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Flexible Microstrip Circuits for Superconducting Electronics

Improved wiring geometry should further reduce the size of the wiring while also reducing the crosstalk among wire pairs.

Flexible circuits with superconducting wiring atop polyimide thin films are being studied to connect large numbers of wires between stages in cryogenic apparatus with low heat load. The feasibility of a full microstrip process, consisting of two layers of superconducting material separated by a thin dielectric layer on 5 mil (≈0.13 mm) Kapton sheets, where manageable residual stress remains in the polyimide film after processing, has been demonstrated. The goal is a 2-mil (≈0.051-mm) process using spin-on polyimide to take advantage of the smoother polyimide surface for achieving high-quality metal films. Integration of microstrip wiring with this polyimide film may require high-temperature bakes to relax the stress in the polyimide film between metallization steps.

Focal planes of cryogenic detectors typically have detectors at the lowest temperature stage with bias and readout located at higher temperature stages to reduce the cooling power requirement on the refrigerator stage that achieves the base temperature for the detectors. Large numbers of wires between cryogenic stages are often necessary and need to be designed to maintain a manageable heat load to each stage. A microstripline wiring configuration is also desired to suppress thermal crosstalk into the detectors due to amplifier switching and bias changes. With the size of focal planes increasing into the range of thousands of biased elements, and a further need for compactness of the focal plane architecture, a technology is needed that can accommodate thousands of superconducting wires between cryogenic components.

Flexible niobium wiring has been demonstrated on Kapton pieces where the impedance of the line was set by the distance between the Nb wires and the dielectric properties of the Kapton. This work proposes to fabricate microstrip Nb wiring consisting of a narrow Nb trace atop a wider trace separated by a thin dielectric layer. This wiring geometry, in comparison to the coplanar designs, should further reduce the size of the wiring while also reducing the crosstalk among wire pairs. Further, the use of a thin polyimide layer will enable lower heat loads between stages for a similar length of flexible wiring.

It was shown that 5-mil (≈0.13-mm) sheets could be readily mounted smoothly onto the substrate. The substrates were taken through all process steps, including Nb deposition and etch, oxide deposition, and aluminum deposition and etch. In all cases, the heat-release tape held the Kapton onto the substrate, indicating that the processes could be run serially to complete a full microstrip process. A Kapton film with a patterned Nb layer on it was released, and showed that the film was superconducting at a temperature close to the expected critical temperature of Nb. Polyimide layers that were free of roughness and pitting were generated through a spin-on process that used successive spins and bakes, with gradual heat lip and cooldown cycles, to build up the film to a full thickness of 2 mils (»0.051 mm). The full thickness film is baked at elevated temperatures to relieve residual stress in the film.

This work was done by James Chervenak of Goddard Space Flight Center, and Jennette Mateo of SB Microsystems. GSC-16718-1

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