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Journal of Physics Conference Series

Study on Microwave Kinetic Inductance Detector with Lithium Niobate Substrate

Ryota Ito, Tohru Taino, S. Mima, Yuto Kamei, Chiko Otani

Published Jul 1, 2025• 1 citation
Publisher

Abstract

Abstract We are developing Microwave Kinetic Inductance Detectors (MKIDs) on Lithium Niobate (LN) substrates. The goal of this research is to realize phonon-mediated superconducting detectors with the high phonon collection efficiency. As a first step, we designed and fabricated MKIDs on LN. The relative permittivity of LN was experimentally obtained at 150 mK, and the resonant peaks of MKIDs on LN were observed. The values of internal quality factor on LN were about 103 times lower than those on Si. This is primarily attributed to the energy dissipation associated with the generation of bulk acoustic waves within LN. From the reproduction of the observed spectral features in the microwave region, the upper limit of the quality factor due to the loss from piezoelectricity was estimated, that was consistent with the observed values.

DOI
10.1088/1742-6596/3054/1/012023

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•Oct 1, 2019•0 citations
Abstract We are developing Microwave Kinetic Inductance Detectors (MKIDs) on Lithium Niobate (LN) substrates. The goal of this research is to realize phonon-mediated superconducting detectors with the high phonon collection efficiency. As a first step, we designed and fabricated MKIDs on LN. The relative permittivity of LN was experimentally obtained at 150 mK, and the resonant peaks of MKIDs on LN were observed. The values of internal quality factor on LN were about 103 times lower than those on Si. This is primarily attributed to the energy dissipation associated with the generation of bulk acoustic waves within LN. From the reproduction of the observed spectral features in the microwave region, the upper limit of the quality factor due to the loss from piezoelectricity was estimated, that was consistent with the observed values.
We designed and fabricated a microwave kinetic inductance detector (MKID) using a niobium nitride (NbN) symmetric spiral resonator array. Previously we revealed that a rewound spiral structure works as not only a high-Q half-wavelength resonator but also as a broadband terahertz antenna. We conducted simulations for a 9 resonator array assuming NbN as the superconducting material and sapphire as the dielectric substrate, and obtained a maximum attenuation of over 30 dB and unloaded quality factors of over 2 × 10 5 for frequencies between 4.4 and 4.9 GHz. We fabricated the 9 resonator array MKID using NbN thin film deposited on an m-sapphire substrate by using dc magnetron sputtering. We observed half-wavelength resonances of around 4.5 GHz at 4 K. We measured the optical response of the MKID. The frequency shift was 0.5 MHz when illuminated with 650 nm photons.
We report on the development of microwave kinetic inductance detectors (MKIDs) using platinum silicide as the sensor material. MKIDs are an emerging superconducting detector technology, capable of measuring the arrival times of single photons to better than two microseconds and their energies to around ten percent. Previously, MKIDs have been fabricated using either sub-stoichiometric titanium nitride or aluminum, but TiN suffers from the spatial inhomogeneities in the superconducting critical temperature and Al has a low kinetic inductance fraction, causing low detector sensitivity. To address these issues, we have instead fabricated the PtSi microresonators with the superconducting critical temperatures of 944 ± 12 mK and high internal quality factors (Qi≳106). These devices show typical quasiparticle lifetimes of τqp≈30–40 μs and spectral resolution, R=λ/Δλ, of 8 at 406.6 nm. We compare PtSi MKIDs to those fabricated with TiN and detail the substantial advantages that PtSi MKIDs have to offer.
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.
Thin-film lithium niobate (LN) has become an enabling platform in the development of quantum integrated circuits, for applications including microwave-to-optical converters and electromechanical transducers. Incorporation of high-quality planar superconducting resonators will further elevate the performance of these devices. However, alongside the desired interactions originated from LN, its piezoelectric nature also introduces parasitic coupling between microwave and phononic modes, resulting in excessive dielectric loss. In this work, we investigate such coupling by studying niobium nitride resonators directly deposited on patterned LN thin films. By varying device configuration, we pinpoint that overlapping electric field with LN can inadvertently excite bulk acoustic modes within the substrate, inducing microwave loss independent of the acoustic dissipation rate. The experimental results are supported by simulation and theoretical modeling. Our approach can be applied to different piezoelectric systems and provide insights into the design of microwave components for quantum transduction devices.
We report on the design and performance of microwave kinetic inductance detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 μΩ cm with RRR=1.6. Resonators on the device displayed internal quality factors of around 200 000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process, which resulted in pixels with decay times of about 40 μs and resolving powers of ∼9 at 800 nm.
Microwave kinetic inductance detectors (MKID) are superconducting broadband direct detectors. If a superconducting thin film patterned into a resonator is illuminated with photons, Cooper pairs break up and the kinetic inductance of the film changes. This shifts the resonance frequency downwards and the dissipation of the resonator increases, which can be detected in the phase and amplitude of a transmitted signal. \n \nIn this thesis, a waveguide coupled MKID is designed and simulated. 350 GHz radiation is coupled into a copper waveguide via a horn antenna. A waveguide probe leads the signal onto a 9 µm silicon membrane where it is absorbed in a 40 nm superconducting aluminum film which is the inductive part of a superconducting resonator. The capacitor of the resonator is made of niobium. Two versions of the device are developed, a transmission line resonator and a lumped element resonator design. The new design will enable an accurate understanding of the optical sensitivity of MKIDs and allow comparison of different resonator materials. Since two different resonator types are used, the direct comparison of transmission line and lumped resonators will also be possible. \n \nA measurement setup is developed including an ADR cryostat to cool MKID devices down to below 40 mK. An analog readout system using either a vector network analyzer or an IQ mixer is designed and built. The necessary software to control the ADR, the temperature monitoring, the readout signal generation and detection and the signal acquisition of the setup is also developed. \n \nThe waveguide coupled MKID design is fabricated in our in-house microfabrication laboratory and measured in the new setup. In my measurements, the devices do not show a resonance feature. The VNA and DC measurements are analyzed to determine a plausible hypothesis of the failure. The devices are diagnosed for critical issues during fabrication which will be addressed in a future iteration. \n \nTo verify the measurement setup, a test device of the IRAM NIKA 2 MKID camera is measured and the complete analysis pipeline is exercised. The quasi-particle lifetime in the aluminum thin films is measured and a dark and optical noise equivalent power are determined. The results are within the uncertainty comparable to measurements of IRAM themselves. \n \nA test chip designed in my master thesis that contains eight NbTiN resonators is measured in the new setup and non-linear effects under strong readout power are studied. Parametric amplification and oscillation of a resonator are shown qualitatively.