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IEICE Technical Report; IEICE Tech. Rep.

Development of Nb/Al bi-layer MKID camera -- Temperature dependence of quality factor

Masakazu Sekine, Yutaro Sekímoto, Takashi Noguchi, Miyachi Akihira, K. Karatsu, Nitta Tom, Shigeyuki Sekiguchi, Takashi Okada, Masato Naruse

Published Jan 16, 2014• 1 citation
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Agnès Dominjon, Masakazu Sekine, K. Karatsu +5 more

Due to their multiplexing capability and their good sensitivity to radiation from submillimeter to X-ray wavelengths, microwave kinetic inductance detectors (MKIDs) are increasingly used in the field of astrophysics. The Advanced Technology Center of the National Astronomical Observatory of Japan is developing MKIDs for astronomical observations such as CMB B-mode search with LiteBIRD. MKIDs are made of superconductors whose energy gap determines the detector frequency range. The energy gap depends on Tc, the critical temperature of the superconductor. It is thus important to be able to adjust Tc in order to choose the suitable frequency range. When using a single-layer MKID, the Tc is fixed by the superconducting gap energy of the unique component and cannot be changed. One possibility is to make a bilayer MKID using the proximity effect to adjust its critical temperature. This paper presents our new study on MKIDs made of superconductor/metal bilayers. We investigated niobium and copper bilayers (Nb/Cu) and fabricated different bilayers in our clean room. The critical temperature of each of them has been measured. We show that the Tc depends on the ratio between Nb and Cu thicknesses and that we are able to control it. Then, we characterized one of these Nb/Cu bilayers (Nb = 8 nm and Cu = 22 nm) once integrated in a MKID. We measured the temperature dependence of the resonant frequency, and we achieved quality factors as high as 2 × 104. The measurement of the noise spectrum provided a lower limit equal to -85 dBc/Hz, and the calculation of the noise equivalent power has shown that the sensitivity of the Nb/Cu bilayer MKID is not very far from that of an Al monolayer MKID.

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Superconducting coplanar waveguide resonators are essential components in quantum processors, where their internal quality factor (Qi) constrains qubit coherence and readout fidelity. In niobium devices, microwave losses at millikelvin temperatures are strongly influenced by two-level systems (TLS) associated with the complex NbOx surface oxide. To mitigate these losses, we investigate a surface-engineering approach in which Nb films are capped in situ with a thin tantalum layer to suppress Nb2O5 formation and replace the native NbOx interface with a Ta-based oxide. We fabricate Nb/Ta bilayer and reference Nb resonators on high-resistivity silicon using identical DC sputtering and wet etching conditions, and characterize their performance at millikelvin temperatures. Fresh Ta-encapsulated devices exhibit internal quality factors up to 2.4 x 10^6 in the near-single-photon regime, with power dependence consistent with reduced TLS-related loss at the metal-air interface. A control Nb device fabricated under the same process shows comparatively lower Q_TLS, consistent with the beneficial effect of the Ta capping layer. Furthermore, ageing tests performed on Nb/Ta resonators after six months reveal a moderate reduction in Q_TLS relative to their initial values, yet the performance remains superior to newly fabricated Nb-only devices. These results suggest that thin Ta encapsulation enhances interface quality and contributes to improved temporal stability while remaining compatible with Nb-based fabrication workflows.

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The Japan Society of Applied Physics•Jul 9, 2020•0 citations
Due to their multiplexing capability and their good sensitivity to radiation from submillimeter to X-ray wavelengths, microwave kinetic inductance detectors (MKIDs) are increasingly used in the field of astrophysics. The Advanced Technology Center of the National Astronomical Observatory of Japan is developing MKIDs for astronomical observations such as CMB B-mode search with LiteBIRD. MKIDs are made of superconductors whose energy gap determines the detector frequency range. The energy gap depends on Tc, the critical temperature of the superconductor. It is thus important to be able to adjust Tc in order to choose the suitable frequency range. When using a single-layer MKID, the Tc is fixed by the superconducting gap energy of the unique component and cannot be changed. One possibility is to make a bilayer MKID using the proximity effect to adjust its critical temperature. This paper presents our new study on MKIDs made of superconductor/metal bilayers. We investigated niobium and copper bilayers (Nb/Cu) and fabricated different bilayers in our clean room. The critical temperature of each of them has been measured. We show that the Tc depends on the ratio between Nb and Cu thicknesses and that we are able to control it. Then, we characterized one of these Nb/Cu bilayers (Nb = 8 nm and Cu = 22 nm) once integrated in a MKID. We measured the temperature dependence of the resonant frequency, and we achieved quality factors as high as 2 × 104. The measurement of the noise spectrum provided a lower limit equal to -85 dBc/Hz, and the calculation of the noise equivalent power has shown that the sensitivity of the Nb/Cu bilayer MKID is not very far from that of an Al monolayer MKID.
Superconducting coplanar waveguide resonators are essential components in quantum processors, where their internal quality factor (Qi) constrains qubit coherence and readout fidelity. In niobium devices, microwave losses at millikelvin temperatures are strongly influenced by two-level systems (TLS) associated with the complex NbOx surface oxide. To mitigate these losses, we investigate a surface-engineering approach in which Nb films are capped in situ with a thin tantalum layer to suppress Nb2O5 formation and replace the native NbOx interface with a Ta-based oxide. We fabricate Nb/Ta bilayer and reference Nb resonators on high-resistivity silicon using identical DC sputtering and wet etching conditions, and characterize their performance at millikelvin temperatures. Fresh Ta-encapsulated devices exhibit internal quality factors up to 2.4 x 10^6 in the near-single-photon regime, with power dependence consistent with reduced TLS-related loss at the metal-air interface. A control Nb device fabricated under the same process shows comparatively lower Q_TLS, consistent with the beneficial effect of the Ta capping layer. Furthermore, ageing tests performed on Nb/Ta resonators after six months reveal a moderate reduction in Q_TLS relative to their initial values, yet the performance remains superior to newly fabricated Nb-only devices. These results suggest that thin Ta encapsulation enhances interface quality and contributes to improved temporal stability while remaining compatible with Nb-based fabrication workflows.