Nanomirror array for high-efficiency soft x-ray spectroscopy

A new type of transmission grating promises to improve the performance of future x-ray-spectroscopy telescopes by a factor of five as compared to the Chandra X-ray Observatory.
27 August 2008
Ralf Heilmann, Minseung Ahn, and Mark Schattenburg

Measuring the light spectra of astrophysical objects with high resolution provides astronomers with invaluable quantitative data for testing theoretical models. For example, astronomers want to use spectra in the soft x-ray band to determine the properties of the warm-hot-intergalactic medium (WHIM), outflows from active galactic nuclei (AGN), and plasma emissions from stellar coronae. Because x-rays are absorbed by earth's atmosphere, x-ray telescopes and spectrometers must be based in space. But even in space, most x-ray signals are very weak, so we want instruments with the highest possible efficiency. In the soft x-ray band, high-resolution spectroscopy is best achieved through the use of wavelength-dispersive diffraction gratings, with a fine pitch of 5000lines/mm or more. In the past, however, this required choosing between either the low mass and relaxed alignment and assembly tolerances of transmission gratings or the high diffraction efficiency with more mass of blazed reflection gratings.

Blazed reflection gratings have high diffraction efficiency because the individual grating facets reflect x-rays very well at shallow angles of grazing incidence, akin to skipping stones on water—see Figure 1(a). However, the direction of the so-called diffraction orders depends strongly on the orientation and figure quality of a reflection grating, similar to reflecting light off a mirror. The grazing incidence geometry and the challenging surface figure requirements tend to make reflection gratings long and thick (and thus heavy) and limit the geometrical throughput of a reflection grating array (such as the one on the European Space Agency x-ray observatory XMM-Newton).

Figure 1. Diffraction grating schematics. (a) Blazed reflection grating. (b) Membrane-supported transmission grating. (c) Critical angle transmission (CAT) grating. Diffraction is enhanced at an angle 2αrelative to the straight-through 0th order, corresponding to mirror reflection from the grating bar sidewalls. This example shows the case where the diffraction angle for the mth order, βm, is 2α.

Transmission gratings only need to be a few micrometers thick and are very forgiving in terms of figure and alignment, similar to sending light through a glass window. NASA's Chandra X-ray Observatory employs ‘phase-shifting’ transmission gratings that work well at higher x-ray energies, but provide only about 10% efficiency in the soft x-ray band, split up into positive and negative diffraction orders.1 The low efficiency is primarily because most soft x-rays are strongly absorbed when transmitted through even a micrometer of most suitable materials, as shown in Figure 1(b). We have developed a new approach that works in transmission, and mostly avoids the absorption problem.

Critical-angle transmission gratings

As Figure 1(c) shows, our critical-angle transmission (CAT) grating features thin and tall grating bars with wide open spaces between them.2 X-rays are incident at small angles α of grazing incidence (below the ‘critical angle for total external reflection’) relative to the sidewalls of the grating bars, which act as mirrors. Diffraction is enhanced in the direction of specular reflection (in much the same way as in blazed reflection gratings) off of the sidewalls, except that the transmission geometry provides low sensitivity to grating misalignment and non-flatness. The dimensions of the grating bars are determined by the grating period p, the critical angle, and the requirement that every x-ray incident between two neighboring grating bars should undergo one reflection from a sidewall. A typical configuration that would fulfill the requirements for NASA's next-generation x-ray telescope Constellation-X would feature a grating period of 100nm with 20nm-wide and 3μm-tall grating bars. In such a broadband x-ray spectroscopy telescope, a CAT grating features the added advantage that high energy x-rays penetrate it with little absorption or diffraction and can be collected by an energy-dispersive detector at the telescope focus.3

Figure 2. CAT grating fabrication results. (a) Schematic showing the integrated silicon support bars in gray, and the CAT grating bars in white. (b) Top view scanning electron micrograph of a CAT grating with 200nm period. The top of the support bars can be seen in the upper left and lower right corners. (c) Cleaved cross section through a CAT grating shows the high aspect ratio of the grating bars and the smooth sidewalls. The destructive cleaving process often leads to ‘stiction’ between bars.
CAT grating fabrication and x-ray tests

Simulations predict that such a grating would exceed 50% diffraction efficiency over the 1–4nm wavelength band with peak efficiencies of almost 70%. But bringing such a geometry from paper into the real world required help from Mother Nature in the form of the highly anisotropic etch properties of silicon crystals in potassium hydroxide (KOH) solutions. Our fabrication process starts with a <110> silicon-on-insulator wafer with a device layer thickness equal to the desired grating bar height.4 We first pattern a support mesh (which holds each grating bar on two sides) and then pattern the CAT grating bars in careful alignment with the silicon <111> direction via scanning-beam interference lithography.5 Then we etch away most of the handle wafer, followed by the KOH etch of the device layer. Finally the mask and buried oxide are removed, and the grating undergoes a supercritical drying process.

The grating bar sidewalls consist primarily of atomically smooth {111} silicon planes (see Figure 2). With this process we have achieved grating bar aspect ratios of >140 at a period of 574nm. Despite some visible defects, we obtained the expected blazing effect and 65–80% of the theoretically predicted efficiencies for the 2–50nm wavelength band.2 We have since improved our process and adapted it to the fabrication of 200nm-period CAT gratings with 40nm-wide grating bars. Very recent x-ray tests are currently being analyzed (see Figure 3).

Figure 3. Example diffraction intensity (not normalized) from a 200nm-period CAT grating at a wavelength of 3.5nm. The straight-through zeroth order is strongly suppressed, and higher orders around the blaze angle of ∼5.2°are strongly enhanced.

The CAT grating design combines the best features of transmission and reflection gratings for soft x-rays and extreme ultraviolet light. It promises high-efficiency, high-resolution spectroscopy for future x-ray telescopes with low weight. Current fabrication efforts focus on higher aspect-ratio grating bars and smaller support mesh structures.

This work was supported by grants from NASA and a Samsung Fellowship. X-ray tests of these prototypes were performed at the Advanced Light Source of Lawrence Berkeley National Laboratory with the help of Eric Gullikson.

Ralf Heilmann, Minseung Ahn, Mark Schattenburg 
Cambridge, MA

Ralf Heilmann is a research scientist at the MIT Kavli Institute for Astrophysics and Space Research (MKI), and the associate director of the Space Nanotechnology Laboratory (SNL). His main research interests are in the areas of advanced diffraction gratings, nanofabrication, interference lithography, and thin-foil x-ray optics.

Minseung Ahn is a graduate student in the Department of Mechanical Engineering at MIT and the recipient of a Samsung Fellowship. He currently works on the fabrication of critical-angle transmission gratings.

Mark Schattenburg is a senior research scientist at MKI and director of the SNL. His main research interests are in ultra-high precision lithography, diffraction grating fabrication, and lightweight high-resolution x-ray optics.

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