An outstanding technique in point of ultra-precision as well as economical production of mirrors is Single Point
Diamond Turning (SPDT). The unique properties of the diamonds are used to get optical surfaces with roughness values
down to 5 nm rms (root mean square) and very precise form accuracy down to 70 nm rms and 500 nm p.-v. (peak to
valley) value over an area of 200 mm x 200 mm. This quality level is typical for applications in the Near Infrared (NIR)
and Infrared (IR) range.
For applications in the VIS and UV range the turning structures must be removed with a smoothing procedure in order to
minimize the scatter losses. Favorable is an aluminium base body plated with a thick-film of Nickel-Phosphorus alloy
(NiP). This alloy can be polished with computer assistance. Ion Beam Figuring (IBF) is the final manufacturing step. The
properties after the finishing process are better than 1 nm rms for roughness and down to 15 nm rms respectively 100 nm
p.-v. regarding the surface irregularity for complex optical shapes.
The techniques SPDT, polishing and IBF ensures a high quality level for large mirrors with plan, spherical or aspherical
surfaces. The manufacturing chain will be analyzed by surface characterisation based on 2D profilometry and white light
interferometry to measure the roughness and 3D-profilometry and interferometry to monitor the shape irregularity.
Scattering light analysis deepens these investigations.
This paper summarizes technologies and measurement results for SPDT and surface finish of metal mirrors for novel
optical applications.
Driven by the increasing demands on optical components for DUV lithography, a system for angle resolved as well as total scatter measurements (ARS and TS respectively) at 193 nm and 157 nm has been developed at the Fraunhofer Institute in Jena. Extremely low background scattering levels of 10-6 for the TS measurements and more than 12 orders of magnitude dynamic range for ARS have been accomplished. The variety of components to be measured extends from
super-smooth substrates with sub-nanometer roughness to multilayer systems with pronounced nanostructures. Examples are presented for scatter analysis of DUV dielectric multilayers as well as for roughness analysis of super-smooth EUV mirrors.
Driven by the increasing requirements for optical surfaces, components, and systems, scattering techniques for the analysis of optical losses, roughness and defects face novel challenges for high sensitivity and flexibility. In this paper we present set-ups developed at the Fraunhofer Institute in Jena for total scattering (TS) and angle resolved scattering (ARS) measurements from the vacuum ultraviolet (VUV) and deep ultraviolet (DUV) over the visible (VIS) up to the infrared (IR) spectral regions. Extremely high sensitivities down to 0.05 ppm have been achieved for TS measurements and a dynamic range of up to 15 orders of magnitude for ARS. The performance is demonstrated by examples for roughness analysis of smooth surfaces and scatter analysis of multilayer coatings and diamond-turned mirrors.
To meet the ever increasing requirements for high quality optical surfaces, components and systems, high-sensitive and flexible analysis of optical losses, nano-roughness and defects is necessary. In this paper, we present the set-ups developed at the Fraunhofer Institute in Jena for total and angle resolved scattering measurements from the VUV and VIS up to the IR spectral regions. Examples are presented for light scattering investigations of the texture of ultraprecision mirrors produced by diamond turning. Experiments are described which were carried out to test the possibilities for nano- and micro defect detection using these set-ups. Furthermore, the relationship between the scattering properties of dielectric multilayer mirrors and their interface texture is discussed for highly reflective DUV mirrors.
Light scattering techniques allow a comprehensive characterization of surfaces and thin film coatings. Driven by the increasing demands on optical components for DUV lithography at 193 nm, a system for ARS and TS measurements at 193 nm and 157 nm has been developed at the Fraunhofer Institute in Jena. The set-up and measurement examples are
presented ranging from ARS of low-scattering substrates and dielectric multilayers to the roughness analysis of EUV mirrors. To follow the recent developments of semiconductor industry towards EUV lithography at 13.5 nm, a new EUV scattering measurement system is developed. The current status is reported.
Combined measurements of transmission T, absorption A and total scattering TS revealed the high accuracy of all applied measurement techniques by obtaining a sum T+A+TS+R = (100±0.3)% (R denotes the Fresnel reflection). In order to investigate CaF2 at high fluences, a variety of samples from high purity excimer grade to research grade was irradiated (80 ... 150 mJ/cm2, 2*106...7*106 pulses) and characterized before and after irradiation by total scattering, laser induced fluorescence (LIF) and transmission measurements. Total scattering mappings showed negligible and
measurable scattering in excimer grade and some research samples of minor purity, respectively. For the first time to our knowledge, laser induced fluorescence measurements revealed increasing (580nm, 740 nm) as well as decreasing (313 nm, 333 nm) emissions. The small increases of the linear absorption, obtained in all samples by transmission measurements, were used to distinguish high from minor quality material. For high quality samples the linear absorption change scales with NH3 (N: number of pulses), whereas for minor quality research samples a NH2-scaling was found.
We report on an arrangement that measures angle resolved scattering (ARS), total scatter (TS), transmittance (T), and reflectance (R) at 157 nm and 193 nm. The ARS set-up is based on a high precision double goniometer arrangement, which can be inserted into the measurement chamber without removing the TS set-up. The TS set-up for detection of forward scatter and backscatter with extremely low background scatter levels of 1 ppm consists of an excimer laser, a Coblentz sphere with detection system, and a beam preparation path. The sphere and preparation path are housed in vacuum chambers allowing operation in vacuum or purge gas. The measurement options altogether constitute a multifunctional system: VULSTAR (VUVLight Scatter, Transmittance, and Reflectance). We present total scatter measurements on deep ultraviolet (DUV) and vacuum ultraviolet (VUV) substrates and optical components with antireflective (AR) and highly reflective (HR) coatings, angle resolved measurements on optical components and R and T measurements on substrates for VUV optical coatings.
CaF2 has become the most important substrate for vacuum ultraviolet (VUV) optical coatings. The popularity of CaF2 relies on its transparency range, which extends down to 120 nm. A major drawback of the material is, however, its mechanical softness. Hence careful assessment of the surface quality is necessary in order to obtain low loss components. By combining roughness data from Nomarski Microscopy and Atomic Force Microscopy (AFM) measurements with total and angle resolved scattering measurements at 157 nm scatter losses due to interface roughness as well as bulk inhomogeneities are being studied. Measured losses are furthermore compared with theoretically predicted results. Results are also presented on VUV coatings. The question of the coupling between the substrate surface finishing quality and the optical properties of the coatings is specially addressed.
We have developed a system that measures total and angle resolved light scattering, reflectance and transmittance at 193 nm and 157 nm. This system allows the investigation of substrates and coatings for vacuum-ultraviolet (VUV) components with high sensitivity, down to scattering levels of 1 ppm for total scatter measurement. The dynamic range of the angle resolved scatter measurement set-up exceeds 9 orders of magnitude. Methods for evaluating the quality of CaF2 substrates for low loss optical components for 157 nm are presented. By using roughness data from Atomic Force Microscopy (AFM) measurements combined with scattering measurements surface roughness as well as inhomogeneities in the bulk of the material can be studied. Results are also presented of anti-reflective (AR) and highly reflective (HR) multiplayer coatings on CaF2.
A system is presented that measures total and angle resolved light
scattering, reflectance and transmittance at 193 nm and 157 nm.
Substrates and coatings for VUV lithography components can be
investigated with high sensitivity, down to scattering levels of 1 ppm.
Within the EUREKA-project EU 2359 (Instruments and Standard Test Procedures for Laser Beam and Optics Characterization) a "Round-Robin"-experiment on reflectivity measurements at the wavelength λ = 1.06 μm has been carried out. The topic of this "Round-Robin"-experiment was the evaluation of the Working Draft ISO/WD 13697 and Committee Draft ISO/CD 15368 and the comparison to simple transmission and reflectivity measurements.
For the determination of transmittance and reflectance of laser components, commercially available spectrophotometers are used in industry to monitor the quality at a medium accuracy level. If a higher level of precision is required, especially to resolve the reflectivity of high-reflecting dielectric mirrors, the measurement set-up described in ISO/WD 13697 should be preferred. This standard procedure was elaborated in the framework of the CHOCLAB project.
This paper analyzes the results of the "Round-Robin"-experiment; the accuracy and comparability of measurements according to different methods will be given.
Methods for evaluating the quality of CaF2 substrates for vacuum ultraviolet (VUV) low loss optical components are presented. Today superpolished CaF2 is available. However, major differences might still occur between batches and careful control is therefore necessary. By using roughness data from AFM measurements combined with total scattering measurements at 193 nm and 157 nm surface roughness as well as inhomogeneities in the bulk of the material can be studied. Results are also presented of anti-reflective (AR) and highly reflective (HR) multilayer coatings on CaF2 where reduced total backscatter scattering was found for the AR-coating as compared to the substrate.
This paper reports on an instrument designed to measure the total backward and forward scattering of optical components down to the DUV/VUV spectral region. The system is based on a Coblentz sphere imaging the light scattered into the backward or forward hemispheres within an angular range from 2 degree(s) to 85 degree(s) onto the detector according to ISO/DIS 13696. The equipment divides into two set-ups, one operating in air at several wavelengths from 10.6 micrometers to 193 nm, the other one working in a vacuum/nitrogen at 157 nm and 193 nm. The system is fully automated and capable of scanning large sample areas. Both a deuterium lamp and an excimer laser can be used as radiation sources at 193 nm and 157 nm. Results of measurements on fluoride multilayer coatings and CaF2 substrates are presented.
We discuss principles of optical surface quality assessment. The micro topography of well polished fused silica, CaF2 and Si surfaces was examined locally and by covering large sample areas. Power Spectral Densities (PSD) were used for consistent roughness description. Subsurface damage was detected by a modified white light interferometer technique and total scattering measurement.
Oxide single layers of niobium oxide and silicon dioxide with different film thicknesses and multilayers consisting of this both components were deposited by reactive sputtering on silicon substrates. The loss of optical performance by structural imperfections was characterized measuring the total light scattering in comparison to AFM roughness investigations. The measured scattered light and the rms roughness values calculated from the AFM images were found to correlated clearly with the density of macroscopic defects in the Nb2O5 and SiO2 single layers. With multilayers, similar effects were observed.
A variety of technical applications require surface roughnesses to be measured and characterized over a wide range of scale. Therefore, it is inevitable to combine different measurements methods, both optical and nonoptical. In this paper, we present results of white light interferometry, atomic force microscopy, and angle resolved scattering. In addition we use total scattering to control the surface roughness and its inhomogeneities over large sample areas. Power spectral density functions are used to combine the roughness data obtained from the measurements over difference scales and to characterize the surface by means of model PSDs. We present results from different statistically rough surfaces, such as quartz and black glass, Si-wafers, and polished brass surfaces.
The measurement of total scatter losses is a major prerequisite for the development, optimization and commercialization of high quality optical components. Especially in laser technology, optical scattering gained of importance in the source of the development of laser system with ever increasing output power and improved beam parameters. Besides its influence on the efficiency of laser systems and the beam steering arrangement, total scattering is an important safety aspect for application of these laser systems in materials processing, medicine and fundamental research. As a consequence of this global trend, working groups of TC 172/SC 9 initialized the development of an International Standard for the measurement of total scattering in optical components.
An apparatus for total integrated backscattering measurement is described that operates in the UV to IR spectral region. Background levels smaller than 0.1 ppm at 633 nm have been achieved. During the measurement, the sample surface is scanned automatically, yielding one- or two-dimensional scattering diagrams. From the latter, small defects on supersmooth surfaces can be localized. Results are reported of measurements on samples with different surface qualities such as supersmooth Si-wafers with sub-angstrom roughness, CaF2 substrates, thin film optical coatings and rough engineering surfaces. The equipment is involved in standardization project ISO/CD 13696'.
An instrument is described which enables the detection of backscattered light within a wide spectral region. The apparatus is based on a Coblentz sphere and is equipped with light sources from the UV to IR spectral region (248 nm to 10.6 micrometer). Results are reported of measurements on samples with different surface qualities such as supersmooth Si-wafers with sub-angstrom surface roughness, glass substrates, thin film optical components and machined surfaces. The set-up complies with a corresponding ISO project.
We investigated thin MgF2/LaF3- and LaF3/MgF2-layers on opaque glass with angle resolved light scattering to analyze the different scattering contributions. We already showed the possibility of separating the volume scattering from the interface contribution for a single MgF2-layer on BK7. In this work we concentrated on layers with thicknesses common for multilayer mirrors in the UV. The roughness of the interfaces was included in the calculations by the power spectral density measured with the atomic force microscope. This has proven to be better than a Gaussian or exponential correlation function ansatz. In order to obtain as much information as possible we also investigated the uncoated substrates and the corresponding single layer systems. We found again that the investigated layers, which have columnar structure, showed generally non-negligible volume scattering. Additionally crosscorrelation between the different interfaces must be taken into consideration.
A total integrated scattering measurement set-up is described which enables the detection of scattered light within a spatial frequency range from 0.0041 micrometers-1 to 4 micrometers-1. The apparatus is based on a Coblentz sphere and is equipped with light sources in the wavelength range from the UV to IR. Examples are presented of measurements on samples with rms- roughnesses from angstroms to microns.
A theoretical formalism and experimental methods are presented, which enable statistical fluctuations of the films bulk and interface roughness properties to be estimated from volume scattering and roughness scattering, respectively. The theoretical model is implemented in a numerical algorithm that allows one to optimize experimental strategies and to determine morphological parameters from measured scattering curves. Angle resolved scattering (ARS) measurements are performed on MgF2 films on glass substrates while varying the illumination and observation parameters. Atomic force microscopy (AFM) provides helpful additional information on the surface morphology.
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