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IEEE Electronics Packaging Technology Conference (EPTC)

Exploring Materials for RF Transmission Lines in Cryogenic Systems

Daniel Lau, Vignesh Shanmugam Bhaskar, Yong Chyn Ng, Yiyu Zhang, Kuan Eng Johnson Goh, Hongyu Li

Published Dec 3, 2024• 0 citations
Publisher

Abstract

Superconducting systems operating in cryogenic environments represent a rapidly evolving domain with diverse applications spanning from quantum computing to medical diagnostics. To optimize and enhance the performance of such systems, a comprehensive understanding of material properties and their responses under cryogenic conditions is imperative. This work investigates the RF and DC electrical performance of passive structures, specifically transmission lines, at both room and cryogenic temperatures. S-parameter measurements were conducted on Coplanar Waveguides (CPWs) transmission lines, which are crucial for interaction with qubit chips and the resonators to read out the qubit states. Additionally, DC electrical testing is performed on the CPWs to analyse the resistance across the signal and ground metal lines. A series of test structures using different substrates and metal lines are fabricated for s-parameter testing at both room and cryogenic temperatures. This paper presents the impact of the superconducting environment on material RF loss and resistance.

DOI
10.1109/eptc62800.2024.10909922

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Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
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Flexible interconnects with superconducting (SC) traces and polyimide dielectric can provide low signal loss and low thermal conductivity at cryogenic temperatures, and are expected to find use in densely integrated cryogenic electronics systems. The ultra-low loss of SC interconnects at microwave frequencies can be accurately characterized through the use of weakly coupled SC resonators. The high-quality factor resonators provide sensitive measurements of the aggregate loss properties of the conductor and dielectric, as functions of frequency, temperature, and signal power. In this work, resonators with different linewidths were fabricated and tested to study how the linewidth impacts the overall internal resonator loss, in order to better characterize and separate the individual loss components (i.e., dielectric loss vs. conductor loss). The results of this work are important for understanding loss and transmission properties of similarly designed and fabricated transmission line interconnects.