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AIP conference proceedings

Superconducting planar filter design

Martin Baránek, P. Neilinger, Daniel Manca, M. Grajcar

Published Jan 1, 2021• 0 citations
Publisher

Abstract

Superconducting planar resonators of thin molybdenum carbide (MoC) films were fabricated by optical lithography. The films were deposited via reactive magnetron sputtering and by changing the deposition parameters, the sheet resistance of the films was varied. The microwave properties of the resonators were studied by broadband coplanar waveguide spectroscopy in flip-chip configuration. The kinetic inductance of MoC films governs the high frequency response of the resonators and is determined by the complex conductivity of the superconducting film. Well below the critical temperature of transition, kinetic inductance is proportional to film sheet resistance in normal state. The temperature-dependent resonances in the broadband spectra below critical temperature are described by a lumped LC model, where the inductance consists of geometric and kinetic inductance of the resonator. The temperature and frequency dependent complex conductivity of the superconducting film is calculated by the Dynes model for dirty superconductors. Furthermore, a numerical model of the kinetic planar resonator in Sonnet software is presented, which can be utilized to design resonators with desired properties, based on highly disordered superconductors. An on-chip design of a kinetic planar resonator filter is presented. The high kinetic inductance of the superconducting films allows us to minimize microwave filter dimensions.

DOI
10.1063/5.0067634

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Superconducting planar resonators of thin molybdenum carbide (MoC) films were fabricated by optical lithography. The films were deposited via reactive magnetron sputtering and by changing the deposition parameters, the sheet resistance of the films was varied. The microwave properties of the resonators were studied by broadband coplanar waveguide spectroscopy in flip-chip configuration. The kinetic inductance of MoC films governs the high frequency response of the resonators and is determined by the complex conductivity of the superconducting film. Well below the critical temperature of transition, kinetic inductance is proportional to film sheet resistance in normal state. The temperature-dependent resonances in the broadband spectra below critical temperature are described by a lumped LC model, where the inductance consists of geometric and kinetic inductance of the resonator. The temperature and frequency dependent complex conductivity of the superconducting film is calculated by the Dynes model for dirty superconductors. Furthermore, a numerical model of the kinetic planar resonator in Sonnet software is presented, which can be utilized to design resonators with desired properties, based on highly disordered superconductors. An on-chip design of a kinetic planar resonator filter is presented. The high kinetic inductance of the superconducting films allows us to minimize microwave filter dimensions.
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.
Advancements in the fabrication of superconducting quantum devices have highlighted tantalum as a promising material, owing to its low surface oxidation microwave loss at low temperatures. However, tantalum films exhibit significantly larger kinetic inductances compared to materials such as aluminum or niobium. Given the inevitable variations in film thickness, this increased kinetic inductance leads to considerable, uncontrolled frequency variances and shifts in components like superconducting coplanar waveguide (SCPW) resonators. Achieving high precision in resonator frequencies is crucial, particularly when multiple resonators share a common Purcell filter with limited bandwidth in superconducting quantum information processors. Here, we tackle this challenge from both fabrication and design perspectives, achieving a reduction in resonator frequency fluctuation by a factor of more than 100. Concurrently, the internal quality factor of the SCPW resonator remains at high level. Our findings open up new avenues for the enhanced utilization of tantalum in large-scale superconducting chips.
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.
Microwave kinetic inductance detectors (MKID) are superconducting broadband direct detectors. If a superconducting thin film patterned into a resonator is illuminated with photons, Cooper pairs break up and the kinetic inductance of the film changes. This shifts the resonance frequency downwards and the dissipation of the resonator increases, which can be detected in the phase and amplitude of a transmitted signal. \n \nIn this thesis, a waveguide coupled MKID is designed and simulated. 350 GHz radiation is coupled into a copper waveguide via a horn antenna. A waveguide probe leads the signal onto a 9 µm silicon membrane where it is absorbed in a 40 nm superconducting aluminum film which is the inductive part of a superconducting resonator. The capacitor of the resonator is made of niobium. Two versions of the device are developed, a transmission line resonator and a lumped element resonator design. The new design will enable an accurate understanding of the optical sensitivity of MKIDs and allow comparison of different resonator materials. Since two different resonator types are used, the direct comparison of transmission line and lumped resonators will also be possible. \n \nA measurement setup is developed including an ADR cryostat to cool MKID devices down to below 40 mK. An analog readout system using either a vector network analyzer or an IQ mixer is designed and built. The necessary software to control the ADR, the temperature monitoring, the readout signal generation and detection and the signal acquisition of the setup is also developed. \n \nThe waveguide coupled MKID design is fabricated in our in-house microfabrication laboratory and measured in the new setup. In my measurements, the devices do not show a resonance feature. The VNA and DC measurements are analyzed to determine a plausible hypothesis of the failure. The devices are diagnosed for critical issues during fabrication which will be addressed in a future iteration. \n \nTo verify the measurement setup, a test device of the IRAM NIKA 2 MKID camera is measured and the complete analysis pipeline is exercised. The quasi-particle lifetime in the aluminum thin films is measured and a dark and optical noise equivalent power are determined. The results are within the uncertainty comparable to measurements of IRAM themselves. \n \nA test chip designed in my master thesis that contains eight NbTiN resonators is measured in the new setup and non-linear effects under strong readout power are studied. Parametric amplification and oscillation of a resonator are shown qualitatively.
Abstract We investigate thin films of conducting aluminium-oxide, also known as granular aluminium, as a material for superconducting high quality, high kinetic inductance circuits. The films are deposited by an optimised reactive DC magnetron sputter process and characterised using microwave measurement techniques at milli-Kelvin temperatures. We show that, by precise control of the reactive sputter conditions, a high room temperature sheet resistance and therefore high kinetic inductance at low temperatures can be obtained. For a coplanar waveguide resonator with 1.5 kΩ sheet resistance and a kinetic inductance fraction close to unity, we measure a quality factor in the order of 700 000 at 20 mK. Furthermore, we observe a sheet resistance reduction by gentle heat treatment in air. This behaviour is exploited to study the kinetic inductance change using the microwave response of a coplanar wave guide resonator. We find the correlation between the kinetic inductance and the sheet resistance to be in good agreement with theoretical expectations.
Superconducting planar circuits possess properties that can be very different from their normal-metal counterparts, which makes modeling efforts more complicated for superconducting components. In this paper we present closed-form expressions for the resistance and internal inductance per unit length of superconducting planar transmission lines in terms of the relevant material parameters. We also include a simple calculation of the nonlinear response of superconducting devices based on a current-dependent superconducting penetration depth. Taken together, these two descriptions can be used to predict the linear and nonlinear response of superconducting planar devices in a manner that is conducive to use in circuit design. The results of these calculations compare well with 76 K measurements of coplanar waveguide transmission lines and resonators fabricated from thin films of the high T/sub c/ superconductor YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//.