Flexible superconducting Nb transmission lines on thin film polyimide for quantum computing applications

Abstract

We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film niobium/polyimide ribbon cables could provide a physically compact and ultra-low thermal conductance alternative to the rapidly increasing number of discrete coaxial cables that are currently used by quantum computing experimentalists to transmit signals between the several low-temperature stages (from ∼4 K down to ∼20 mK) of a dilution refrigerator. We have concluded that these structures are technically feasible to fabricate, and so far they have exhibited acceptable thermo-mechanical reliability. S-parameter results are presented for individual 2-metal layer Nb microstrip structures having 50 Ω characteristic impedance; lengths ranging from 50 to 550 mm were successfully fabricated. Solderable pads at the end terminations allowed testing using conventional rf connectors. Weakly coupled open-circuit microstrip resonators provided a sensitive measure of the overall transmission line loss as a function of frequency, temperature, and power. Two common microelectronic-grade polyimide dielectrics, one conventional and the other photo-definable (PI-2611 and HD-4100, respectively) were compared. Our most striking result, not previously reported to our knowledge, was that the dielectric loss tangents of both polyimides, over frequencies from 1 to 20 GHz, are remarkably low at deep cryogenic temperatures, typically 100× smaller than corresponding room temperature values. This enables fairly longdistance (meter-scale) transmission of microwave signals without excessive attenuation, and also permits usefully high rf power levels to be transmitted without creating excessive dielectric heating. We observed loss tangents as low as 2.2×10 −5 at 20 mK, although losses increased somewhat at very low rf power levels, similar to the well-known behavior of amorphous inorganic dielectrics such as SiO 2 . Our fabrication techniques could be extended to more complex structures such as multiconductor cables, embedded microstrip, 3-metal layer stripline or rectangular coax, and integrated attenuators and thermalization structures.

Transmission line printed using silver nanoparticle ink on FR-4 and polyimide substrates

“…In conclusion, a transmission lines have almost fabricated using a vacuum deposition, electroplating and electroless plating [9,10], however these transmission lines are difficult to be fabricated as the application in regard to flexible electronics device in the future [11]. Thus, we fabricated the transmission line on FR-4 and PI substrates and investigated their RF performance in order to know an influence of substrate in regard to transmission lines.…”

Microwave Superconductivity

“…Another important issue for microwave superconductivity is the ability to get high frequency signals back and forth to the cryogenic environment without compromising the integrity of the signal or the efficiency of the cryogenic cooling system. This has led to development of low-microwave loss transmission lines that are simultaneously a small heat-load on the cryogenic environment [126]. Examples of such structures include flexible dielectric tapes with an array of superconducting or low-loss metallic transmission line structures.…”

Design and Characterization of a 28-nm Bulk-CMOS Cryogenic Quantum Controller Dissipating Less Than 2 mW at 3 K

“…A more tractable solution is to thermalize the control electronics in the range of 3-4 K and use superconductive transmission lines to couple the control signals down to the 10-mK stage [15], [17]. By using superconducting interconnects, it is possible to achieve excellent electrical and thermal performance in a small cross-sectional area [39]. In the following, we will assume that the quantum control electronics are thermalized in the range of 3-4 K.…”