Development of High-Density 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 current imaging spectrometer satellite missions such as Chandra and XMM. Instruments based on the microcalorimeters can help answer a broad range of important scientific questions as noted in the X-ray mission concepts study reported commissioned by NASA's Program Office for the Physics of the Cosmos. "Lynx" has been listed by NASA as one of the four major mission concepts to be studied in the next Astrophysics Decadal Review. One of the key instruments on such a mission would be a very large format X-ray microcalorimeter array, with an array size of over 100,000 pixels. Currently, one of the main limitations on the achievable microcalorimeter array size is due to the challenge of fabricating high-density, high-yield microstrip superconducting wiring between all the pixels in the array, especially when the pixel pitch becomes very small. In order to realize the large focal plane array envisaged for the Lynx X-ray Microcalorimeter (LXM), with over 100,000 pixels (which is a factor of 100 larger than current state-of-the-art), with pixel pitches as small as 25 microns (corresponding to 0.5 arc-seconds) in some regions, fine-pitch multi-level superconducting wiring with high yield is essential. To achieve this, we propose to demonstrate the viability of this approach by building prototype large-format microcalorimeter arrays as a collaboration between MIT Lincoln Laboratory (MIT/LL) and NASA/GSFC. In this approach, MIT/LL would fabricate the base-layer multi-level superconducting wiring layer, taking advantage of over a decade of investment in infrastructure and process development that makes MIT/LL a leader in this field of development. GSFC would then fabricate and test the microcalorimeter arrays, integrating with the MIT/LL wiring. MIT Lincoln Laboratory recently demonstrated a functional chip with a world record integration of over 800,000 Josephson Junctions, in a process utilizing eight levels of superconducting wiring layers each with submicron resolution. The goal of the proposed effort is to demonstrate the integration of the high-density wiring capability with GSFC's new advanced X-ray microcalorimeters using wiring designs consistent with being able to read out full arrays of the scale needed for the LXM. Since there are currently two leading microcalorimeter technologies under development focused on meeting LXM requirements, transition edge sensors (TES) and metallic magnetic calorimeters (MMC), we will develop separate designs for each, and fabricate these designs together for efficiency. If the proposal is accepted, GSFC will support this development as part of an existing work package titled, "Advanced X-ray Microcalorimeters". We will optimize the process and design parameters for both detector technologies, and increase sensitivity for the MMCs by using as thin an insulation layer as possible between the sensor and pick-up coil. We will also generate more advanced wiring designs for hybrid arrays utilizing more layers of wiring, incorporating superconducting ground-plane layers in-between micro-strip wiring to avoid cross-talk, and allowing heat-sinking fabrication as would be needed for a full scale array (MMCs and TESs). Further benefits of high-density wiring, such as miniaturization of components, will be investigated.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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