and surfaces equipped with anti-reflective moth-eye-structures. For diffuse reflection, micrometer sized cavities are created in bulk fused silica during a sol-gel process. In contrast, carbon black particles are added to get the highly absorbing material. The moth-eye-structures are prepared by block copolymer micelle nanolithography (BCML), followed by a reactive-ion-etching (RIE) step. The moth-eye-structures drastically reduce the specular reflectance on both diffuse reflecting and highly absorbing samples across a wide spectral range from 250 nm to 2500 nm and for varying incidence angles. The adjustment of the height of the moth-eye-structures allows us to select the spectral position of the specular reflectance minimum, which measures less than 0.1%. Diffuse Lambertian-like scattering and absorbance appear nearly uniform across the selected spectral range, showing a slight decrease with increasing wavelength.

Light scattering-based characterization techniques are fast, sensitive, and robust, which can be exploited for roughness measurements or homogeneity and defect assessment. However, only compact scattering instruments enable even large freeform surfaces to be inspected close to fabrication or even within fabrication processes. The combination of a light scattering sensor with an industrial robot, as well as its application for full surface evaluation of aspheriscal and freeform surfaces, is addressed. In addition to the technical considerations regarding the sensor, the robotic handling system, or the sample curvature, examples of application to different optical components fabricated at the Fraunhofer Institute for Applied Optics and Precision Engineering using state-of-the-art techniques are presented. The results include the discussion of anisotropic surface structures induced by diamond turning or polishing as well as the analysis of the influence of a protection layer. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)