Development of a Through Silicon Via -enabled HREXI Detector Module (HDM) (TSV)

We propose to continue our APRA program to optimize the development of a High Resolution Energetic X-ray Imager (HREXI) with a new generation of CdZnTe (CZT) detectors, each read out by NuSTAR ASICs (NuASICs) with Through Silicon Vias (TSVs) rather than fragile wire bonds (WBs). We have designed and built an optimal close-tiled Detector Crystal Array (DCA) board composed of 2 x 2 close-tiled 2 x 2 x 0.3cm CZTs. We have designed and are ready to fabricate a DCA controller board that will also serve as a prototype for the FPGA Mezzanine Board (FMB) that will provide full control, commanding and high speed readout of a 2 x 2 close-tiled array of DCAs. We have developed the first robust implementation of TSVs in NuASICs with a leading 3-D interconnects firm (Micross) that will allow minimal gap CZT tiling (lower backgrounds), lower noise (than WBs; as we have demonstrated), and flip-chip bonding for more robust and simplified integration and testing than with the 87 WB connections on each ASIC for data, commanding and power as flown on NuSTAR. The Micross TSV ASIC fabrication revealed high inter-TSV leakage which will be fixed by increased thickness of barrier layers on each TSV and/or possibly by fabrication of NuASICs on higher resistivity NuSTAR Si wafers. We now propose to design and build the final module of a larger imaging CZT array: the FPGA Mezzanine Board (FMB) to command, control and read out 2 x 2 DCA boards. This hierarchical design (only 2 boards high) enables much larger total area and lower power imagers as required for our ultimate goal: a 1024 cm^2 CZT array consisting of 4 x 4 FMBs that interface to a single Detector Mother Board (DMB) for prompt burst image processing of a coded aperture imaging telescope with a 0.87 sr Field of View to provide prompt ~20'' source positions for GRBs and transients with the current Nu ASIC (600um pixels) and ~10'' positions with the proposed 300 um pixel ASIC being developed by our Caltech partners. This HREXI detector plane would provide the first broad band (3 - 300 keV) time-resolved spectra from stacked images. We propose to build and test 1 fully populated FMB (with 2 x 2 DCAs) under this APRA proposal. The first two DCAs (Yr 2) will employ WB-NuASICs for initial tests as we eliminate Micross TSV leakage with thicker TSV barrier layers or higher resistivity Si wafers for fabrication of TSV-NuASICs. A second pair of DCAs follows (Yr3) equipped with TSV-NuASICs. Both types of DCAs will be fully tested (TVAC and shake tests) simultaneously while mounted together to an FMB to bring the electronics and TSV technology to TRL 6 within 3 years. This enables a full test of TSV vs. WB readouts to fully demonstrate TSV-NuASICs are as reliable as WB-NuASICs, which are flight-proven by NuSTAR. This would enable development of the compact wide-field (0.87 sr), broad band (3 – 300 keV) X-ray telescope on a SmallSat platform. This will do impressive new science (below) to achieve NASA and Astro2020 priority objectives in Time-domain Astrophysics (TDA) and would enable a future continuous full-sky, broad-band, time-domain hard/soft X-ray imaging survey with a Constellation (8 - 12 SmallSats) mission as the 4pi X-ray Imaging Observatory (4piXIO). Such a mission, possible as an “Incremental Explorer” (launched 2-4 at a time) or a low-cost Probe mission, would address numerous key NASA and Astro 2020 objectives, from Exoplanet habitability from rates of X-ray flares from M dwarf host stars, to the study of the Early Universe revealed by high-z GRBs, physics and evolution of black holes (on all scales), and high resolution <10” source positions (with the future 300um pixel ASIC) and spectra for TDA and Multi-messenger Astrophysics for, particularly, LIGO/VIRGO/KAGRA GW events.

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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