Nikolay A. Artemiev

Advanced environmental control as a key component in the development of ultrahigh accuracy ex situ metrology for x-ray optics

Abstract. The advent of fully coherent free-electron laser and diffraction-limited synchrotron radiation storage ring sources of x-rays is catalyzing the development of new ultrahigh accuracy metrology methods. To fully exploit these sources, metrology needs to be capable of determining the figure of an optical element with subnanometer height accuracy. The major limiting factors of the current absolute accuracy of ex situ metrology are drift errors due to temporal instabilities of the lab’s environmental conditions and systematic errors inherent to the metrology instruments. Here, we discuss in detail work at the Advanced Light Source X-Ray Optics Laboratory on building of advanced environmental control that is a key component in the development of ultrahigh accuracy ex situ metrology for x-ray optics. By a few examples, we show how the improvement of the environmental conditions in the lab allows us to significantly gain efficiency in performing ex situ metrology with high-quality x-ray mirrors. The developed concepts and approaches, included in the design of the new X-Ray Optics Laboratory, are described in detail. These data are essential for construction and successful operation of a modern metrology facility for x-ray optics, as well as high-precision measurements in many fields of experimental physics.

Methodology for Optimal In Situ Alignment and Setting of Bendable Optics for Diffraction-Limited Focusing of Soft X-Rays

Abstract. We demonstrate a comprehensive and broadly applicable methodology for the optimal in situ configuration of bendable soft x-ray Kirkpatrick-Baez mirrors. The mirrors used for this application are preset at the Advanced Light Source Optical Metrology Laboratory prior to beamline installation. The in situ methodology consists of a new technique for simultaneously setting the height and pitch angle of each mirror. The benders of both mirrors were then optimally tuned in order to minimize ray aberrations to a level below the diffraction-limited beam waist size of 200  nm (horizontal)×100  nm (vertical). After applying this methodology, we measured a beam waist size of 290  nm (horizontal)×130  nm (vertical) with 1 nm light using the Foucault knife-edge test. We also discuss the utility of using a grating-based lateral shearing interferometer with quantitative wavefront feedback for further improvement of bendable optics.

Automated suppression of errors in LTP-II slope measurements with x-ray optics

Systematic error and instrumental drift are the major limiting factors of sub-microradian slope metrology with state-of-the-art x-ray optics. Significant suppression of the errors can be achieved by using an optimal measurement strategy suggested in [Rev. Sci. Instrum. 80, 115101 (2009)]. Here, we report on development of an automated, kinematic, rotational system that provides fully controlled flipping, tilting, and shifting of a surface under test. The system is to be integrated into the Advanced Light Source long trace profiler, LTP-II, allowing for complete realization of the advantages of the optimal measurement strategy method. We describe in detail the systems specification, design operational control and data acquisition. The performance of the system is demonstrated via the results of high precision measurements with a number of super-polished mirrors.

A new x-ray optics laboratory (XROL) at the ALS: mission, arrangement, metrology capabilities, performance, and future plans

The X-Ray Optics Laboratory (XROL) at the Advanced Light Source (ALS), a unique optical metrology lab, has been recently moved to a new, dedicated clean-room facility that provides improved environmental and instrumental conditions vitally required for high accuracy metrology with state-of-the-art X-ray optics. Besides the ALS, the XROL serves several DOE labs that lack dedicated on-site optical metrology capabilities, including the Linac Coherent Light Source (LCLS) at SLAC and LBNL’s Center for X-Ray Optics (CXRO). The major role of XROL is to proactively support the development and optimal beamline use of x-ray optics. The application of different instruments available in the lab enables separate, often complementary, investigations and addresses of different potential sources of error affecting beamline performance. At the beamline, all the perturbations combine to produce a cumulative effect on the performance of the optic that makes it difficult to optimize the optics operational performance. Ex situ metrology allows us to address the majority of the problems before the installation of the optic at a beamline, and to provide feedback on design and guidelines for the best usage of optics. We will review the ALS XROL mission, lab design and arrangement, ex situ metrology capabilities and performance, as well as the future plans for instrumentation upgrades. The discussion will be illustrated with the results of a broad spectrum of measurements of x-ray optics and optical systems performed at the XROL.

Correlation analysis of surface slope metrology measurements of high quality x-ray optics

We discuss an application of correlation analysis to surface metrology of high quality x-ray optics with the aim of elicitation and, when possible, suppression of the instrumental systematic errors in the final metrology results. We describe and present the mathematical foundation for a novel method consisting of the randomization of the systematic error by the averaging of multiple measurements, specially arranged to mutually anti-correlate. We also discuss the possibility to apply correlation analysis to the entire residual surface slope distribution in order to find anti-correlation parameters of the distribution. In this case, repeated measurements with the corresponding change of the experimental arrangement (position of the surface and/or its overall tilt) can be used to identify the origin of the observed anticorrelation features by analyzing the difference between the measurements. If the corresponding minimum of the autocorrelation function is due to a systematic error, averaging over the repeated measurements will provide an efficient suppression of the systematic error. If the observed anti-correlation properties are due to the polishing process, and therefore belong to the surface itself, we suggest that the possibility of re-polishing the surface based on the correlation analysis be considered. Throughout the present work we have discussed correlation analysis of surface slope metrology data. However, a similar consideration can be applied to surface topography in the height domain measured with other metrology instrumentation, for example: interferometers and interferometric microscopes.

Fabrication of x-ray gratings by direct write mask-less lithography

Fabrication of diffraction grating for x-rays is a very challenging problem due to the exacting requirements of surface quality, groove position, and groove profile. Traditional fabrication techniques have significant limitations and do not cover all the necessary requirements. For example, classical holographic recording is limited in the type of groove patterns that can be produced. This is particularly important in the design of wide aperture high resolution spectrometers, where aberration correction using complex groove patterns is necessary. We are pioneering the use of direct-write mask-less optical lithography to make grating patterns of arbitrary complexity. In this work we report on the first results from our direct-write mask-less approach, including quality assessment of the patterns using interferometric techniques.

Experimental methods for optimal tuning of bendable mirrors for diffraction-limited soft x-ray focusing

We report on hands-on experimental methods developed at the Advanced Light Source (ALS) for optimal tuning of mechanically bendable x-ray mirrors for diffraction-limited soft x-ray nano-focusing. For ex situ tuning of the benders for optimal beam-line performance, we use a revised version of the method of characteristic functions recently developed at the ALS optical metrology laboratory. At-wavelength optimal tuning of bendable optics consists of a series of wavefront-sensing tests with increasing accuracy and sensitivity, including modified scanning-slit Hartmann tests. The methods have been experimentally validated at ALS test beamline 5.3.1 and the micro-diffraction beamline 12.3.2 in applications to optimally set bendable Kirkpatrick-Baez mirrors designed for sub-micron focusing.

Design optimization of bendable x-ray mirrors

Convenience and cost often lead to synchrotron beamlines where the final bendable Kirkpatrick-Baez focusing pair must relay the final image to different samples at different image distances e.g., [Proc. FEL2009, 246-249 (2009)] either for different experimental chambers, or diagnostics. We present an initial analytical approach, starting from, and extending the work of Howells et al. [OE 39(10), 2748-62 (2000)] to analyze the trade-offs between choice of mirror, bending couples and the given, shaped sagittal width of the optic. Both experimentally and in simulation, we have found that after an appropriate re-bending, sagittally shaped optics can perform with high quality at significantly different incidence angles and conjugate distances. We present one successful demonstration from the ALS Optical Metrology Beamline 5.3.1, and review some new closed form analytical solutions with a view towards understanding our results.

Ex situ tuning of bendable x-ray mirrors for optimal beamline performance

We extend analytical and numerical methods recently developed at the Advanced Light Source (ALS) optical metrology laboratory (OML) for optimal tuning and calibration of bendable x-ray optics based on ex situ measurements with surface slope profilers [Opt. Eng. 48(8), 083601 (2009); Proc. SPIE 8141, 8141-19 (2011)]. We minimize the rms variation of residual slope deviations from ideal surface figure. Previously, our adjustment assumed the deviations were weighted equally across the optic. In this work, we analyze the case when the mirror length is significant with respect to the imaging conjugate. This corresponds, for example, to high de-magnification by bendable Kirkpatrick Baez mirror pairs, used near the ends of synchrotron and free electron laser beamlines for micro- and nano-focusing that often results in a very short mirror to image distance, of the same order of magnitude as the mirror’s length. In this case, contributions to focal distortion of residual errors of mirror surface figure (appearing due to mechanical alignment tolerances, sagittal shaping errors, and the limited number of adjustable parameters inherent in a two-couple bender) strongly depend on position across the optic. Specifically, the downstream deviations from exact shape should be weighted less because the rays have a shorter path to travel to the image. Here, we derive an analytical expression for the weighting function and present a mathematical background for the bending adjustment procedure for optimization of the mirror’s beamline performance. The efficacy of the optimization is demonstrated for a short-focus mirror used for diffraction limited focusing at ALS beamline 12.3.2.

The developmental long trace profiler (DLTP) optimized for metrology of side-facing optics at the ALS

The autocollimator and moveable pentaprism based DLTP [NIM A 616 (2010) 212-223], a low-budget, NOM-like profiler at the Advanced Light Source (ALS), has been upgraded to provide fast, highly accurate surface slope metrology for long, side-facing, x-ray optics. This instrument arrangement decreases sensitivity to environmental conditions and removes the gravity effect on mirror shape. We provide design details of an affordable base tool, including clean-room environmental arrangements in the new ALS X-ray Optics Laboratory with advanced temperature stabilization and turbulence reduction, that yield measurements in under 8 hours with accuracy better than 30 nanoradians (rms) for super polished,190 mm flat optics, limited mainly by residual temporal instability of the experimental set-up. The upgraded DLTP has been calibrated for highly curved x-ray optics, allowing same day measurements of a 15 m ROC sphere with accuracy of better than 100 nanoradians (rms). The developed calibration procedure is discussed in detail. We propose this specific 15 m ROC sphere for use as a round-robin calibration test optic.