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arXiv:1201.3384 · Applied Physics Letters

Planar superconducting resonators with internal quality factors above one million

A. Megrant, C. Neill, R. Barends, B. Chiaro, Yu Chen, Ludwig Feigl, J. Kelly, Erik A. Lucero, M. Mariantoni, P. J. J. O’Malley, D. Sank, A. Vainsencher, J. Wenner, T. White, Yi Yin, Jie Zhao, C. J. Palmstrøm, John M. Martinis, Andrew N. Cleland

Published Mar 12, 2012• 479 citations
arXivHTMLPDF

Abstract

We describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 107 at high microwave powers and over 106 at low powers, with the best low power results approaching 2×106, corresponding to ∼1 photon in the resonator. These quality factors are achieved by controllably producing very smooth and clean interfaces between the resonators’ aluminum metallization and the underlying single crystal sapphire substrate. Additionally, we describe a method for analyzing the resonator microwave response, with which we can directly determine the internal quality factor and frequency of a resonator embedded in an imperfect measurement circuit.

DOI
10.1063/1.3693409

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We describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 107 at high microwave powers and over 106 at low powers, with the best low power results approaching 2×106, corresponding to ∼1 photon in the resonator. These quality factors are achieved by controllably producing very smooth and clean interfaces between the resonators’ aluminum metallization and the underlying single crystal sapphire substrate. Additionally, we describe a method for analyzing the resonator microwave response, with which we can directly determine the internal quality factor and frequency of a resonator embedded in an imperfect measurement circuit.
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Abstract Dielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5 × 106 at high powers and 2 × 106 (with the best value of 4.4 × 106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7–10.5 um center trace on high-resistivity silicon substrates commonly used in Josephson-junction based quantum circuit. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges’ positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.
In this master's thesis, I present the design, fabrication, and characterisation of superconducting microwave resonators. These resonators are quarter-wavelength coplanar waveguide resonators, operating at resonance frequencies between 4 GHz and 8 GHz. This kind of resonator is a key component in several applications, ranging from quantum computation devices to parametric amplifiers. Minimising the losses (achieving high internal quality factors) in these resonators is important to maximise performance of the applications they are used for. During the thesis work, we have studied several possible changes to improve the resonator quality.\n\nFabrication of the resonators was done in the Nanofabrication Laboratory at Chalmers University of Technology, and consist of the following steps: cleaning a sapphire or silicon wafer, depositing a layer of superconducting niobium, photolithography, and etching the waveguide. We then measured the resonators in a cryogen-free dilution refrigerator, at a temperature of 10 mK. The measurements consisted of a power sweep, in order to quantify the quality factor at high and low excitation powers. As an example, the first resonator we fabricated had a quality factor of 4.3 x 10^4 at high power (about 10^5 photons in the resonator), and quality factor of 3.7 x 10^4 at the single-photon level.\n\nWe then investigated different loss mechanisms that we expect to negatively affect the quality factor, and how to reduce this loss. Firstly, to reduce radiation losses and magnetic vortex losses, we reduced the lateral dimensions of the resonator. Secondly, as two-level system (TLS) losses are believed to mainly reside at the surfaces of the materials and interfaces between them, we etched into the substrate to selectively move a part of the TLS away from the resonator. We also studied the effect of the meandering of the resonator on the quality factor, as well as using two different etching gasses.\n\nBy gathering all this data, we were then able to compare the different combinations to see the effect of the changing parameters. In conclusion, the reduced lateral dimensions and the removal of part of the substrate indeed reduce the losses, as expected from the theory. We also found interesting behaviour with regards to the meandering, namely that having a meandered structure negatively affects the quality factor.\n\nCombining all results, we can give a guideline for the fabrication of resonators in the future, which gave us a quality factor up to 4.9 x 10^5 at high excitation power, and 3.0 x 10^5 at the single photon level.
The loss in superconducting microwave resonators at low-photon numbers and low temperatures is not well understood but has implications for achievable coherence times in superconducting qubits. We have fabricated single-layer resonators with a high quality factor by patterning a superconducting aluminum film on a sapphire substrate. Four resonator geometries were studied with resonant frequencies ranging from 5 to 7 GHz: a quasi-lumped element resonator, a coplanar strip waveguide resonator, and two hybrid designs that contain both a coplanar strip and a quasi-lumped element. Transmitted power measurements were taken at 30 mK as a function of frequency and probe power. We find that the resonator loss, expressed as the inverse of the internal quality factor, decreases slowly over four decades of photon number in a manner not merely explained by loss from a conventional uniform spatial distribution of two-level systems in an oxide layer on the superconducting surfaces of the resonator.
Superconducting coplanar waveguide (SCPW) resonators have a wide range of applications due to the combination of their planar geometry and high quality factors relative to normal metals. However, their performance is sensitive to both the details of their geometry and the materials and processes that are used in their fabrication. In this paper, we study the dependence of SCPW resonator performance on materials and geometry as a function of temperature and excitation power. We measure quality factors greater than 2 × 106 at high excitation power and 6 × 105 at a power comparable to that generated by a single microwave photon circulating in the resonator. We examine the limits to the high excitation power performance of the resonators and find it to be consistent with a model of radiation loss. We further observe that while in all cases the quality factors are degraded as the temperature and power are reduced due to dielectric loss, the size of this effect is dependent on resonator materials and geometry. Finally, we demonstrate that the dielectric loss can be controlled in principle using a separate excitation near the resonance frequencies of the resonator.
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.