High-Density Superconducting Cables for Advanced ACTPol
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Advanced ACTPol (AdvACT) is an upcoming Atacama Cosmology Telescope (ACT) receiver upgrade, scheduled to deploy in 2016, that will allow measurement of the cosmic microwave background polarization and temperature to the highest precision yet with ACT. The AdvACT increase in sensitivity is partly provided by an increase in the number of transition-edge sensors (TESes) per array by up to a factor of two over the current ACTPol receiver detector arrays. The high-density AdvACT TES arrays require 70 \(\upmu \)m pitch superconducting flexible cables (flex) to connect the detector wafer to the first-stage readout electronics. Here, we present the flex fabrication process and test results. For the flex wiring layer, we use a 400-nm-thick sputtered aluminum film. In the center of the cable, the wiring is supported by a polyimide substrate, which smoothly transitions to a bare (uncoated with polyimide) silicon substrate at the ends of the cable for a robust wedge wire-bonding interface. Tests on the first batch of flex made for the first AdvACT array show that the flex will meet the requirements for AdvACT, with a superconducting critical current above 1 mA at 500 mK, resilience to mechanical and cryogenic stress, and a room temperature yield of 97\(\,\%\).
KeywordsAtacama Cosmology Telescope ACTPol Detector Flex Superconductor Wire bonding
This work was supported by the U.S. National Science Foundation through awards AST-0965625, PHY-1214379, and AST-1440226. The NIST authors would like to acknowledge the support of the NIST Quantum Initiative. The work of EG, BJK, BLS, SMS, and JTW was supported by NASA Space Technology Research Fellowship awards. We had helpful conversations with Marcel Bruijn about his experience with flex fabrication that helped us begin this research. We would also like to thank Bert Harrop and the rest of the Princeton University MNFL staff for their advice and guidance during the flex fabrication process development.
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