Alexander von Finck

Instrument for close-to-process light scatter measurements of thin film coatings and substrates

Scatter analysis is an effective method for the characterization of thin film components. The new highly sensitive table top system ALBATROSS-TT (3D-Arrangement for Laser Based Transmittance, Reflectance and Optical Scatter Measurement-Table Top) has been developed at the Fraunhofer Institute in Jena to meet the specific requirements for close-to-process applications. Extremely high sensitivity with a noise equivalent angle resolved scatter level of 2×10(-8) sr(-1), full three-dimensional spherical measurement capability, and an instrument size as small as 0.8 m×0.8 m×0.8 m have been achieved. Details of specifications, optical components, and software are presented, including a comparison to our laboratory system. Anisotropy analysis of diamond-turned aluminum substrates as well as substrate and coating characterization are demonstrated as examples of application.

Characterization of optical coatings using a multisource table-top scatterometer

Light scattering measurement and analysis is a powerful tool for the characterization of optical and nonoptical surfaces. To enable a more comprehensive postmeasurement characterization, three visible laser sources were recently implemented in a highly sensitive table-top scatterometer with 3D spherical detection capability. Based on wavelength scaling, the instrument is utilized to characterize thin-film coatings and their substrates with respect to surface roughness, roughness growth, and contamination. Topographic measurement techniques are used to verify the results. The spectral sensitivity to contamination (scatter loss) is demonstrated to be significantly different for single surfaces and interference coatings. In addition, power losses of a highly reflective coating are analyzed.

Sophisticated light scattering techniques from the VUV to the IR regions

Light scattering based characterization techniques are well suited to meet the challenging requirements for fast and sensitive finish assessment of optical surfaces. Further advantages are the high flexibility and robustness which enable the inspection of large geometries and freeform optics that are sometimes too complex for characterization techniques like atomic force microscopy or white light interferometry. In this paper, we report on the development of instruments for total and angle resolved light scattering measurements at wavelengths ranging from the vacuum ultraviolet to the infrared spectral regions. Extremely high sensitivities equivalent to surface roughness levels of below 0.1 nm and dynamic ranges of up to 15 orders of magnitude have been achieved. In addition to laboratory-based equipment, compact and table-top tools are discussed which enable the advantages of light scattering metrology to be used for characterization tasks close to or even in manufacturing processes. Instructive examples of applications are presented ranging from the characterization of diamond-turned and polished substrates to interference coatings, diffraction gratings, and IR window materials.

Estimating hemispherical scatter from incident plane measurements of isotropic samples scattering from both bulk and surface irregularities

In the mid-1970’s it became apparent that incident plane scatter data could be represented by simple two or three parameter expressions. This realization made possible the generation of stray light estimation codes which are used on everything from military weapons, to telescopes, to car headlights to flat panel display systems. Almost all of these applications estimate hemispherical scatter from incident plane measurements. The authors’ 2012 review of this process was limited to samples scattering just from surface roughness. In this paper hemispherical measurements are compared to calculations made from incident plane measurements using isotropic samples that scatter from bulk irregularities as well as surface scatter. The issue of non-isotropic samples is briefly introduced. The data is also analyzed to investigate reciprocity.

Light scattering characterization of optical components - BRDF, BTDF and scatter losses

Light scattering caused by imperfections of optical components can critically affect the performance of optical systems in terms of losses and image degradation. Because of the numerous potential sources of scattering such as roughness, surface and sub-surface defects, bulk inhomogeneities, as well as coatings, scattering properties must be carefully specified and measured at the wavelengths of application. Bidirectional Reflectance and Transmittance Distribution Functions (BRDF / BTDF) are used to quantify the angle resolved scattering properties. The data can be used as an input for optical engineering software just as FRED, ASAP, ZEMAX for stray light modeling. In addition, analyzing the scattered light can provide valuable information about the relevant imperfections. The presentation provides an overview of instrumentation for light scattering measurements at wavelengths ranging from the visible to the extreme ultraviolet and the infrared spectral regions. Examples of applications will be discussed ranging from superpolished mirrors to diffraction gratings, interference coatings, and black absorbing coatings.

Estimating hemispherical scatter from incident plane measurements of isotropic samples

In the mid-1970’s it became apparent that incident plane scatter data could be represented by simple two or three parameter expressions. This realization made possible the generation of stray light estimation codes which are used on everything from military weapons, to telescopes, to car headlights to flat panel display systems. Almost all of these applications estimate hemispherical scatter from incident plane measurements. For most of these applications estimates that are good to within a factor of three - or even an order of magnitude - are acceptable and can result in dramatic product improvements. This paper takes a closer look at how the out-of-incident-plane estimates are made and evaluates how accurate the process is for the case of optically rough isotropic surfaces. The estimating process is reviewed and estimates are compared to hemispherical scatter measurements. It will be shown how the empirical observations that led to the procedures now widely used can be understood using well-known scatter models that link the surface roughness and scattering properties.

Reducing light scattering from surface contaminations by thin film design

Abstract In most applications, contamination of optical thin film coatings is inevitable over time. State-of-the art approaches to tackle this problem are usually based on two strategies – avoiding contamination or removing already existing contamination. We demonstrate that the coating design can be tailored to reduce light scattering and stray light arising from particle contamination. This allows reducing the optical symptoms (light scattering) rather than trying to address the inevitable cause of the problem (contamination) itself. This new approach can consequently be easily combined with state-of-the-art approaches.

Standardization of light scattering measurements

Abstract In every advanced optical system, light scattering caused by the imperfections of optical components sooner or later becomes an issue that needs to be addressed. Light scattering can be a critical factor for both the throughput and the imaging quality of optical systems. On a component level, the quantities to describe these effects are the scatter loss or total scattering (TS) and the scattering distribution function or angle-resolved light scattering (ARS). In the last decades, a number of instruments have been developed worldwide for the measurement of TS and ARS. However, numerous pitfalls have to be avoided to obtain objective, reliable, and reproducible measurement results. This is, in particular, true for low scatter levels of high-end optical components. Standard procedures that have to be both concise and easy to implement are thus of crucial importance for the optics community. This paper tries to give an overview on existing standards as well as an outlook on new standards that are still being developed. Special emphasis is put on ISO standards jointly developed, reviewed, and revised by the international experts in the field.

Light scattering of interference coatings from the IR to the EUV spectral regions

Abstract Light scattering of optical components caused by residual imperfections can be a critical factor for their practical application. In particular, the scattering properties of optical interference coatings are rather complex. Yet, simple theoretical models and comparisons with experimental results provide valuable insight into the main impact factors and mechanisms. The magnitude of scattering and the dominating factors strongly depend on the wavelength of application in connection with the types of coatings used in the corresponding ranges. The paper, therefore, gives an overview of the scattering properties of coatings in different spectral regions including the visible, deep ultraviolet, and extreme ultraviolet and discusses strong in-band variations of the scattering characteristics that have been neglected so far.

Angle and wavelength resolved light scattering measurement of optical surfaces and thin films

Light scattering metrology has become more and more important with the development of cutting-edge optical components and systems. Light scattering is also a very versatile tool for the characterization of nanostructures and defects. While optical engineering and manufacturing are striving for ever increasing resolution of optical devices and lowest optical losses, the demands for highly resolved light scattering metrology have become extremely challenging. In this sense, “highly resolved” means: (i) measurements with high angular resolution, not just in one plane but within the entire scattering sphere, (ii) small near-angle limits, (iii) highest sensitivities and lowest instrument signatures close to or even below the Rayleigh scattering limit, as well as (iv) at-wavelength operation and, more recently, spectral resolution. Instruments for scatter measurements developed at Fraunhofer IOF to meet these demands are presented together with practical examples of application comprising roughness, sub-surface damage, and defects of polished surfaces and thin film coatings. Compact tools like a table-top 3D scatterometer and a CMOS-based scatter sensor are presented. Finally, we report on the development of a new instrument for spectroscopic angle resolved scatter measurements based on an OPO tunable laser.