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Japanese Journal of Applied Physics

Microwave loss of DC sputtered NbTiN microstrip lines

Wenlei Shan, Yutaro Sekímoto, Takashi Noguchi

Published Aug 5, 2015• 5 citations
Publisher

Abstract

Abstract Amorphous NbTiN is widely applied in superconducting electronic devices owing to its merits of wide energy gap, high T c , and mitigated requirement in fabrication. Its highly disordered feature yields considerable kinetic inductance, making it an ideal material for parametric amplification. To build up the correspondence between the fabrication conditions and the desirable electrical performance, we designed, fabricated, and measured NbTiN microstrip resonators at microwave frequencies. The principal result is that the unloaded Q factor of the resonators does not depend on the film normal state resistivity, which largely depends on the sputtering pressure. This result is consistent with the dirty superconductor theories.

DOI
10.7567/jjap.54.090303

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Abstract Amorphous NbTiN is widely applied in superconducting electronic devices owing to its merits of wide energy gap, high T c , and mitigated requirement in fabrication. Its highly disordered feature yields considerable kinetic inductance, making it an ideal material for parametric amplification. To build up the correspondence between the fabrication conditions and the desirable electrical performance, we designed, fabricated, and measured NbTiN microstrip resonators at microwave frequencies. The principal result is that the unloaded Q factor of the resonators does not depend on the film normal state resistivity, which largely depends on the sputtering pressure. This result is consistent with the dirty superconductor theories.
Abstract A wide-bandwidth and low-noise amplification chain in the microwave regime is crucial for the efficient read-out of quantum systems based on superconducting detectors, such as Microwave Kinetic Inductance Detectors (MKIDs), Transition Edge Sensors (TESs), Magnetic Microcalorimeters (MMCs), and RF cavities, as well as qubits. Kinetic Inductance Travelling Wave Parametric Amplifiers (KI-TWPAs) operated in a three-wave mixing fashion have demonstrated exceptional dynamic range and low-noise performance, approaching the quantum limit. These amplifiers can be fabricated using a single layer of a high kinetic inductance film as weakly dispersive artificial transmission lines, with the ability to control the phase-matched bandwidth through dispersion engineering. In this study, we present the optimisation of the rf sputter-deposition process of NbTiN films using a Nb80%Ti20% target, with the goal of achieving precise control over film characteristics, resulting in high kinetic inductance while maintaining a high transition temperature. The parameter landscape related to the different sputtering conditions, such as pressure, power, and nitrogen flow, has been explored and the film thickness has been used as a fine-tuning parameter to adjust the properties of the final NbTiN films used for the fabrication of KI-TWPAs. As a final result, we have obtained a NbTiN film with a kinetic inductance of 8.5 pH/sq which we have exploited to fabricate KI-TWPA prototype devices, showing promising amplification performance.
High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ~670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconducting computing technologies. In this paper, we report on the growth of high-quality face-centered cubic (FCC) NbTiN films through the use of an alternative materials deposition technology-reactive bias target ion beam deposition. This novel technique offers more degrees of freedom compared to conventional sputtering, notably through decoupling the plasma generation and target bias, through use of a hollow-cathode ion source and independently pulse biased sputtering targets. The reported films are of high quality, with increased transition temperatures, reduced normal state resistivity, and surface roughness, compared to our previously reported reactive magnetron sputtered NbTiN films. In particular, at UVA, we are investigating these NbTiN films for wide-bandgap all-NbTiN superconducting-insulating-superconducting mixers, kinetic inductance detectors, and traveling-wave kinetic inductance parametric amplifier devices.
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We present a lab-on-chip experiment to accurately measure losses of superconducting microstrip lines at microwave and sub-mm wavelengths. The microstrips are fabricated from NbTiN, which is deposited using reactive magnetron sputtering, and amorphous silicon which is deposited using plasma-enhanced chemical vapor deposition (PECVD). Sub-mm wave losses are measured using on-chip Fabry-P{é}rot resonators (FPR) operating around $350\ $GHz. Microwave losses are measured using shunted half-wave resonators with an identical geometry and fabricated on the same chip. We measure a loss tangent of the amorphous silicon at single-photon energies of tan⁡δ=3.7±0.5×10−5\tanδ=3.7\pm0.5\times10^{-5}tanδ=3.7±0.5×10−5 at $6\ $GHz and tan⁡δ=2.1±0.1×10−4\tanδ= 2.1\pm 0.1\times10^{-4}tanδ=2.1±0.1×10−4 at $350\ $GHz. These results represent very low losses for deposited dielectrics, but the sub-mm wave losses are significantly higher than the microwave losses, which cannot be understood using the standard two-level system loss model.
Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.