Emergence of ultrafast EUV to X-ray sources, Free Electron Laser, High harmonic generation, betatron and Compton, has opened new paradigms in physical, chemical, biological and medical sciences by either producing ultrahigh intensities or for ultrafast imaging or by enabling pump-probe experiments at new timescales. Most of these experiments require an excellent or at least a properly defined wavefront (WF). A number of WF sensing techniques have been proposed in the X-Rays, including grating-based interferometry, speckle tracking, pencil beam deflectometry, or curvature sensors. Among these techniques, Hartmann WF sensing demonstrated a number of advantages, such as insensitivity to vibrations, achromaticity and very large dynamic range. Furthermore, the phase and intensity maps are directly retrieve and nearly instantaneously without the use of complex and long algorithms. Imagine Optic developed EUV to X-ray WF sensors since more than 15 years taking benefit of decades of experience in the visible range.
We will show several experiments using our EUV sensors for metrology and optimization of ultrafast EUV sources. We recently developed a Hartmann sensor in the 5 - 25keV range. The device is based on a custom scintillator-to-detector optical relay system, as well as on an optimal Hartmann array geometry, providing 20µm spatial WF sampling resolution, over a 3x3 mm² pupil. We show the results of first experiments on a synchrotron beamline at 10 keV, achieving 4pm WF repeatability.
Since its creation in 1996, Imagine Optic designed and manufactured high performance Shack-Hartmann wavefront (WF) sensors for many kinds of applications such as telescope alignment, laser characterization, optics qualification or adaptive optics, and for many different fields such as space optics, microscopy, high power lasers or lithography. Since 2003, Imagine Optic is actively developing EUV to X-ray Hartmann WF sensors for applications on metrology beams emitted by synchrotrons, free-electron lasers, plasma-based soft X-ray lasers and high harmonic generation. Our most recent developments include the realization of a EUV sensor adapted to strongly convergent or divergent beams having numerical aperture as high as 0.15, as well as the production of a hard X-ray sensor working above 10 keV, providing outstanding repeatability as good as 4 pm rms. Our sensors have demonstrated their high usefulness for the metrology of EUV to X-ray optics from single flat or curved mirrors to more complex optical systems (Schwarzschild, Kirkpatrick-Baez static or based on bender technology or with activators). In terms of optics qualification is a clear advantage of actually measuring the wavefront at-wavelength. Also, we show active Kirkpatrick-Baez alignment in few minutes using our WF sensor in both manual and automatic loops at the benefit of strong improvement of the beam focalization on the sample. Recently we started developing our own compact deformable grazing incidence mirror bender. We present a review of the developed sensors, as well as experimental demonstrations of their benefits for optical metrology of various EUV and X-ray optics.
Imagine Optic (IO) is actively developing EUV to X-ray wavefront (WF) sensors since 2003 for applications on metrology of EUV to X-ray beams emitted by synchrotrons, free-electron lasers, plasma-based soft X-ray lasers and high harmonic generation. Our sensors have demonstrated their high usefulness for metrology of EUV to X-ray optics from single flat or curved mirrors to more complex optical systems (Schwarzschild, Kirkpatrick-Baez static or based on bender technology or with activators). Our most recent developments include the realization of a EUV sensor adapted to strongly convergent or divergent beams having numerical aperture as high as 0.15, as well as the production of a hard X-ray sensor working at 10 keV and higher energies, providing repeatability as good as 4 pm rms. We present a review of the developed sensors, as well as experimental demonstrations of their benefits for various metrology and WF optimization requirements.
We report on the design and performances of a test prototype active X-ray mirror developed for the French national
synchrotron radiation facility SOLEIL in collaboration with a French company ISP System. The active mirror uses 11
mechanical actuators: one actuator for the main curvature and 10 actuators along the mirror surface for correction of the
residual shape errors. Its radius of curvature can be adjusted from infinity down to 50 m, with residual slope errors in
correction less than 0.6 μrad RMS over a 300 mm useful length. A dedicated X-ray Hartmann wavefront sensor, based
on YAG:Ce wavelength conversion to visible light, was developed for feedback control of the mirror. Closed-loop
experiments were performed at 10 keV on the Metrology and Tests Beamline at SOLEIL.
New lithography technologies, as for example Extreme Ultra Violet (EUV), require high flatness on the exposure surfaces
as the depth of focus is impacted. It is essential for semiconductor manufacturing to measure and control flatness of
wafer surfaces at nanometer scale. In-plane geometrical defects on wafer surfaces following Chemical Mechanical Planarization
(CMP) processing in the lateral millimeter range and in vertical dimensions in the nanometer range are of increasing
importance. They will become a severe yield limiting factor in the 32 nm generations and below. This paper
shows the result from improvement and optimization of metrology using wavefront sensing methods according to Makyoh
and Shack Hartmann. Magnification and increased density of measurement points were identified to improve the
existing performance with respect to vertical resolution significantly below 100 nm. The achieved lateral resolution on
the wafer surface was 750 μm. The accuracy of the measurement on patterned wafer surfaces was determined to be less
than 15 nm. The accuracy was determined by repeating the topography measurement and filtering of the according data
of the sensors using cross section analysis and spatial processing with double Gaussian filters. The samples were taken
from different manufacturing steps, such as Shallow Trench Isolation (STI) and interconnect metallization. Wavefront
sensing based on methods according to Makyoh and Shack Hartmann enabled instantaneous and non-destructive flatness
measurement of surfaces.
We present a full optimization of the high harmonics wave-front thanks to the use of a soft x-ray Hartmann sensor. The
sensor was calibrated using high harmonics source with a λ/50 accuracy. We observed relatively good high harmonics
wave-front, two times the diffraction-limit, with astigmatism as the dominant aberration for any interaction parameters.
By slightly clipping the unfocused beam, it is possible to produce a diffraction-limited beam containing about 90% of the
incident energy. The influence of high harmonic generation parameters was also studied in particularly the influence of
the infra-red wave-front. In particular we studied the correlation between the infrared wave-front use to create high
harmonics and the high harmonic wave-front. We also report wave-front measurements of a high order harmonic beam
into an x-ray laser plasma amplifier at 32.8 nm.
In this article, a stitching Shack-Hartmann profilometric head is presented. This instrument has been developed to answer
improved needs for surface metrology in the domain of short-wavelength optics (X/EUV). It is composed of a highaccuracy
Shack-Hartmann wavefront sensor and an illumination platform. This profilometric head is mounted on a
translation stage to perform bidimensional mappings by stitching together successive sub-aperture acquisitions. This
method ensures the submicroradian accuracy of the system and allows the user to measure large surfaces with a submillimetric
We particularly emphasize on the calibration method of the head; this method is validated by characterizing a super-flat
reference mirror. Cross-checked tests with the Soleil's long-trace profiler are also performed. The high precision of
profilometric head has been validated with the characterization of a spherical mirror. We also emphasize on the large
curvature dynamic range of the instrument with the measurement of an X-ray toric mirror.
The instrument, which performs a complete diagnostic of the surface or wavefront under test, finds its main applications
in metrology (measurement of large optics/wafers, post-polishing control and local surface finishing for the industry,
spatial quality control of laser beam).
In 2002, first experiments at the Advanced Light Source (ALS) at Berkeley, allowed us to test a first prototype of EUV Hartmann wave-front sensor. Wave-front measurements were performed over a wide wavelength range from 7 to 25 nm. Accuracy of the sensor was proved to be better than λ<sub>EUV</sub>/120 rms (λ<sub>EUV</sub> = 13.4 nm, about 0.1 nm accuracy) with sensitivity exceeding λ<sub>EUV</sub>/600 rms, demonstrating the high metrological performances of this system.
At the Swiss Light Source (SLS), we succeeded recently in the automatic alignment of a synchrotron beamline by Hartmann technique. Experiments were performed, in the hard X-ray range (E = 3 keV, λ = 0.414 nm), using a 4-actuators Kirkpatrick-Baez (KB) active optic. An imaging system of the KB focal spot and a hard X-ray Hartmann wave-front sensor were used alternatively to control the KB. The imaging system used a genetic algorithm to achieve the highest energy in the smallest spot size, while the wave-front sensor used the KB influence functions to achieve the smallest phase distortions in the incoming beam. The corrected beam achieved with help of the imaging system was used to calibrate the wave-front sensor. With both closed loops, we focused the beam into a 6.8x9 μm<sup>2</sup> FWHM focal spot. These results are limited by the optical quality of the imaging system.
Metrology of XUV beams and more specifically X-ray laser (XRL) beam is of crucial importance for development of applications. We have then developed several new optical systems enabling to measure the x-ray laser optical properties. By use of a Michelson interferometer working as a Fourier-Transform spectrometer, the line shapes of different x-ray lasers have been measured with an unprecedented accuracy (δλ/λ~10<sup>-6</sup>). Achievement of the first XUV wavefront sensor has enable to measure the beam quality of laser-pumped as well as discharge pumped x-ray lasers. Capillary discharge XRL has demonstrated a very good wavefront allowing to achieve intensity as high 3*10<sup>14</sup> Wcm<sup>-2 </sup>by focusing with a f = 5 cm mirror. The measured sensor accuracy is as good as λ/120 at 13 nm. Commercial developments are under way.
We present real-time measurements of the wave front distortion induced by a variable focal lens. This lens, called Varioptic, is made of a transparent cell filled with twin liquids. We submit a 4.5mm in diameter lens upon a 110V voltage step inducing a optical power shift of about 25 dioptries (m<sup>-1</sup>) . Characteristic response time is shown to be of the order of a few 1/100s, the lens recovering its full quality after 5/100s. We present a scaling analysis of this response time versus lens size.