The surface figure of x-ray mirrors can be improved by differential deposition of thin films. To achieve the required corrections, WSi2 layers of variable thickness were deposited through beam-defining apertures of different openings. The substrates were moved in front of the particle source with specific velocity profiles that were calculated with a deconvolution algorithm. Two different DC magnetron sputter systems were used to investigate the correction process. Height errors were evaluated before and after each iteration using off-line visible light surface metrology. Four 300mm long flat Si mirrors were used to study the impact of the initial shape errors on the performance of the correction approach. The shape errors were routinely reduced by a factor of 20 to 30 down to levels below 0.5nm RMS.
The surface figure of an x-ray mirror was improved by differential deposition of WSi2 layers. DC magnetron sputtering through beam-defining apertures was applied on moving substrates to generate thin films with arbitrary longitudinal thickness variations. The required velocity profiles were calculated using a deconvolution algorithm. Height errors were evaluated after each correction iteration using off-line visible light surface metrology. WSi2 was selected as a promising material since it conserves the initial substrate surface roughness and limits the film stress to acceptable levels. On a 300 mm long flat Si mirror the shape error was reduced to less than 0.2 nm RMS.
The process of differential deposition is currently applied at the ESRF in order to correct figure errors of x-ray optics substrates, prior to multilayer deposition. The substrate is moved at a controlled speed in front of a sputtering source to precisely control the deposition profile. This work will describe the concept of differential deposition at the ESRF as well as recent results of its implementation to correct a real mirror substrate surface. Finally, initial studies using a synchrotron beamline characterization technique based on x-ray total reflection are presented.
The figure errors of an x-ray mirror were reduced by differential deposition of C/Pt layered structures. Different apertures were inserted into the particle beam to correct height errors on variable length scales down to less than 10 mm. The required velocity profile was calculated using a deconvolution algorithm. The film thickness profiles were measured directly by xray reflectivity. Height errors were evaluated using visible light surface metrology. The results of these different techniques are compared and discussed. After two iterations the shape error of a 300 mm long flat Si mirror was reduced by a factor of 5 to less than 1 nm RMS. This work describes the experimental techniques and discusses the achieved accuracy. It also addresses open questions such as roughness evolution, layer stress, and the interpretation of metrology data.
Differential deposition techniques were applied to reduce the figure error of x-ray mirrors. Cr layers were sputtered on flat substrates that were moved with variable speed in front of a beam defining aperture. The required velocity profile was calculated using a deconvolution algorithm. The Cr thickness profiles were derived in two ways: directly, using x-ray reflectivity and indirectly, by measuring the surface figure before and after the deposition. After two iterations the mirror surface figure could be improved by almost one order of magnitude. This work will describe the experimental techniques and discuss the achieved accuracy. It will also address open questions such as layer stress, roughness evolution, and limitations of the available instrumentation.
X-ray mirror figure errors are commonly measured in the synchrotron community using Long Trace Profiler (LTP) or Nanometer Optical measuring Machine (NOM) instruments, both providing 2D slope measurement. 3D reconstruction is possible but time consuming, and requires a high stability of environmental conditions over long periods which is not easy to achieve. Characterisation of the complete topography of the mirror surface is essential for the application of deterministic figure correction techniques and also to reveal undesired stresses or deformations, such as twist, introduced by optomechanical mounting. At the ESRF metrology laboratory Fizeau stitching methods are under development. A full automated mechanical setup dedicated to stitching measurement of long flat mirrors is now operational. We have previously demonstrated accurate reconstruction by stitching 2D profiles acquired from Fizeau subaperture measurements. This work is focused on 3D reconstruction of flat mirror surfaces up to one meter long. Repeatability, accuracy and in particular the influence of the transmission element will be discussed.
The upgraded ESRF end station ID16A was equipped with a new Kirkpatrick-Baez (KB) nano-focusing setup. The figured KB mirrors were coated with steeply graded W/B4C multilayers to account for the variable angle of reflection along the beam footprint. The multilayers were deposited at the ESRF Multilayer Facility by DC magnetron sputtering in dynamic mode, where the substrates move in front of the sputter cathodes. The present work deals with the design, the fabrication, and the characterization of the coatings. First results obtained during commissioning experiments on ID16A complement the report.
The design, manufacture and characterization of a Kirkpatrick-Baez (KB) configuration mirror system for high-throughput
nanofocusing down to 50 nm beam sizes are described. To maximize the system aperture whilst retaining
energy tunability, multilayer coated optics are used in conjunction with 2 dynamically figured mirror benders. This
approach, which has been developed at the ESRF for many years, allows the focusing performance to be optimized when
operating the system in the 13-25 keV photon energy range. Developments in the key technologies necessary for the
production of mirror bending systems with dynamic figuring behavior close to the diffraction limit requirements are
discussed. These include system optimization via finite element analysis (FEA) modeling of the mechanical behavior of
the bender-mirror combination, manufacturing techniques for precisely-shaped multilayer substrates, multilayer
deposition with steep lateral gradients and the stitching metrology techniques developed for the characterization and
figure optimization of strongly aspherical surfaces. The mirror benders have been integrated into a compact and stable
assembly designed for routine beamline operation and results of the initial performance of the system at the ESRF
ID22NI endstation are presented demonstrating routine focusing of 17 keV X-rays to sub-60 nm resolution.
The degree of coherence preservation of x-ray multilayers was investigated using Talbot imaging on the ESRF undulator
beamline ID06. Several W/B4C multilayer mirrors with differing d-spacings were studied with monochromatic light at
various photon energies. To understand the respective influence of the underlying substrate and the multilayer coatings,
measurements were made under total reflection, at different Bragg peaks, and on the bare substrates. In addition, samples
with different substrate quality were compared. The relation between spatial coherence preservation and the visibility of
characteristic line structures in the x-ray beam will be discussed.
The ESRF has initiated an ambitious ten-year upgrade program involving the construction of eight new beamlines and
significant refurbishment of existing instruments. The availability of high-precision X-ray optical elements will be a key
factor in ensuring the successful implementation of these beamline projects. Particular challenges are to ensure the
necessary optical quality for X-ray beam coherence preservation and high numerical-aperture high focusing systems.
Surface optical metrology is a key tool, not only for the quality control, but also in improving the manufacturing
processes of such components. Amongst the most demanding tasks is the characterisation of the surface topography of
highly aspheric surfaces for reflective nanofocusing technologies which typically require measurement of shape errors in
the nm range.
In order to satisfy these new demands, the ESRF metrology laboratory has recently been equipped with two new
instruments: a Fizeau interferometer and a micro-interferometer. In parallel the long trace profiler has been continuously
developed to increase both stability and accuracy.
In this paper we will present the new instrumentation and associated techniques like micro-stitching interferometry used
to measure typical high quality X-ray mirrors.
We will also focus on the parameters that can affect repeatability and accuracy of the radius of curvature assessment of
flat optical surfaces, in particular when measuring with the long trace profiler. Finally an example of the power spectral
density function based on our instrument measurements of a typical high quality x-ray mirror will be shown.
The first series of metrology round-robin measurements carried out in 2005 at the APS, ESRF and SPring-8 metrology
laboratories involving two flat x-ray mirrors and a cylindrical x-ray mirror has shown excellent agreement among the
three facilities' Long Trace Profilers (LTP) despite their architectural differences. Because of the growing interest in
diffraction-limited hard x-ray K-B focusing mirrors, it was decided to extend the round robin measurements to spherical
and aspheric x-ray mirrors. The strong surface slope variation of these mirrors presents a real challenge to LTP. As a
result, new LTP measurement protocol has to be developed and implemented to ensure measurement accuracy and
consistency.
In this paper, different measurement techniques and procedures will be described, the results will be discussed,
and comparison will be extended to micro-stitching interferometry measurements performed at Osaka University, Japan.
R. Burns, A. Chumakov, G. Carbone, S. Connell, D. Dube, H. Godfried, J. Hansen, J. Härtwig, F. Masiello, M. Rebak, A. Rommeveaux, R. Setshedi, P. Van Vaerenbergh, A. Gibaud
There is currently interest in low strain HPHT diamond due to its expected application as various types of X-ray optical
elements at Synchrotrons, where the X-ray intensity is becoming progressively too severe for the existing materials. The
diamond crystals need to be synthesised with unprecedented lattice quality. In recent measurements at the ID19
beamline of European Synchrotron Radiation Facility (ESRF), the strain sensitivity of the (quantitative) X-ray plane
wave monochromatic topography was increased to the level of 10-8 using the double crystal technique with successively
higher order reflections and correspondingly higher energy X-rays. At this level the strain fields of certain defects have
a clearly visible macroscopic extent. In particular, both compressive and tensile strain fields of sparse single
dislocations are well observed, as are long range strain fields due to isolated surface scratches. The surface processing
of diamond for low roughness and good near surface crystal quality is a priority. A study of the progress towards this
goal using the X-ray techniques of reflectivity, Grazing Incidence small angle X-ray Scattering (GISAXS) and Grazing
Incidence X-ray Diffraction (GID) has been undertaken. The ability of diamond X-ray optical elements to process X-ray
beams while preserving the coherence properties of the beam is essential to establish, and measurements of this via the
Talbot effect have been carried out. This contribution will detail some of the latest results and comment on future
prospects.
Periodic multilayers exposed to a non-destructive annealing sequence have shown reversible and irreversible structural
modifications. In-situ x-ray reflectometry experiments at the ESRF bending magnet beam line BM5 demonstrate that
the overall periodic structure remains stable during the annealing process. At the same time, initially present
asymmetric interdiffusion layers have been reduced, in particular, in Ru/B4C.
The controlled thermal treatment of multilayer optics before its installation on synchrotron beam lines can help to
avoid alterations during their use as optical elements. An important issue is the reduction of stress introduced during
the coating process. The evolution of stress in multilayer test coatings deposited on wafers was worked out from
measurements done by optical metrology before and after coating and annealing.
The investigation of the influence of a thermal action on their reflectivity response is a real challenge. We will present
our experimental approach: deposition technique, multilayer choice, isothermal sequence, reflectivity and stress
measurements. We will also discuss compromises made to keep both reflectivity and stress optimized versus thermal
treatment. Future studies will have to deal with the impact of radiation on multilayer optics and its distinction from
annealing effects.
Laterally graded W/B4C multilayers were conceived for the focusing of hard X rays at 3rd generation synchrotron sources. They were deposited using a differential sputter coating technique. The multilayer mirror was bent to the correct shape on a dynamical bending device applying automated alignment routines. During experiments on the ESRF beamline ID19 the undulator source was focused vertically to a 41 nm (FWHM) wide line using a photon energy of 24 keV. The measured line width can be attributed to the finite source size, to diffraction effects, and to slope errors of the mirror. The potential impact of beam penetration into the multilayer will be discussed.
This paper presents the first series of round-robin metrology measurements of x-ray mirrors organized at the Advanced Photon Source (APS) in the USA, the European Synchrotron Radiation Facility in France, and the Super Photon Ring (SPring-8) (in a collaboration with Osaka University,) in Japan. This work is part of the three institutions' three-way agreement to promote a direct exchange of research information and experience amongst their specialists. The purpose of the metrology round robin is to compare the performance and limitations of the instrumentation used at the optical metrology laboratories of these facilities and to set the basis for establishing guidelines and procedures to accurately perform the measurements. The optics used in the measurements were selected to reflect typical, as well as state of the art, in mirror fabrication. The first series of the round robin measurements focuses on flat and cylindrical mirrors with varying sizes and quality. Three mirrors (two flats and one cylinder) were successively measured using long trace profilers. Although the three facilities' LTPs are of different design, the measurements were found to be in excellent agreement. The maximum discrepancy of the rms slope error values is 0.1 μrad, that of the rms shape error was 3 nm, and they all relate to the measurement of the cylindrical mirror. The next round-robin measurements will deal with elliptical and spherical optics.
The ESRF optics metrology laboratory was created 15 years ago. Various measurement devices have been progressively installed and the present status of available equipment will be briefly presented. Since the beginning of the first beamline construction, all X-ray mirrors have been tested before their installation. Most of the mirrors are mounted on mechanical bending systems, and it is mandatory to characterize optical elements under working conditions and to calibrate the systems before their installation on a beamline. These calibrations are now part of the acceptance tests whenever a system is delivered. Optics tests carried out on the Long Trace Profiler (LTP) will be described, with particular emphasis on the special configuration developed for mirrors facing down. Measurement reproducibility and accuracy achieved with the LTP will be discussed.
The emerging micro focusing needs at ESRF have promoted the development of Kirkpatrick-Baez systems. Precise metrology plays an important role to control the mirror clamping using interferometry techniques and to predict the performance of the system using the LTP. The automatic shaping procedure will be described.
In the framework of the European COoperation in the field of Scientific and Technical research action on "X-ray and Neutron Optic" (COST P7) and following the decision announced in the last International Workshop on Metrology for X-ray and neutron optic (Grenoble, April 2004), the metrology facilities of four European synchrotrons, Bessy, Elettra, ESRF and Soleil, have decided and started a program of instrument inter-comparison. Other synchrotrons are joining us and further interested Institutions are invited to participate in this open measurement comparison. The metrology instruments involved are different kinds of direct slope measurement devices, like the well known Long Trace Profiler (in house made or modified) and the Bessy N.O.M..
The Round Robin was started with 2 flat and 2 spherical mirrors (three made of Zerodur and one of fused silica) made available by Bessy and Elettra. A short radius of curvature spherical mirror of Silicon from SOLEIL was later added. First results show a very close match between the measurements of all facilities provided that the same procedures are followed. In particular, a special attention has to be given to the way of supporting the reference objects, as it will be illustrated by some examples. Another important issue is the characterization of the systematic errors of the different instruments and how they can be reduced or eliminated.
The paper expects to open a discussion on the performances of different commercial and custom made or modified profilometers, and over standard procedures for calibration testing, including the definition of standard reference surfaces.
Daniel Rossetti, Ulrich Lienert, Claude Pradervand, Roman Schneider, Ming Shi, Sasa Zelenika, Michel Rossat, Olivier Hignette, Amparo Rommeveaux, Clemens Schulze-Briese
The mirror bender installed at the SLS protein crystallography beamline is designed to be capable of adjusting the vertical phase space of the undulator to the acceptance of protein crystals, i.e. to produce micro-beams as well as essentially parallel beams. The two-moment bender is based on the flexural hinge design pioneered at the ESRF but adapted to high-vacuum by making use of in-vacuum motors and high resolution worm-gears. Special care was taken in the design and fabrication of the clamps and the application of the clamping torque. The Rh-coated fused silica substrate (Zeiss) has a free length of 400 mm and a thickness of 30 mm. Metrology tests at ESRF indicate the high quality of the mirror and the bender. Over the useful length of 350 mm the meridional slope error was found to be 0.17-0.3 μrad (rms) prior to, and less than 0.5 μrad after clamping to the bender. In practice this allows the full central cone of an in-vacuum undulator to be focussed to 7 μm at an image distance of 7.1 m and to 2.1 μm at 1.75 m, corresponding to effective slope errors of less than 0.25 μrad. The bending is very reproducible and is well described by an interaction matrix. Finally, the long range tails were measured in the context of the generation of fs-pulses by means of bunch slicing. Their level cannot be attributed to the surface roughness of 2.9 Å (rms) but rather to scattering from other beamline components such as Be-windows.
ID09 is a dual-purpose beamline dedicated to time-resolved and high-pressure experiments. The time-resolved experiments use a high-speed chopper to isolate single pulses of x-rays. The chopper is installed near the sample (focal spot) and the shortest opening time depends on the height of the tunnel in the chopper, i.e. the sharpness of the vertical focus. In the 16-bunch mode, the opening window of the chopper has to be smaller than 0.352 μs in order to isolate single pulses of x-rays. This requires reducing the height of the tunnel to 0.143 mm. To ensure a reasonable transmission though the tunnel, we have designed a very precise toroidal mirror that focuses the beam 22.4 m downstream with a magnification M = 0.677. The 1.0 m long silicon mirror is curved by gravity into a nearly perfect toroid with a meridional radius of 9.9 km. The curvature is fine-tuned by a stepper motor that pushes via a spring from below the mirror. The overall figure error from the gravity sag and the corrective force is less than 0.3 μrad. The polishing error is 0.7 μrad (rms) averaged over the central 450 mm of the 1000 mm long mirror. The measured size of the polychromatic focus is 0.100 mm × 0.070 mm (h x v) in agreement with the prediction from the ESRF long trace profiler data. The small focal spot, which integrates the full central cone of the U17 undulator, is the result of very high optical quality, curvature fine-tuning, strain-free mount, vibration free cooling and careful alignment.
This paper describes the mechanical design of Kirkpatrick-Baez (KB) mirror systems that have been developed at the ESRF over several years. These very compact and stable though flexible focusing devices for synchrotron x-ray beams are based on bending an initially flat, superpolished plate, which permits to vary the focusing conditions. Nowadays a whole family of mechanical benders exists at the ESRF that allows us to choose the most adapted system according to the properties defined by the experiment such as the energy and energy range, the focusing parameters such as magnification and focusing distance and the substrate coatings, i.e. single-layer mirrors or multilayers. The geometrical characteristics of these KB systems can be chosen in terms of focal distances ranging from 0.1 to 3 m and circular or elliptical bending radii from 20 to 1000m. Mirror substrates such as silicon or pyrex, single-layer or multilayer coatings require different motorisations and deformation systems. The very challenging requirements for mechanical resolution and sensitivity have led to the development of several generations of micro-motors. The ESRF has built a special multi-purpose micropusher that provides the required resolution and linearity, a thrust up to 80 N and finally a good position latching. Issues such as mounting interfaces, stress in the bent mirror and the dynamic bender, local mirror deformation and curvature stability had to be addressed and were solved. The ESRF has developed mirror clamping technologies controlled by mechanical and optical metrologies. The dynamic stability and reproducibility requirements to achieve a spot size variable from sub-micron to tens of microns required by various beamlines necessitate a very high degree of stiffness.
We describe Kirkpatrick-Baez (KB) reflecting mirror systems that have been developed at the European Synchrotron Radiation Facility (ESRF). They are intended to be used mainly in the hard x-ray domain from 10 KeV to 30 KeV for microfluorescence, microdiffraction and projection microscopy applications. At 19 KeV a full width at half maximum (FWHM) spot size of 200x600 nanometers has been measured and with an estimated irradiance gain of 3.5x105. The alignment and bending processes of the system are automated based on the wavefront information obtained by sequentially scanning slits and reading a position-sensitive device located in the focal plane. The sub-microradian sensitivity of this method allows us to predict the spot size and ot provide a metrology map of the surfaces for future improvements of the performances. A novel device based on specular reflection by a micromachined platinum mirror has been used to determine the spot size with an equivalent slit size of less than 100 nanometers. Projection phase images of submicron structures are presented which clearly show both the high potential and also the present limitations of the system. First microfluorescence images obtained at 20.6 KeV are shown. Finally, a roadmap towards diffraction-limited performance with metal and multilayer surfaces is presented.
Surface flatness of optical elements like mirrors and multilayers may significantly influence on the experiments exploiting coherence properties of x rays. A wavy mirror surface can be considered as a random-phase object. In the article, brief notes on the coherence propagation downstream from a random-phase object are presented. Image formation of a wavy surface at grazing incidence is analyzed. An approach to the inverse problem solution of phase retrieval from the mirror images in coherent x rays is shown. A performed experiment demonstrates general consistency in surface topography obtained by means of x rays and LTP. The estimates on the coherence degradation due to surface roughness are obtained.
Beamline optical components and their dynamic holding and bending systems are qualified at the ESRF in the optical metrology laboratory, which has been moved recently into the ESRF experimental hall. Software and hardware enhancements of the five instruments used to perform these ex situ characterizations are described. In situ beamline mirror metrology using a wavefront analyzer has been introduced in ID24 and ID20 beamlines. The design of the analyzer, its architecture with respect to the mirror chambers and its use in active optics control with mechanical and bimorph benders are described in this paper.
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