Development of Next-Generation Wiring Capability for Advanced X-Ray Microcalorimeters

Microcalorimeters offer exciting opportunities for X-ray astronomy, with energy resolution that is about a factor of 100 better than the silicon CCDs, which is the current work-horse detector technology being used in X-ray imaging spectrometer satellite missions such as Chandra and XMM. The recent astrophysics decadal survey endorsed an X-ray mission as one of the three new NASA Great Observatories (NGO) missions that should proceed in development through the next decade. This is a proposal to further develop X-ray microcalorimeter technology, continuing to work in close collaboration with the X-ray microcalorimeter group at NASA’s Goddard Space Flight Center, with a view to providing an instrument with even greater capabilities than have ever been envisaged before, and potentially with properties that could help reduce the estimated cost of the future X-ray NGO. This will be a direct follow-on to an existing APRA program. GSFC will collaborate with this development as part of a GSFC astrophysics work package titled, "Advanced X-ray Microcalorimeters". One of the main limitations on the achievable microcalorimeter array size had been the challenge of fabricating high-density, high-yield microstrip superconducting wiring between all the pixels in the array. Utilizing decades of investment in infrastructure and process development at MIT/LL for superconducting electronics, a new approach was developed to solve this problem. In this approach, MIT/LL fabricated the base-layer multi-level superconducting wiring layer, and GSFC fabricated and tested the microcalorimeter arrays, integrating with the MIT/LL wiring. The approach successfully demonstrated a prototype 50 kpixel array for transition edge sensors (TESs) and a 100 kpixel array for magnetic microcalorimeters (MMCs). In this program we propose to take this development to the next step, providing even greater advances in detector array fabrication capability. We propose to investigate advanced next-generation wiring capabilities for X-ray microcalorimeters that may enable a microcalorimeter instrument to meet most of an X-ray NGO observatory’s instrument requirements. The following aspects of the superconducting metal lines will be developed: 1. Smaller features to enable smaller pixel pitch. 2. Additional wiring layers. 3. Larger regions with high density wiring. Currently the metal line width and spacing of 400 nm has been demonstrated. In this proposal, we will reduce this to 300 nm line and space definition to enable higher wiring density. Second, additional metal layers will enable additional shielding and inductance in a given area, which can help with the array design and performance. Third, we have demonstrated the 400 nm metal line features for the central 22x22 mm region of the wiring wafer. By either stitching or using another DUV lithography tool with a larger exposure field, we can increase the central region which has fine features. This will enable patterning a detector array that can cover a wider field of view. The additional wiring capabilities have the potential to enable new array capabilities with pixel pitches of 0.3 arc-seconds or less, a field-of-view of one arc-second pixels that is greater than ten arc-minutes, and some pixels with resolving powers greater than 5000. If this research is successful, an X-ray NGO that will enable us to reveal the hidden universe could become significantly more stream-lined and less expensive than the Lynx observatory design submitted to the astrophysics Decadal Survey.

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