Abstract Body

A Microwave Kinetic Inductance Detector (MKID) [1] is a superconducting detector and consists of single microwave readout line, resonators and antennas. The MKID is fabricated by depositing and patterning a thin superconducting film on a dielectric substrate, e.g., silicon (Si) or sapphire (Al₂O₃). The Cooper pairs breaking due to the irradiating photon is detected as a change in the kinetic inductance, i.e., a change in the microwave transmission characteristics. A substrate absorption type superconducting detector has been proposed for particle physics experiments, e.g., searching for dark matter. This device uses a superconductor to detect phonons generated in a substrate by irradiation photon. In our past experiments with superconducting tunnel junctions [2], we experimentally found single-crystal lithium niobate (LiNbO₃:LN), generally used as piezoelectric materials and nonlinear optical materials, had high phonon propagation anisotropy. To utilize this material as a substrate, it is expected the phonon collection efficiency can be improved. In this work, to apply a LN substrate to a MKID, we study characteristics of the Nb thin film and the MKID on a LN substrate.

Fabricating a MKID, we used Z-cut single-crystal stoichiometric LN substrates with 10 mm×10 mm×500 μmt, produced by Oxide Corporation. The relative permittivity of Z-cut LN substrates was obtained about 24 (cf. the value of Si is about 11.8). Using this permittivity, we designed a microwave readout line and λ/2 resonators with a microstrip structure. A superconducting material Nb was deposited by DC magnetron sputtering. The MKID design was patterned by a photolithography technique, reactive ion etching process or liftoff process.

We measured properties of 200 nmt Nb thin film, such as crystalline, surface roughness, critical temperature (T_c) and residual resistivity ratio (RRR), on Si or LN substrates. We performed X-ray diffraction measurements and found the Nb thin films deposited on each substrate had same [1 1 0] orientation. The results of atomic force microscope with dynamic force mode showed the thin films had similar roughness. To evaluate the T_c and RRR of the Nb thin film, we mounted fabricated devices in an open dewar filled with liquid helium and cooled them to 4.2 K. During the cooling process, we measured the temperature dependence of Nb thin film resistivity using the 4 terminal method (Fig.1). This measurement showed the T_c and RRR of each thin film were much the same. These results suggested the properties of Nb thin film on LN substrate was equivalent to them on Si substrate.

After the Nb thin film evaluation, we mounted the fabricated MKID (Fig.2) in a dilution refrigerator and cooled them to about 150 mK. We measured microwave transmittance in the frequency domain with a vector network analyzer. We confirmed the MKID on the LN substrate worked as resonators as well as on the Si substrate. Fitting resonant peaks, we obtained the internal quality factor about 200 whereas it reached about 600,000 on Si substrate. Since the properties of Nb thin film are comparable, we have concluded the LN substrate limits the device performance. As mentioned in Ref.[3], we also observed overlapping bulk acoustic mode resonances with the microwave transmission. This mode was excited by overlapping electric field with piezoelectric materials, e.g., LN, and induced microwave loss. We will report details of the MKID evaluation with a LN substrate.

References

[1] P. K. Day et al., Nature, 425, 817, 2003
[2] T. Taino et al., IEEE Trans. Appl. Supercond., 15, 2, 2005
[3] L. Yang et al., Phys. Rev. Appl., 20, 054026, 2023

Acknowledgment

This work was supported by RIKEN Junior Research Associate Program and KAKENHI Grant Number 22K18991, 21K18150, 20H01937 and 19H05809. Equipment shared by RIKEN CEMS Material Characterizations Support Team and Semiconductor Science Research Support Team was used.

Fig.1 Temperature dependence of Nb thin film resistivity on Si or LN substrates.
Fig.2 Appearance of the LN substrate MKID mounted on oxygen-free copper jig.

Keywords: microwave kinetic inductance detector, substrate absorption type superconducting detector, lithium niobate