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Superconductor Science and Technology

Ultra-high-Q resonators for low-noise, microwave signal generation using sapphire buffer layers and superconducting thin films

F Abbas, L.E. Davis, John C. Gallop

Published Jul 1, 1994• 6 citations
Publisher

Abstract

An analysis has been completed to provide guidelines for producing planar microwave resonators with Q-factors of the order of 10 6 -10 8 at a temperature of 4 K and a frequency of 1-20 GHz using high-T c superconducting thin films on sapphire shielded by suitable buffer layers. Buffer layers not only overcome the problems of film-substrate interactions and reduction of microcracks, but can also confine the field into the sapphire which enhances the Q r and Q c of the resonator.

DOI
10.1088/0953-2048/7/7/009

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Journal of Physics Conference Series•Feb 1, 2008•11 citations
An analysis has been completed to provide guidelines for producing planar microwave resonators with Q-factors of the order of 10 6 -10 8 at a temperature of 4 K and a frequency of 1-20 GHz using high-T c superconducting thin films on sapphire shielded by suitable buffer layers. Buffer layers not only overcome the problems of film-substrate interactions and reduction of microcracks, but can also confine the field into the sapphire which enhances the Q r and Q c of the resonator.
Abstract We demonstrate the operation of superconducting coplanar microwave resonators in a very large frequency range up to 50 GHz. The resonators are fabricated from niobium thin films on sapphire substrates and have fundamental frequencies of 5 GHz or 10 GHz. We study numerous harmonics of the resonators at temperatures between 1.5 K and 6 K, and we determine quality factors of up to 22 600 at 1.5 K. As an example for spectroscopy applications of such resonators we detect the superconducting transition of a bulk tin sample at multiple probing frequencies.
Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2 × 107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have deduced that one of the causes of TLS loss is the amorphous silicon layer at the film–substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS–TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.
Superconducting resonators are widely used in fields spanning from quantum computing to electron spin resonance (ESR) spectroscopy. With the goal of realizing superconducting resonators, a broad variety and combination of superconducting materials, substrates and fabrication processes have been used and thoroughly reported in the literature. High temperature superconductors such as YBCO and low temperature superconductors such as Nb, NbN, NbTiN and Al are the major actors in the domain. In this work, we investigate the possibility to extend the family of suitable low temperature superconductors for the realization of planar superconducting microwave resonators for future ESR applications. In particular, this study focuses on NbTi, a widely used material to realize superconducting cables but not investigated for planar resonating structures at GHz frequencies. A 150 nm thick film of NbTi is sputtered and patterned on top of an Al2O3 substrate. For devices resonating around 6.8 GHz quality factors greater than 10,000 are observed at 3 K and in magnetic fields up to 250 mT.
We present measurements and simulations of superconducting Nb co-planar waveguide resonators on sapphire substrate down to millikelvin temperature range with different readout powers. In the high temperature regime, we demonstrate that the Nb film residual surface resistance is comparable to that observed in the ultra-high quality, bulk Nb 3D superconducting radio frequency cavities while the resonator quality is dominated by the BCS thermally excited quasiparticles. At low temperature both the resonator quality factor and frequency can be well explained using the two-level system models. Through the energy participation ratio simulations, we find that the two-level system loss tangent is ∼10−2\sim 10^{-2}∼10−2, which agrees quite well with similar studies performed on the Nb 3D cavities.
Using molecular beam epitaxy, we have realized thin films of titanium nitride (TiN) on c-plane sapphire that exhibit the lowest observed full-width at half maximum X-ray rocking curve width of 18 arcsec. Though the (111) oriented TiN exhibits an abrupt and crystalline interface with sapphire, for the first time we observe sub-surface defects in the sapphire substrate, which nucleate structural defects in the epitaxial TiN layer. Using quarter-wavelength coplanar waveguide (CPW) resonators in a 3 \textmu m/6 \textmu m/3 \textmu m gap/strip/gap lines in a hanger geometry, we find the internal quality factor of the TiN resonators to be >106>10^{6}>106 in the single-photon ⟨n⟩∼1\langle n \rangle \sim 1⟨n⟩∼1 limit at 5.8 GHz and 10 mK, rising to >20×106>20 \times 10^{6}>20×106 at ⟨n⟩∼106\langle n \rangle \sim 10^{6}⟨n⟩∼106. The results are of high interest for applications of superconducting TiN in several areas, and provide a path towards epitaxial Josephson junctions with crystalline barriers in the future for high coherence qubits.
The sensitivity of a kinetic inductance detector is determined by the performance of the cavity circuit and therefore a high quality factor and low phase noise are required. We developed superconducting niobium coplanar waveguide resonators for kinetic inductance detectors at a resonance frequency of 6 GHz. The length of the resonators is adjusted to a quarter of a wavelength which corresponds to ≈ 5.3 mm in our geometry. Different layouts with coplanar waveguide transmission line topology for the resonator were designed and simulated. Some parameters of the device fabrication process were varied. The niobium thin films were deposited by magnetron sputtering on sapphire substrates kept at ambient temperature. The resonators were patterned by photolithography and reactive ion etching and measured at helium temperatures. Because of the quarter wave geometry with one shorted end the reflection parameter was measured. We have studied the influence of the coupling capacitances on quality factor. The optimization includes microwave simulations and implementation in different resonator structures. The measurements of optimized devices showed a very high quality factor above one million (10 6 ). The obtained results and perspective on further development of resonators will be discussed.