Fehllicht in LIGA-Mikrospektrometern [online]

Kurzfassung Koppelt man monochromatisches Licht in ein Spektrometer ein, erwartet man für ein ideales Spektrometer in der Spektralverteilung eine einzige Linie bei der eingekoppelten Wellenlänge. Aufgrund von Fertigungstoleranzen tritt jedoch auch an anderen Stellen Licht auf, das als Fehllicht bezeichnet wird. Der Anteil des Fehllichts im gemessenen Spektrum bestimmt die technische Einsetzbarkeit von Mikrospektrometern. Um den kommerziellen Erfolg der LIGAMikrospektrometer in weiteren Anwendungsgebieten zu gewährleisten, müssen die Systeme bezüglich ihrer optischen Eigenschaften optimiert und der Fehllichtanteil reduziert werden. Ziel dieser Arbeit war es, die Ursachen für Fehllicht in LIGA-Mikrospektrometern zu identifizieren, zu bewerten und daraus Ansatzpunkte für prozesstechnische Verbesserungen zu erarbeiten. Dazu war es notwendig, neue Mess- und Auswertemethoden zu entwickeln, die der geringen Größe und der Bauform der LIGA-Gitterspektrometer gerecht werden. Aufgrund fotografischer Aufnahmen der Spektrometer konnten Unzulänglichkeiten im Spektrometerlayout als eine Quelle des Fehllichts ausgemacht werden. Dieser Anteil konnte durch Änderungen des Spektrometerlayouts im Bereich der Lichteinkoppelbereichs und des Lichtauskoppelspiegels deutlich reduziert werden. Die Auswertung der gemessenen Spektren zeigte, dass ein Großteil des Fehllichts nicht durch Reflexionen an rauen Oberflächen oder durch Mehrfachreflexionen auf den Detektor gelangt, sondern durch Unregelmäßigkeiten im Gitter verursacht wird. Um die Größe und Art der Abweichungen der Gitterform von der Sollform zu bestimmen, wurden verschiedene Methoden eingesetzt. Rasterelektronenmikroskopische und rasterkraftmikroskopische Aufnahmen lieferten Aussagen über die Einhaltung der Form der einzelnen Gitterzähne. Dabei wurde eine mit der Zahl der Prozessschritte zunehmende Verrundung der einzelnen Gitterzähne festgestellt. Eine leichte Zunahme der Oberflächenrauigkeit der Zahnflanken konnte ebenfalls festgestellt werden. Beides führt jedoch nicht zu dem beobachteten Anstieg des Fehllichtanteils. Zur präzisen Vermessung der Position einzelner Gitterzähne wurde ein Elektronenstrahlschreiber mit seinem laserinterferometrisch kontrollierten Probentisch als Rasterelektronenmikroskop genutzt. Dabei wurden lokale Abweichungen von der Gittersollform von im Mittel 23 nm bis 32 nm gemessen, die stark zum Fehllichtaufkommen beitragen. Verschiedene, eigens angepasste fotografische Aufnahmenmethoden zeigten, dass sowohl die LIGA-Seitenwände als auch die Gitterstrukturen nichtperiodische und periodische Strukturabweichungen aufweisen. Mit Hilfe einer fourieroptischen Auswertung solcher Aufnahmen wurden die Periodenlängen dieser Störungen ermittelt. Die ermittelten Periodenlängen von etwa 4,3 µm und 500 µm decken sich mit der maximalen Trapezfeldgröße (einem Parameter im ersten Strukturierungsschritt) und der Hauptablenkfeldgröße (die Länge, nach welcher der Probentisch bei der Maskenstrukturierung verschoben wird) des Elektronenstrahlschreibers. Zur Kompensation des Fehllichtanteils in gemessenen Spektren wurde im Rahmen dieser Arbeit ein iterativer Algorithmus entwickelt, der es erlaubt, den Fehllichtanteil rechnerisch weitgehend zu unterdrücken. Der Algorithmus beruht darauf, dass sich durch Entfaltung der Spektren auf das eingekoppelte Spektrum zurückschließen lässt. Um den Fehllichtanteil der Mikrospektrometer weiter zu senken, ist es notwendig, eine LIGA-Maskentechnik zu entwickeln, die es erlaubt, die Goldabsorberstrukturen mit einer absoluten lateralen Positionsgenauigkeit von besser 20 nm auf der Trägermembran zu platzieren. Abstract "Misguided Light in LIGA-Mikrospectrometers" A perfect spectrometer is expected to yield a single line only, which is representative of the wavelength of incoming monochromatic light. Due to production tolerances, however, light also appears at other points. This light is referred to as misguided light. The proportion of misguided light in the spectrum measured determines the technical applicability of microspectrometers. To ensure commercial success of LIGA microspectrometers in further fields of application, improved optical properties with a minimum of misguided light are therefore essential. The aims of this dissertation were to identify the sources of misguided light in LIGA microspectrometers and to implement improvements. To reach this objective, novel measurement and analysis methods tailored to the small size and to the design of the LIGA microspectrometers had to be developed. On the basis of photographies, deficiencies in the layout of the spectrometers were identified as a source of misguided light. The corresponding proportion was clearly reduced by changing the spectrometer layout in the area where the light is coupled in respectively in the area of the mirror reflecting the light to the detector. Evaluation of the spectra measured revealed that a vast fraction of the misguided light is not passed to the detector by reflection at rough surfaces or multiple reflection at side walls, but caused by irregularities of the grating. Therefore, various methods were employed to determine the extent and characteristics of grating deviations from the geometrical optimum. A scanning electron microscope and atomic force microscope were used to determine the characteristics of the individual teeth of the grating. It was found that rounding of the teeth increased with each process step. In addition, an increase of the surface roughness was observed. However, even these imperfections could not account for all the misguided light. An electron beam writer with a laser-interferometrically controlled table was used (in its scanning electron microscope mode) to precisely measure the positions of the individual grating teeth. Local deviations were found to range from 23 nm to 32 nm on the average. It is now known that such local deviations account for most of the misguided light. Various photographic exposure techniques (each one specifically adapted to the microspectrometer) showed periodic and erratic structural deviations for both the LIGA side walls and the grating. A fourieroptical analysis of these photographies yielded periods of 4.3 µm and 500 µm, which correspond to the maximum trapezoid field size (a parameter in the first structuring step) and the size of the main field deflection of the electron beam writer (the distance the table with the sample is moved stepwise when structuring the mask), respectively. To compensate for misguided light in the measured spectra, an iterative algorithm was developed, allowing for a suppression of the preponderating part of the misguided light. Since the properties of the system are known, the algorithm was based on calculations of the original spectrum by deconvolution of the measured spectra. Further reduction of misguided light requires the development of a LIGA mask technique, by means of which the absorbing gold structures are placed on the carrier membrane with an absolute lateral positioning accuracy of < 20 nm

Grid-enhanced X-ray coded aperture microscopy with polycapillary optics

Polycapillary devices focus X-rays by means of multiple reflections of X-rays in arrays of bent glass capillaries. The size of the focal spot (typically 10–100 $\mu$ m) limits the resolution of scanning, absorption and phase-contrast X-ray imaging using these devices. At the expense of a moderate resolution, polycapillary elements provide high intensity and are frequently used for X-ray micro-imaging with both synchrotrons and X-ray tubes. Recent studies have shown that the internal microstructure of such an optics can be used as a coded aperture that encodes high-resolution information about objects located inside the focal spot. However, further improvements to this variant of X-ray microscopy will require the challenging fabrication of tailored devices with a well-defined capillary microstructure. Here, we show that submicron coded aperture microscopy can be realized using a periodic grid that is placed at the output surface of a polycapillary optics. Grid-enhanced X-ray coded aperture microscopy with polycapillary optics does not rely on the specific microstructure of the optics but rather takes advantage only of its focusing properties. Hence, submicron X-ray imaging can be realized with standard polycapillary devices and existing set-ups for micro X-ray fluorescence spectroscopy

Multi-Lens Array Full-Field X-ray Microscopy

X-ray full-field microscopy at laboratory sources for photon energies above 10 keV suffers from either long exposure times or low resolution. The photon flux is mainly limited by the objectives used, having a limited numerical aperture NA. We show that this can be overcome by making use of the cone-beam illumination of laboratory sources by imaging the same field of view (FoV) several times under slightly different angles using an array of X-ray lenses. Using this technique, the exposure time can be reduced drastically without any loss in terms of resolution. A proof-of-principle is given using an existing laboratory metal-jet source at the 9.25 keV Ga Kα-line and compared to a ray-tracing simulation of the setup

Quantitative characterization of X-ray lenses from two fabrication techniques with grating interferometry

Refractive X-ray lenses are in use at a large number of synchrotron experiments. Several materials and fabrication techniques are available for their production, each having their own strengths and drawbacks. We present a grating interferometer for the quantitative analysis of single refractive X-ray lenses and employ it for the study of a beryllium point focus lens and a polymer line focus lens, highlighting the differences in the outcome of the fabrication methods. The residuals of a line fit to the phase gradient are used to quantify local lens defects, while shape aberrations are quantified by the decomposition of the retrieved wavefront phase profile into either Zernike or Legendre polynomials, depending on the focus and aperture shape. While the polymer lens shows better material homogeneity, the beryllium lens shows higher shape accuracy

Development of an Array of Compound Refractive Lenses for Sub-Pixel Resolution, Large Field of View, and Time-Saving in Scanning Hard X-ray Microscopy

A two-dimensional array of compound refractive lenses (2D array of CRLs) designed for hard X-ray imaging with a 3.5 mm$^{2}$ large field of view is presented. The array of CRLs consists of 2D polymer biconcave parabolic 34 × 34 multi-lenses fabricated via deep X-ray lithography. The developed refractive multi-lens array was applied for sub-pixel resolution scanning transmission X–ray microscopy; a raster scan with only 55 × 55 steps provides a 3.5 megapixel image. The optical element was experimentally characterized at the Diamond Light Source at 34 keV. An array of point foci with a 55 µm period and an average size of ca. 2.1 µm × 3.6 µm was achieved. In comparison with the conventional scanning transmission microscopy using one CRL, sub-pixel resolution scanning transmission hard X-ray microscopy enables a large field of view and short scanning time while keeping the high spatial resolution

Simulation of aperture-optimised refractive lenses for hard X-ray full field microscopy

The aperture of refractive X-ray lenses is limited by absorption and geometry. We introduce a specific simulation method to develop an aperture-optimized lens design for hard X-ray full field microscopy. The aperture-optimized lens, referred to as Taille-lens, allows for high spatial resolution as well as homogeneous image quality. This is achieved by the individual adaptation of the apertures of hundreds of lens elements of an X-ray imaging lens to the respective microscopy setup. For full field microscopy, the simulations result in lenses with both a large entrance and exit aperture and lens elements with smaller apertures in the middle of the lens

Parabolic gratings enhance the X-ray sensitivity of Talbot interferograms

In grating-based X-ray Talbot interferometry, the wave nature of X-ray radiation is exploited to generate phase contrast images of objects that do not generate sufficient contrast in conventional X-ray imaging relying on X-ray absorption. The phase sensitivity of this interferometric technique is proportional to the interferometer length and inversely proportional to the period of gratings. However, the limited spatial coherency of X-rays limits the maximum interferometer length, and the ability to obtain smaller-period gratings is limited by the manufacturing process. Here, we propose a new optical configuration that employs a combination of a converging parabolic micro-lens array and a diverging micro-lens array, instead of a binary phase grating. Without changing the grating period or the interferometer length, the phase signal is enhanced because the beam deflection by a sample is amplified through the array of converging-diverging micro-lens pairs. We demonstrate that the differential phase signal detected by our proposed set-up is twice that of a Talbot interferometer, using the same binary absorption grating, and with the same overall inter-grating distance

Determination of the packing fraction in photonic glass using synchrotron radiation nanotomography

Photonic glass is a material class that can be used as photonic broadband reflectors, for example in the infrared regime as thermal barrier coating films. Photonic properties such as the reflectivity depend on the ordering and material packing fraction over the complete film thickness of up to 100 μm. Nanotomography allows acquiring these key parameters throughout the sample volume at the required resolution in a non-destructive way. By performing a nanotomography measurement at the PETRA III beamline P05 on a photonic glass film, the packing fraction throughout the complete sample thickness was analyzed. The results showed a packing fraction significantly smaller than the expected random close packing giving important information for improving the fabrication and processing methods of photonic glass material in the future