Broadband high-energy resolution hard x-ray spectroscopy using transition edge sensors at SPring-8.

We have succeeded in operating a transition-edge sensor (TES) spectrometer and evaluating its performance at the SPring-8 synchrotron x-ray light source. The TES spectrometer consists of a 240 pixel National Institute of Standards and Technology (NIST) TES system, and 220 pixels are operated simultaneously with an energy resolution of 4 eV at 6 keV at a rate of ∼1 c/s pixel-1. The tolerance for high count rates is evaluated in terms of energy resolution and live time fraction, leading to an empirical compromise of ∼2 × 103 c/s (all pixels) with an energy resolution of 5 eV at 6 keV. By utilizing the TES's wideband spectroscopic capability, simultaneous multi-element analysis is demonstrated for a standard sample. We conducted x-ray absorption near-edge structure (XANES) analysis in fluorescence mode using the TES spectrometer. The excellent energy resolution of the TES enabled us to detect weak fluorescence lines from dilute samples and trace elements that have previously been difficult to resolve due to the nearly overlapping emission lines of other dominant elements. The neighboring lines of As Kα and Pb Lα2 of the standard sample were clearly resolved, and the XANES of Pb Lα2 was obtained. Moreover, the x-ray spectrum from the small amount of Fe in aerosols was distinguished from the spectrum of a blank target, which helps us to understand the targets and the environment. These results are the first important step for the application of high resolution TES-based spectroscopy at hard x-ray synchrotron facilities.

Grinding and chemical mechanical polishing process for micropore x-ray optics fabricated with deep reactive ion etching.

Silicon micropore optics using deep reactive ion etching of silicon wafers has been being developed for future x-ray astronomy missions. Sidewalls of the micropores through a thin wafer with a typical thickness of hundreds of micrometers and a pore width of ∼20 µm are used for x-ray mirrors. However, burr structures observed after etching with a typical height of a few micrometers at the micropore edges are known to significantly reduce x-ray reflectivity. A new grinding and chemical mechanical polishing process is introduced to remove the burr structures. Both sides of the silicon wafer were ground and precisely polished after etching. X-ray reflectivity measurements confirmed an increase of reflectivity by 2-15 times at incident angles of 0.8-0.2 deg. The surface microroughness worsened from 2.0±0.2 nm rms to 7.8-0.8+0.6 nm rms; however, an additional annealing recovered the smooth surface and the estimated surface microroughness was <1.4 nm rms. This new process enables not only removing the burr structures but also choosing a flat part of the sidewalls for better angular resolution.

Pt thermal atomic layer deposition for silicon x-ray micropore optics.

We fabricated a silicon micropore optic using deep reactive ion etching and coated by Pt with atomic layer deposition (ALD). We confirmed that a metal/metal oxide bilayer of Al2O3∼10 nm and Pt ∼20 nm was successfully deposited on the micropores whose width and depth are 20 µm and 300 µm, respectively. An increase of surface roughness of sidewalls of the micropores was observed with a transmission electron microscope and an atomic force microscope. X-ray reflectivity with an Al Kα line at 1.49 keV before and after the deposition was measured and compared to ray-tracing simulations. The surface roughness of the sidewalls was estimated to increase from 1.6±0.2 nm rms to 2.2±0.2 nm rms. This result is consistent with the microscope measurements. Post annealing of the Pt-coated optic at 1000°C for 2 h showed a sign of reduced surface roughness and better angular resolution. To reduce the surface roughness, possible methods such as the annealing after deposition and a plasma-enhanced ALD are discussed.

Iridium-coated micropore x-ray optics using dry etching of a silicon wafer and atomic layer deposition.

To enhance x-ray reflectivity of silicon micropore optics using dry etching of silicon (111) wafers, iridium coating is tested by use of atomic layer deposition. An iridium layer is successfully formed on sidewalls of tiny micropores with a pore width of 20 µm and depth of 300 µm. The film thickness is ∼20 nm. An enhanced x-ray reflectivity compared to that of silicon is confirmed at Ti Kα 4.51 keV, for what we believe to be the first time, with this type of optics. Some discrepancies from a theoretical reflectivity curve of iridium-coated silicon are noticed at small incident angles <1.3°. When a geometrical shadowing effect due to occultation by a ridge existing on the sidewalls is taken into account, the observed reflectivity becomes well represented by the modified theoretical curve. An estimated surface micro roughness of ∼1 nm rms is consistent with atomic force microscope measurements of the sidewalls.