Superconducting RF Inductors on 200-mm routing platform

We present compact RF superconducting inductors fabricated on a multi-layer routing platform using niobium nitride (NbN) thin films deposited on 200 mm oxidized silicon wafers. Two 100 nm-thick NbN layers, separated by SiO₂ dielectric layers and connected through tungsten vias, are used to realize spirals inductors with a 300 nm trace width and various number of turns employing one or both superconducting layers.

Two-port S-parameters of the devices are measured at 4.7 K up to 25 GHz using a cryogenic probe station with GSG probes after a full calibration and de-embedding procedure.

The extracted parameters of the components show inductances values ranging from nH to µH, consistent with both the designed geometrical inductance and the kinetic inductance of the superconducting films. The high resonance frequencies of the single-layer inductors indicate very low parasitic capacitances in the order of few femtofarads. Two-layer stacked inductors offer a more compact design but exhibit higher series capacitances and thus lower resonance frequencies. Quality factors up to 1,000 are measured. At low frequencies, the quality factor is limited by the resistance of the metallic vias, while at high frequencies, dielectric losses become dominant.

As a use case, these inductors are integrated into a multiplexed gate-based reflectometry setup for the readout of silicon spin qubits. The substantial reduction in parasitic capacitance, combined with a high quality factor, enables state-of-the-art single-shot readout performance. Discrimination between singlet and triplet states in a double quantum dot configuration is achieved with a fidelity exceeding 99% using a 5 µs integration time, despite a relatively small lever arm of 0.14 eV/V between the resonator and the qubit.

In conclusion, we demonstrate a reliable, high-quality 200 mm superconducting fabrication route for multilayer structures. The combination of flexible design, high material quality, and robust process integration yields high-performance RF inductors, which are further leveraged to achieve state-of-the-art spin readout fidelity using gate-based reflectometry.