Broadside-Coupled Niobium Flexible Cables

We have developed fine-pitch, multilayer, superconducting wiring for routing around a ninety-degree corner terminated with wirebonding interfaces. The component-level testbed for the Advanced Telescope for High Energy Astrophysics (ATHENA) X-Ray Integral Field Unit (X-IFU) focal plane requires compact, high-density, low-crosstalk wiring fanout to connect the detectors in the focal plane array with NIST-fabricated SQUID time domain multiplexing (TDM) readout chips. The full assembly baselines two interface chips: a flexible interface chip bending around the corner and a planar silicon carrier chip. The TDM readout is indium bump-bonded to the silicon carrier and afterward the flexible chip is clipped in place and wirebonded to the detector and fanout wiring on the carrier. This assembly is repeated for each side of the hexagonal focal plane structure. As conventional commercial cables are not able to achieve the fine-pitch, low-crosstalk, superconducting wiring required, we fabricate these flexible interface chips in-house via lithographic patterning and etching of sputter deposited thin films to create broadside-coupled superconducting niobium microstrips. Within the chip, the wiring on the flexible polyimide region transitions to silicon substrate for the closely spaced wirebond pads. The Nb microstrip wiring climbing a thick polyimide sidewall presents a fabrication challenge which we shall discuss in this paper. We describe the function of these components to build an effective engineering testbed for the ATHENA X-IFU.