Light scattering and roughness analysis of optical surfaces and thin films

Die in-line Charakterisierung optischer Oberflächen erfordert robuste, berührungslose und schnelle Verfahren mit hoher Sensitivität. Zur Erfüllung dieser Anforderungen bei der Bewertung von Oberflächenrauheit und Defekten waren die Entwicklung einer Streulichtsensorik sowie angepasster Analysemethoden notwendig. Diese Kombination ermöglicht einerseits die Bewertung des durch Oberflächenunvollkommenheiten hervorgerufenen Störlichts. Andererseits können durch modelltheoretische Zusammenhänge Information über die das Streulicht verursachenden Strukturen selbst gewonnen werden. Das realisierte Sensorkonzept für 3D winkelaufgelöste Streulichtmessungen ermöglicht Rauheitsmessungen an optischen Oberflächen glatter als 0.3 nm in weniger als einer Sekunde. Dabei reicht der erfasste Streuwinkelbereich von weniger als 10° um den spekularen Reflex aus, um Rauheitsspektren in einem Ortsfrequenzbereich von eineinhalb Dekaden zu analysieren Die aus Sensormessungen gewonnenen Informationen ermöglichen insbesondere eine zeiteffiziente Charakterisierung anisotroper Oberflächenstrukturen. Darauf aufbauend konnten die charakteristischen Strukturen von Fertigungsverfahren wie Diamantdrehen oder magnetorheologischem Polieren untersucht werden. Durch die Implementierung von zwei Beleuchtungskanälen sowie die Anwendung von Rauheitsmodellen, konnte die Rauheitsentwicklung von Titanschichten untersucht werden. Dabei wurde sogar eine höhere Datengüte erzielt als mit konventionellen topographiebasierten Verfahren. Ein Streulichtmodell für ein Notchfilter-Schichtsystem wurde durch Messung und Modellierung von Rauheit und Streulicht erarbeitet. Auf dieser Basis wurden zusätzliche Anwendungsszenarien für die sensorbasierte in-line Charakterisierung des Filters erarbeitet und erprobt. Die Klassifizierung von Oberflächendefekten mittels Streulichtsensor konnte erfolgreich demonstriert werden. Die Zuverlässigkeit dieser Prozedur wurde zusätzlich in experimentellen und modelltheoretischen Untersuchungen ermittelt

International round-robin experiment for angle-resolved light scattering measurement

International audienc

Light Scattering from Contamination and Defects - Measurement, Analysis, and Modelling

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

Surface characterization of high-end optical components using light scattering

Light scattering is a powerful tool to assess the quality and the performance of high-end optical surfaces. In addition to laboratory tools, a compact scatter sensor has been developed to enable even large freeform surfaces to be analyzed

Impact Of Surface Roughness On The Scatter Losses And The Scattering Distribution Of Surfaces And Thin Film Coatings

The estimation of the impact of surface roughness on the light scattering losses and the scattering distribution is of crucial importance for deriving roughness specifications for optical surfaces. A detailed roughness analysis should always be based on surface Power Spectral Density functions and the band-limited roughness relevant for the application at hand. The scattering from single surfaces can easily be estimated using rather simple formulas. The most commonly used expression to estimate the total scattering, however, is only valid if the roughness is small and the correlation width is large compared to the wavelength of light. A special expression has been used in the thin film community for surface structures with short correlation lengths. It will be demonstrated that distinguishing between these limiting cases is unnecessary simply by using the concept of band-limited roughness. Different models are compared to results of scatter measurements and discussed with respect to their ranges of validity. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)

Comprehensive nanostructure and defect analysis using a simple 3D light-scatter sensor

Light scattering measurement and analysis is a powerful tool for the characterization of optical and nonoptical surfaces. A new 3D scatter measurement system based on a detector matrix is presented. A compact light-scatter sensor is used to characterize the scattering and nanostructures of surfaces and to identify the origins of anisotropic scattering features. The results from the scatter sensor are directly compared with white light interferometry to analyze surface defects as well as surface roughness and the corresponding scattering distributions. The scattering of surface defects is modeled based on the Kirchhoff integral equation and the approach of Beckmann for rough surfaces

Design, additive manufacturing, processing, and characterization of metal mirror made of aluminum silicon alloy for space applications

Metal mirrors are used for spaceborne optical systems, such as telescopes and spectrometers. In addition to the optical performance, the mechanical needs and the mass restrictions are important aspects during the design and manufacturing process. Using the additive manufacturing process, optimized internal lightweight structures are realized to reduce the weight of the system while keeping the mechanical stability. A mass reduction of ≈60.5  %   is achieved. Using the aluminum silicon alloy AlSi40, the thermal mismatch of the mirror base body to a necessary electroless nickel-polishing layer is minimized. Based on an exemplary mirror design, the optimization of the interior lightweight structure is described, followed by the manufacturing process from additive manufacturing to diamond turning, plating, and polishing. Finally, the results of surface metrology and light scattering measurements are presented. A final form deviation below 80 nm p  .    −  v  .   and a roughness of ∼1  nm rms could be demonstrated

Evaluation of subsurface damage by light scattering techniques

Subsurface damage (SSD) in optical components is almost unavoidably caused by mechanical forces involved during grinding and polishing and can be a limiting factor, in particular for applications that require high laser powers or an extreme material strength. In this paper, we report on the characterization of SSD in ground and polished optical surfaces, using different light scattering measurement techniques in the visible and extreme ultraviolet spectral ranges. The materials investigated include fused silica, borosilicate glass, and calcium fluoride. The scattering results are directly linked to classical destructive SSD characterization techniques, based on white light interferometry, optical microscopy, and atomic force microscopy of the substrate topography and cross sections obtained after etching in hydrofluoric acid and fracturing