Towards a 100 000 TES Focal Plane Array: A Robust, High-Density, Superconducting Cable Interface

We describe a new fabrication process carried out at the Princeton University Micro/Nano Fabrication Laboratory (MNFL) for creating high-density superconducting cables designed to interface with ACTPol (a polarization-sensitive receiver for the Atacama Cosmology Telescope) detector arrays. A polyimide substrate about 50 μm thick fabricated by spinning and furnace curing a polyimide precursor (HD Microsystems PI-2611) is used for the flexible part of the cable, which gradually transitions to a bare silicon substrate at each end of the cable for a robust wire bonding interface. Aluminum traces 400 nm thick are sputtered on top of the silicon/polyimide hybrid substrate then wet etched into 50-μm-wide traces at a 100 μm pitch. A 10-μm polyimide passivation layer is formed on top of the wiring layer to protect the aluminum traces. To release the flexible part of the cable from the hard silicon substrate reliably without RIE etching tools, we developed a novel process involving backside dicing of the silicon and a gold release layer. The aluminum traces are superconducting with a critical temperature of 1.2 K +/-0.1 K, and a critical current exceeding 5.7 mA at temperatures 1.0 K and below. Aluminum wires 25 μm thick wedge bonded to the cable bond pads have 9.5 g average pull strength.