Thomas Zeschke

The Nanometer Optical Component Measuring Machine: a new Sub‐nm Topography Measuring Device for X‐ray Optics at BESSY

The Nanometer Optical Component measuring Machine (NOM) has been developed at BESSY for the purpose of measuring the surface figure of optical components up to 1.2 m in length used at grazing incidence in Synchrotron Radiation beamlines. It is possible to acquire information about slope and height deviations and the radius of curvature of a sample in the form of line‐scans and in a three dimensional display format. With the NOM surfaces up to 600 cm2 have been measured with an estimated measuring uncertainty in the range of 0.01 arcsec rms and with a correspondingly high reproducibility. This is a five to tenfold improvement compared to the present state of the art of surface measuring techniques. The engineering conception, the design of the NOM and the first measurements are discussed in detail.

On the characterization of ultra-precise X-ray optical components: advances and challenges in ex situ metrology.

State-of-the-art ex situ metrology for characterizing the quality of ultraprecise reflective synchrotron optics is reported. Beside slope measuring deflecometry the current state of mirror coating technology for single layer and multilayer coatings for very long mirror substrates is discussed.

Proposal for a universal test mirror for characterization of slope measuring instruments

The development of third generation light sources like the Advanced Light Source (ALS) or BESSY II brought to a focus the need for high performance synchrotron optics with unprecedented tolerances for slope error and micro roughness. Proposed beam lines at Free Electron Lasers (FEL) require optical elements up to a length of one meter, characterized by a residual slope error in the range of 0.1 μrad (rms), and rms values of 0.1 nm for micro roughness. These optical elements must be inspected by highly accurate measuring instruments, providing a measurement uncertainty lower than the specified accuracy of the surface under test. It is essential that metrology devices in use at synchrotron laboratories be precisely characterized and calibrated to achieve this target. In this paper we discuss a proposal for a Universal Test Mirror (UTM) as a realization of a high performance calibration instrument. The instrument would provide an ideal calibration surface to replicate a redundant surface under test of redundant figure. The application of a sophisticated calibration instrument will allow the elimination of the majority of the systematic error from the error budget of an individual measurement of a particular optical element. We present the limitations of existing methods, initial UTM design considerations, possible calibration algorithms, and an estimation of the expected accuracy.

Developmental long trace profiler using optimally aligned mirror based pentaprism

A low-budget surface slope-measuring instrument, the developmental long-trace profiler (DLTP), was recently brought into operation at the Advanced Light Source Optical Metrology Laboratory. The instrument is based on a precisely calibrated autocollimator and a movable pentaprism. The capability of the DLTP to achieve submicroradian surface slope metrology has been verified via cross-comparison measurements to other high-performance slope-measuring instruments when measuring the same high-quality test optics. Further improvement of the DLTP is achieved by replacing the existing bulk pentaprism with a specially designed mirror-based pentaprism, which offers the possibility to eliminate systematic errors introduced by inhomogeneity of the optical material and fabrication imperfections of a bulk pentaprism. We provide the details of the mirror-based pentaprism design and describe an original experimental procedure for precision mutual alignment of the mirrors. The algorithm of the alignment procedure and its efficiency are verified with rigorous ray-tracing simulations. Results of measurements of a spherically curved test mirror and a flat test mirror using the original bulk pentaprism are compared to measurements using the new mirror-based pentaprism, demonstrating the improved performance.

Advanced metrology: an essential support for the surface finishing of high performance x-ray optics

The performance of x-ray beamlines at 3rd generation synchrotron radiation sources and Free Electron Lasers (FELs) is limited by the quality of the state of the art optical elements. Proposed FEL beamlines require optical components which are of better quality than is available from the optical manufacturing technology of today. As a result of a joint research project (Nanometer Optik Komponenten - NOK) coordinated by BESSY, involving both metrologists and manufacturers it is possible now to manufacture optical components beyond the former limit of 0.1 arcsec rms slope error [1, 2]. To achieve the surface finishing of optical components with a slope error in the range of 0.04 arcsec rms (for flat or spherical surfaces up to 300 mm in length) by polishing and finally by ion beam figuring technology it is essential that the optical surface be mapped and the mapping data used as input for the multiple ion beam figuring stages. Metrology tools of at least five times superior accuracy to that required of the component have been developed in the course of the project. The Nanometer Optical Component measuring Machine (NOM) was developed at BESSY for line and area measurements of the figure of optical components used at grazing incidence in synchrotron radiation beamlines. Surfaces up to 730 cm2 have been measured with the NOM a measuring uncertainty in the range of 0.01 arcsec rms and a correspondingly high reproducibility [3]. Three dimensional measurements were used to correct polishing errors some nanometers high and only millimeters in lateral size by ion beam treatment. The design of the NOM, measurement results and results of NOM supported surface finishing by ion beam figuring will be discussed in detail. The improvement of beamline performance by the use of such high quality optical elements is demonstrated.

Cleaning of contaminated XUV-optics at BESSY II

Abstract Carbon contaminations as observed on XUV-optics can be removed by an in situ plasma discharge process. The method developed at BESSY is based on waterfree oxygen/argon mixture and avoids water contamination of the UHV-equipment. The radio frequency based plasma cleaning method has been used at several undulator beamlines at BESSY II with a gain in flux at the carbon K-edge. At the UE56-I-plane grating monochromator, a gain in flux up to a factor 20 is observed. No loss in flux has been observed across the whole energy ranges of the “cleaned” beamlines.

Optimal alignment of mirror-based pentaprisms for scanning deflectometric devices

In the recent work [Proc. of SPIE 7801, 7801-2/1-12 (2010), Opt. Eng. 50(5) (2011), in press], we have reported on improvement of the Developmental Long Trace Profiler (DLTP), a slope measuring profiler available at the Advanced Light Source Optical Metrology Laboratory, achieved by replacing the bulk pentaprism with a mirror based pentaprism (MBPP). An original experimental procedure for optimal mutual alignment of the MBPP mirrors has been suggested and verified with numerical ray tracing simulations. It has been experimentally shown that the optimally aligned MBPP allows the elimination of systematic errors introduced by inhomogeneity of the optical material and fabrication imperfections of the bulk pentaprism. In the present article, we provide the analytical derivation and verification of easily executed optimal alignment algorithms for two different designs of mirror based pentaprisms. We also provide an analytical description for the mechanism for reduction of the systematic errors introduced by a typical high quality bulk pentaprism. It is also shown that residual misalignments of an MBPP introduce entirely negligible systematic errors in surface slope measurements with scanning deflectometric devices.

High-resolution constant length Rowland circle monochromator at BESSY

The working principle, the layout and the performance data of a simple Rowland circle soft x-ray monochromator of constant length are described. By translating the grating while scanning the photon energy, defocus, coma and some spherical aberrations vanish, yielding excellent resolution in the optimized energy range between 270 and 590 eV. This result is obtained while using all optical elements at full illumination. The achieved resolution ranks among the best worldwide.

THE BESSY CONSTANT LENGTH ROWLAND CIRCLE MONOCHROMATOR

Abstract The BESSY constant length Rowland circle monochromator (CL-RCM) was designed for high resolution spectroscopy in the soft x-ray region. The principle is based on a spherical grating/plane mirror assembly, which is moved between the fixed entrance and exit slits for photon energy scan. By this means, the Rowland conditions are fulfilled over an energy range of 270–590 eV, yielding excellent resolution. Fast beamline optimization and, most important, the efficient use of beamtime in routine user operation is considerably enhanced by the newly implemented on-the-fly measuring mode, as demonstrated by the first results obtained on the beamline.

High precision tilt stage as a key element to a universal test mirror for characterization and calibration of slope measuring instruments

The ultimate performance of surface slope metrology instrumentation, such as long trace profilers and auto-collimator based deflectometers, is limited by systematic errors that are increased when the entire angular range is used for metrology of significantly curved optics. At the ALS X-Ray Optics Laboratory, in collaboration with the HZB/BESSY-II and PTB (Germany) metrology teams, we are working on a calibration method for deflectometers, based on a concept of a universal test mirror (UTM) [V. V. Yashchuk et al., Proc. SPIE 6704, 67040A (2007)]. Potentially, the UTM method provides high performance calibration and accounts for peculiarities of the optics under test (e.g., slope distribution) and the experimental arrangement (e.g., the distance between the sensor and the optic under test). At the same time, the UTM calibration method is inherently universal, applicable to a variety of optics and experimental arrangements. In this work, we present the results of tests with a key component of the UTM system, a custom high precision tilt stage, which has been recently developed in collaboration with Physik Instrumente, GmbH. The tests have demonstrated high performance of the stage and its capability (after additional calibration) to provide angular calibration of surface slope measuring profilers over the entire instrumental dynamic range with absolute accuracy better than 30 nrad. The details of the stage design and tests are presented. We also discuss the foundation of the UTM method and calibration algorithm, as well as the possible design of a full scale UTM system.