With the rapid evolution of synchrotron x-ray sources, the demand for high-quality precision x-ray mirrors has greatly increased. Single nanometer shape accuracy is required to keep imaging capabilities at the diffraction limit. Ion beam figuring (IBF) has been used frequently for ultra-precision finishing of mirrors, but achieving the ultimate accuracy depends on three important points: careful alignment, accurate dwell time calculation and implementation, and accurate optical metrology. The Optical Metrology Group at National Synchrotron Light Source II has designed and built a position–velocity–time-modulated two-dimensional IBF system (PVT-IBF) with three novel characteristics: (1) a beam footprint on the mirror was used as a reference to align the coordinate systems between the metrology and the IBF hardware; (2) the robust iterative Fourier transform-based dwell time algorithm proposed by our group was applied to obtain an accurate dwell time map; and (3) the dwell time was then transformed to velocities and implemented with the PVT motion scheme. In this study, the technical aspects of the PVT-IBF systems are described in detail, followed by an experimental demonstration of the figuring results. In our first experiment, the 2D RMS in a $ 50\;{\rm mm} \times 5\;{\rm mm} $50mm×5mm clear aperture was reduced from 3.4 to 1.1 nm after one IBF run. In our second experiment, due to a 5 mm pinhole installed in front of the source, the 2D RMS in a $ 50\;{\rm mm} \times 5\;{\rm mm} $50mm×5mm clear aperture was reduced from 39.1 to 1.9 nm after three IBF runs, demonstrating that our PVT-IBF solution is an effective and deterministic figuring process.

Development of a position-velocity-time-modulated two-dimensional ion beam figuring system for synchrotron x-ray mirror fabrication.

We present recent advancements in the Optical Metrology Laboratory (OML) at Diamond Light Source. Improvements in optical manufacturing technology, and demands from beamlines at synchrotron and free electron laser facilities, have made it a necessity to routinely characterize X-ray mirrors with slope errors < 100 nrad rms. The Diamond-NOM profiler can measure large, fully assembled optical systems in a sideways, upwards, or downwards facing geometry. Examples are provided of how it has recently characterized several challenging systems, including: actively bent mirrors; clamped monochromator gratings in a downward-facing geometry; and four, state-of-the-art, elliptically bent, long mirrors with slope errors < 100 nrad rms. The NOM’s components and data analysis procedures are continuously updated to stay ahead of the ever-increasing quality of X-ray optics and opto-mechanics. The OML’s newest instrument is a Zygo HDX 6” Fizeau interferometer. A dedicated support frame and motorized translation and rotation stages enable sub-aperture images to be stitched together using in-house controls and automation software. Cross-comparison of metrology data, including as part of the MooNpics collaboration, provides a valuable insight into the nature of optical defects and helps to push optical fabrication to a new level of quality.

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The Optical Metrology Laboratory at Diamond: pushing the limits of nano-metrology

A metrology method of x-ray optical elements based on an electromagnetic phase stepping x-ray grating interferometer with high slope accuracy is presented in this study. The device consists of an x-ray tube, a phase grating [Formula: see text] for modulating the incoming wavefront, and an absorption grating [Formula: see text] as a transmission mask to produce a broader moiré pattern for the x-ray camera. The focal spot of the microfocus x-ray source is shifted by a magnetic field from a solenoid coil. Electromagnetic phase stepping analysis is used to obtain a pixel-wise map of the wavefront phase distortion to replace the traditional precision mechanical scanning system, improving stability, speed, accuracy, and flexibility. The x-ray grating interferometer can be used as a feedback tool for evaluating the quality of optical elements and detecting defects caused by the x-ray beam or the x-ray optical elements in ordinary laboratories and mirror-processing centers without the need of scheduling synchrotron beam time.

X-ray wavefront sensing and optics metrology using a microfocus x-ray grating interferometer with electromagnetic phase stepping

A Fizeau Interferometry Stitching System to Characterize X-Ray Mirrors with Sub-Nanometre Errors

Advances in accelerator technologies have enabled the continuous development of synchrotron radiation and X-ray free electron laser (XFEL) sources. At the same time, it has been critical to perform in-situ wavefront sensing to aid delivery of high-quality X-ray beams to the end users of these facilities. The speckle-based scanning technique has obtained popularity due to its high spatial resolution and superior sensitivity compared to other wavefront sensing methods. However, these advantages often come at the expense of longer data acquisition times since multiple images have to be collected to derive the necessary wavefront information. Whereas initial speckle tracking techniques could obtain wavefront information relatively quickly, the installation of additional hardware was routinely required to do so. Here, we propose a novel speckle-based approach, termed Alternating Speckle Tracking (AST), to perform fast wavefront sensing within a conventional beamline setup. The wavefront information derived from the new technique has proven to be valuable for many applications that require temporal resolution. Importantly, both horizontal and vertical wavefront information can be simultaneously derived by moving the speckle generator along the diagonal direction. We expect this method will be widely used by the synchrotron radiation and XFEL community in the future.

Fast wavefront sensing for X-ray optics with an alternating speckle tracking technique.

Meeting the ever-increasing performance demands of X-ray beamlines at modern synchrotrons, such as Diamond Light Source (DLS), requires the use of ultra-high-quality X-ray mirrors with surface deviations of less than a few nanometres from their ideal shape. Ion beam figuring (IBF) is frequently used for creating mirrors of this precision, but achieving the highest accuracy is critically dependent on careful alignment and precise metrology of defects on the optical surface. Multiple iterations of measurement and correction are typically required, and convergence towards the requisite shape can be a slow process. DLS have designed and built an in-house IBF system that comprises a large diameter DC gridded ion source, and a 4-axis motion stage for manipulating the mirror being figured. Additionally, a slope measuring profilometer for in-situ metrology, and an imaging system for alignment, are also built into the system. The advantages of incorporating these extra components are twofold: fast metrology feedback after each figuring run will considerably reduce the time required to perform multiple figuring iterations; and alignment and indexing errors will be drastically reduced when transferring the optic. Complemented by the Optical Metrology Laboratory at DLS and at-wavelength X-ray measurements on the Test beamline B16, it is expected that this system will enable rapid development and testing of high-quality mirrors with novel designs for micro- and nano-focussing of X-rays.

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Ion beam figuring and optical metrology system for synchrotron x-ray mirrors

X-ray mirrors are widely used for synchrotron radiation, free-electron lasers, and astronomical telescopes. The short wavelength and grazing incidence impose strict limits on the permissible slope error. Advanced polishing techniques have already produced mirrors with slope errors below 50 nrad root mean square (rms), but existing metrology techniques struggle to measure them. Here, we describe a laser speckle angular measurement (SAM) approach to overcome such limitations. We also demonstrate that the angular precision of slope error measurements can be pushed down to 20nrad rms by utilizing an advanced sub-pixel tracking algorithm. Furthermore, SAM allows the measurement of mirrors in two dimensions with radii of curvature as low as a few hundred millimeters. Importantly, the instrument based on SAM is compact, low-cost, and easy to integrate with most other existing X-ray mirror metrology instruments, such as the long trace profiler (LTP) and nanometer optical metrology (NOM). The proposed nanometrology method represents an important milestone and potentially opens up new possibilities to develop next-generation super-polished X-ray mirrors, which will advance the development of X-ray nanoprobes, coherence preservation, and astronomical physics. A versatile high precision metrology instrument has been developed that surpass the limits of existing metrology techniques and opens up new possibilities to develop next-generation super-polished X-ray mirrors.

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Simone Moriconi

Papers

Ion beam figuring and optical metrology system for synchrotron x-ray mirrors

Nano-precision metrology of X-ray mirrors with laser speckle angular measurement.