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arXiv:1209.4853 · Applied Physics Letters

Proximity-coupled Ti/TiN multilayers for use in kinetic inductance detectors

Michael R. Vissers, Jiansong Gao, Martin Sandberg, Shannon M. Duff, David S. Wisbey, K. D. Irwin, David P. Pappas

Published Jun 10, 2013• 69 citations
arXivHTMLPDF

Abstract

We apply the superconducting proximity effect in TiN/Ti multi-layer films to tune the critical temperature, TC, to within 10 mK with high uniformity (less than 15 mK spread) across a 75 mm wafer. Reproducible TC’s are obtained from 0.8 to 2.5 K. These films had high resistivities, >100μΩ cm, and internal quality factors for resonators in the GHz range, on the order of 100 k and higher. Trilayers of both TiN/Ti/TiN and thicker superlattice films were prepared, demonstrating a well controlled process for films over a wide thickness range. Detectors were fabricated and shown to have single photon resolution at 1550 nm. The high uniformity and controllability coupled with the high quality factor, kinetic inductance, and inertness of TiN make these films ideal for use in frequency multiplexed kinetic inductance detectors and potentially other applications such as nanowire detectors, transition edge sensors, and associated quantum information applications.

DOI
10.1063/1.4804286

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We apply the superconducting proximity effect in TiN/Ti multi-layer films to tune the critical temperature, TC, to within 10 mK with high uniformity (less than 15 mK spread) across a 75 mm wafer. Reproducible TC’s are obtained from 0.8 to 2.5 K. These films had high resistivities, >100μΩ cm, and internal quality factors for resonators in the GHz range, on the order of 100 k and higher. Trilayers of both TiN/Ti/TiN and thicker superlattice films were prepared, demonstrating a well controlled process for films over a wide thickness range. Detectors were fabricated and shown to have single photon resolution at 1550 nm. The high uniformity and controllability coupled with the high quality factor, kinetic inductance, and inertness of TiN make these films ideal for use in frequency multiplexed kinetic inductance detectors and potentially other applications such as nanowire detectors, transition edge sensors, and associated quantum information applications.
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We have grown superconducting TiN films by atomic layer deposition with thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune the resistivity and critical temperature by controlling the film thickness. The microwave properties are measured, using a coplanar-waveguide resonator, and we find internal quality factors above a million, high sheet inductances (5.2-620 pH), and pulse response times up to 100 μs. The high normal state resistivity of the films (>; 100 μΩ · cm) affects the superconducting state and thereby the electrodynamic response. The microwave response is modeled using a quasiparticle density of states modified with an effective pair-breaker, consistently describing the measured temperature dependence of the quality factor and the resonant frequency.
The non-homogeneity in the critical temperature TcT_{c}Tc​ of an Microwave Kinetic Inductance Detector (MKID) could be caused by non-uniformity in the deposition process of the thin superconducting film. This produces low percent yield and frequency collision in the readout of the MKIDs. Here, we show the homogeneity that offers Atomic Layer Deposition (ALD). We report an improvement of up to a factor of 50 in the fractional variation of the TcT_{c}Tc​ for TiN MKIDs fabricated with Atomic Layer Deposition in comparison with MKIDs fabricated with sputtering. We measured the critical temperature of 48 resonators. We extracted the TcT_{c}Tc​ of the MKIDs by fitting the fractional resonance frequency to the complex conductivity of their resonators. We observed uniformity on the critical temperature for MKIDs belonging to the same fabrication process, with a maximum change in the TcT_{c}Tc​ of 60 mK for MKIDs fabricated on different wafers.
We demonstrate photon counting at 1550 nm wavelength using microwave kinetic inductance detectors (MKIDs) made from TiN/Ti/TiN trilayer films with superconducting transition temperature Tc≈ 1.4 K. The detectors have a lumped-element design with a large interdigitated capacitor covered by aluminum and inductive photon absorbers whose volume ranges from 0.4 μm3 to 20 μm3. The energy resolution improves as the absorber volume is reduced. We achieved an energy resolution of 0.22 eV and resolved up to 7 photons per optical pulse, both greatly improved from previously reported results at 1550 nm wavelength using MKIDs. Further improvements are possible by optimizing the optical coupling to maximize photon absorption into the inductive absorber.
We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled, microwave kinetic inductance detectors made of a TiN/Ti/TiN trilayer superconducting film, tuned to have a transition temperature of 1.4 K. Micro-machining of the silicon-on-insulator wafer backside creates a quarter-wavelength backshort optimized for efficient coupling at 250 μm. Using frequency read out and when viewing a variable temperature blackbody source, we measure device noise consistent with photon noise when the incident optical power is >0.5 pW, corresponding to noise equivalent powers >3×10−17 W/Hz. This sensitivity makes these devices suitable for broadband photometric applications at these wavelengths.
Abstract Low microwave loss and regulatable kinetic inductance are two key characteristics of superconducting films to manufacture high-performance quantum devices. Epitaxial titanium nitride (TiN) thin films have been found to exhibit significant advantages in these two aspects. In this paper, we investigated into the dielectric loss and kinetic inductance of epitaxial TiN films, which were prepared on both silicon and sapphire substrates with different temperatures. Crystal structure, surface morphology, and superconducting properties were characterized systematically. The radio-frequency response of microwave resonators made of TiN films was studied at low temperatures to extract the information of kinetic inductance and quality factor. We obtained the highest internal quality factor up to 1.6 × 10 6 at single photon regime and achieved a regulation of kinetic inductance exceeding 16 times by altering the substrate and the sputtering temperature of the TiN films. This work not only provides an essential foundation for the precise design and preparation of low-loss TiN resonators for superconducting quantum bits, but also demonstrates TiN film as an excellent material platform with both low microwave loss and high kinetic inductance that can be applied in other fields such as microwave kinetic inductance detectors.