The rapid developments in optical technologies generate increasingly higher and sometimes completely new demands on the quality of materials, surfaces, components, and systems. Examples for such driving applications are the steadily shrinking feature sizes in semiconductor lithography, nanostructured functional surfaces for consumer optics, and advanced optical systems for astronomy and space applications. The reduction of surface defects as well as the minimization of roughness and other scatter-relevant irregularities are essential factors in all these areas of application. Quality-monitoring for analysing and improving those properties must ensure that even minimal defects and roughness values can be detected reliably. Light scattering methods have a high potential for a non-contact, rapid, efficient, and sensitive determination of roughness, surface structures, and defects.
The continuous development of optical technologies and the accompanying requirements on the manufacturing process place challenging demands on metrology. In addition to highly sensitive and robust measurement techniques, the inspection tools should be fast and capable of characterizing large and complex-shaped surfaces. These aspects can be addressed by light-scattering-based characterization techniques, which also enable a large flexibility for the measurement conditions because of the noncontact data acquisition and are, thus, suited not only for ex situ but also in situ characterization scenarios. Application examples ranging from the roughness characterization of magneto-rheological finished substrates to polished extreme ultraviolet mirror substrates with diameters of more than 600 mm by compact as well as laboratory-based instruments are presented.
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.
Light scattering is one of the loss mechanisms of optical components. It is caused by intrinsic and extrinsic imperfections
such as roughness, index fluctuations, and bulk or surface defects that can all play critical roles for the laser stability of
optical components. Light scattering metrology has proven to be a versatile non-destructive technique to characterize
imperfections. Information can be retrieved with high sensitivity even over large areas. The total scatter levels or
scattering coefficients provide information about scatter losses whereas the angle resolved scattering provides detailed
information about the sources of scattering. An overview of the instruments developed at Fraunhofer IOF will be given
and a variety of examples of application will be discussed comprising roughness and defect maps of lithography optics,
investigations of bulk scattering of optical materials, and enhanced scattering through thickness errors of interference
Laser-induced damage of optical surfaces, thin film coatings, and materials is greatly influenced by imperfections such
as surface and interface roughness and surface or subsurface defects. All these imperfections give also rise to light
scattering. Light scattering techniques are thus well suited to identify and characterize damage-relevant features.
Additionally, they are non-contact, highly sensitive, and enable large sample areas to be investigated.
Conventional characterization techniques are usually confined to small sample areas. A light scattering method will be
presented that provides roughness and defect maps even of large and curved surfaces. Subsurface defects also play a
critical role as damage precursors. Many detection methods are still based on wedging and/or etching the sample surface.
A new non-contact approach to detect subsurface damage using polarized light scattering will be presented.
In addition, a method will be discussed that provides information about the structural and optical properties of multilayer
coatings by analyzing the scattered light distribution. Even small deviations of the illumination and the design
wavelengths or angles can lead to substantial field enhancements inside the coating which can be clearly observed as
resonant scattering wings.
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.
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.
Light scattering from interface imperfections or defects is a topic of persistent interest. On the one hand, light scattering
can be a critical issue that limits throughput and imaging quality of optical components even if high-end polishing and
coating techniques are employed. This in particular holds for multilayer coatings and for short wavelengths of
application. On the other hand, scattered light also carries valuable information about its origins, such as the structural
properties of multilayer coatings.
A large variety of instruments for scatter measurements in the visible, ultraviolet, infrared spectral ranges has been
developed at the Fraunhofer IOF in Jena during the past years. In addition, software tools have been developed based on
existing scattering theories which enable an advanced analysis of the observed scattering properties.
For the rather complicated scattering of thin film coatings, a simplified model was developed recently which introduces
two simple parameters to describe the roughness evolution from interface to interface and optical thickness deviations.
An iterative solution of the inverse scattering problem yields information about alterations of the structural and optical
properties inside multilayer coatings based on angle resolved scatter measurements. Experimental results will be
presented for highly reflective metal fluoride coatings for 193 nm as well as for a Rugate filter after laser damage tests.
It will be discussed how the procedure could be implemented in a laser damage test environment to enable the damage
behavior to be characterized in-situ during irradiation. This can provide valuable information about the fundamental
damage processes even prior to the ultimate damage events.