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arXiv:2311.12681 · Physical Review Applied

Geometry dependence of two-level-system noise and loss in a - Si C : H parallel-plate capacitors for superconducting microwave resonators

Kevin Kouwenhoven, Gwen P.J. van Doorn, Bruno T. Buijtendorp, Steven A. H. de Rooij, Dimitry Lamers, David Johannes Thoen, Vignesh Murugesan, J. J. A. Baselmans, Pieter de Visser

Published Apr 18, 2024• 6 citations
arXivHTMLPDF

Abstract

Parallel-plate capacitors (PPC) significantly reduce the size of superconducting microwave resonators, reducing the pixel pitch for arrays of single-photon energy-resolving kinetic inductance detectors (KIDs). The frequency noise of KIDs is typically limited by tunneling two-level systems (TLS), which originate from lattice defects in the dielectric materials required for PPCs. How the frequency noise level depends on the PPC's dimensions has not been experimentally addressed. We measure the frequency noise of 56 resonators with $a$-$\mathrm{Si}\mathrm{C}$:$\mathrm{H}$ PPCs, which cover a factor of 44 in PPC area and a factor of 4 in dielectric thickness. To support the noise analysis, we measure the resonators' TLS-induced power-dependent intrinsic loss and temperature-dependent resonance frequency shift. From the TLS models, we expect a geometry-independent microwave loss and resonance frequency shift, which is set by the TLS properties of the dielectric. However, we observe a thickness-dependent microwave loss and resonance frequency shift; this is explained by surface layers that limit the performance of PPC-based resonators. For a uniform dielectric, the frequency noise level should scale directly inversely with the PPC area and thickness. We observe that an increase in PPC size reduces the frequency noise, but the exact scaling is, in some cases, weaker than expected. Finally, we derive engineering guidelines for the design of KIDs based on PPC-based resonators.

DOI
10.1103/physrevapplied.21.044036

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Space-based astronomy is in need of a photon-noise limited detector with multiplexing opportunities. The kinetic inductance detector (KID) is capable of both properties. An incoming photon breaks Cooper pairs in the superconducting layer of the KID, which leads to a change in inductance, which can be detected with phase- and amplitude read-out. Photon noise, generation-recombination noise are inevitable noise sources, but two-level system noise can be minimized by adjusting the resonator and its geometry. To gain insight in the noise and loss sources this thesis investigates the difference between microstrip and co-planar waveguide resonators; and the difference between a silicon substrate and a silicon-on-insulator substrate. Numerical analysis shows that: noise in all types of resonators are dielectric bulk dominated; an Al superconducting layer shows has different losses in terms of Q-factor, compared to a NbTiN superconducting layer, but quantitatively the same internal Q-factor in the order of 105 is achieved; an Al superconducting layer in a microstrip has 5dBc/Hz and 2dBc/Hz higher noise in the metal-air and metal-edge interfaces, respectively, compared to a NbTiN superconducting layer; an Al on Si CPW and microstrip have the same Q-factor performance; an Al on Si CPW has a 2-8dBc/Hz more noise at all interfaces compared to an Al on Si microstrip; a buried oxide layer has no influence on the loss in terms of the Q-factor for 100nm Al on Si substrate CPWs; and that the noise of an Al on SOI CPW is bulk dominated, and the dielectric bulk contributes 10dBc/Hz more to the noise, compared to an Al on Si CPW. Experimental analysis shows that a gapwidth of wgap ? 10?m for Al on SOI CPWs shows no response, due to high two-level system noise. Low Q-factors in the order of 104 have been measured for Al on SOI CPWs, which is low compared to literature. A high set-up noise of -85dBc/Hz and divergent relationships with the internal power indicate that TLS-noise does not dominate the measurements, and set-up negatively influences the measurements. In conclusion, the resonator geometry is an important factor for the performance of a KID in terms of noise and Q-factors. A buried oxide layer has no consequence for the Q-factor, and negatively affects the noise of a CPW.

•Sep 24, 2015•0 citations

Noise and loss in superconducting resonators

Werner Karl-Gustav Daalman

Sub-mm astronomy in space calls for an array of photon noise limited detectors, both for imaging and broadband spectroscopy. Microwave Kinetic Inductance Detectors (MKIDs), superconducting resonance circuits, are a suitable candidate for this purpose due to its multiplexing potential, but in literature excess noise in phase readout is encountered and attributed to so-called two-level systems (TLSs). Reduction in TLS induced noise and loss will provide greater flexibility in design and a route towards background limited detector performance. In this thesis, TLSs from surface and bulk sources are modelled, so that their behaviour can be predicted through numerical computations of the field distributions inside the resonators. These calculations not only provide a guide for sensible chip designs, but allow for interpretation of experimental data and determination of dominant TLS sources. It is found that for Al CPW resonators on Si or SiN, the noise is surface dominated but with a non-negligible bulk contribution, while for microstrips on a SiN membrane, the noise is bulk dominated. As the loss in microstrips for narrow microstrips is dominated by the substrate-air interface, the dominant TLS loss and noise sources do not necessarily coincide and should be treated independently. This makes it impossible to determine the dominant CPW surface noise contribution. Additionally, microstrips and CPWs on the same dielectric perform similarly, while Si is better than SiN, both in terms of loss and noise, due to a combination of SiN interface and bulk effects. Finally, material dependent loss and noise parameters have been determined and the importance of thorough Si surface cleaning has been established, yielding the best Al CPW noise ever encountered. For sub-mm astronomy in space, the logical path to improvement would be the use of thorough cleaned Si as a dielectric, overetching and the use of LEKIDS and hybrid resonators, where microstrips are still viable for use. Importantly, having located the important TLS locations for various cases, tackling these problems areas further could provide the step towards background limited performance in space.

•Jan 1, 2014•1 citation
Parallel-plate capacitors (PPC) significantly reduce the size of superconducting microwave resonators, reducing the pixel pitch for arrays of single-photon energy-resolving kinetic inductance detectors (KIDs). The frequency noise of KIDs is typically limited by tunneling two-level systems (TLS), which originate from lattice defects in the dielectric materials required for PPCs. How the frequency noise level depends on the PPC's dimensions has not been experimentally addressed. We measure the frequency noise of 56 resonators with aaa-SiC\mathrm{Si}\mathrm{C}SiC:H\mathrm{H}H PPCs, which cover a factor of 44 in PPC area and a factor of 4 in dielectric thickness. To support the noise analysis, we measure the resonators' TLS-induced power-dependent intrinsic loss and temperature-dependent resonance frequency shift. From the TLS models, we expect a geometry-independent microwave loss and resonance frequency shift, which is set by the TLS properties of the dielectric. However, we observe a thickness-dependent microwave loss and resonance frequency shift; this is explained by surface layers that limit the performance of PPC-based resonators. For a uniform dielectric, the frequency noise level should scale directly inversely with the PPC area and thickness. We observe that an increase in PPC size reduces the frequency noise, but the exact scaling is, in some cases, weaker than expected. Finally, we derive engineering guidelines for the design of KIDs based on PPC-based resonators.
Over the past two decades, microwave kinetic inductance detectors (MKIDs) have gained exceptional importance in millimeter and sub-millimeter astronomy. MKIDs consist of thin strip resonators capable of detecting changes in the surface impedance of superconductor strips, which result from variations in resonance circuit properties. The principal noise in MKIDs comprises excess frequency noise and two-level system noise. In this paper, we propose a technique to mitigate the effect of two-level system (TLS) noise in MKIDs using a parallel plate capacitor with three layers of high ε dielectrics. To achieve this, we employ three layers of Al2O3, HfO2, and TiO2 with equal thickness between the capacitor plates. The experimental results demonstrate a nearly 30% reduction in TLS power spectral density.
At sub-Kelvin temperatures, two-level systems (TLS) present in amorphous dielectrics source a permittivity noise, degrading the performance of a wide range of devices using superconductive resonators such as qubits or kinetic inductance detectors. We report here on measurements of TLS noise in hydrogenated amorphous silicon (a-Si:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) in superconductive lumped-element resonators using parallel-plate capacitors (PPCs). The TLS noise results presented in this article for two recipes of a-Si:H improve on the best achieved in the literature by a factor >5 for a-Si:H and other amorphous dielectrics and are comparable to those observed for resonators deposited on crystalline dielectrics.
At sub-Kelvin temperatures, two-level systems (TLSs) present in amorphous dielectrics source a permittivity noise, degrading the performance of a wide range of devices using superconductive resonators such as qubits or kinetic inductance detectors. We report here on measurements of TLS noise in hydrogenated amorphous silicon (a-Si:H) films deposited by plasma-enhanced chemical vapor deposition in superconductive lumped element resonators using parallel-plate capacitors. The TLS noise results presented in this article for two recipes of a-Si:H improve on the best results achieved in the literature by a factor >5 for a-Si:H and other amorphous dielectrics and are comparable to those observed for resonators deposited on crystalline dielectrics.
Noise performance is one of the most crucial aspects of any detector. Superconducting Microwave Kinetic Inductance Detectors (MKIDs) have an “excess” frequency noise that shows up as a small time dependent jitter of the resonance frequency characterized by the frequency noise power spectrum measured in units of Hz2/Hz. Recent studies have shown that this noise almost certainly originates from a surface layer of two‐level system (TLS) defects on the metallization or substrate. Fluctuation of these TLSs introduces noise in the resonator due to coupling of the TLS electric dipole moments to the resonator’s electric field. Motivated by a semi‐empirical quantitative theory of this noise mechanism, we have designed and tested new resonator geometries in which the high‐field “capacitive” portion of the CPW resonator is replaced by an interdigitated capacitor (IDC) structure with 10–20 μm electrode spacing, as compared to the 2 μm spacing used for our more conventional CPW resonators. Measurements show that this new IDC design has dramatically lower TLS noise, currently by about a factor of ∼29 in terms of the frequency noise power spectrum, corresponding to an improvement of about a factor of 29 in NEP. These new devices are replacing the CPW resonators in our next design iteration in progress for MKIDCam. Opportunities and prospects for future reduction of the TLS noise will be discussed.
Space-based astronomy is in need of a photon-noise limited detector with multiplexing opportunities. The kinetic inductance detector (KID) is capable of both properties. An incoming photon breaks Cooper pairs in the superconducting layer of the KID, which leads to a change in inductance, which can be detected with phase- and amplitude read-out. Photon noise, generation-recombination noise are inevitable noise sources, but two-level system noise can be minimized by adjusting the resonator and its geometry. To gain insight in the noise and loss sources this thesis investigates the difference between microstrip and co-planar waveguide resonators; and the difference between a silicon substrate and a silicon-on-insulator substrate. Numerical analysis shows that: noise in all types of resonators are dielectric bulk dominated; an Al superconducting layer shows has different losses in terms of Q-factor, compared to a NbTiN superconducting layer, but quantitatively the same internal Q-factor in the order of 105 is achieved; an Al superconducting layer in a microstrip has 5dBc/Hz and 2dBc/Hz higher noise in the metal-air and metal-edge interfaces, respectively, compared to a NbTiN superconducting layer; an Al on Si CPW and microstrip have the same Q-factor performance; an Al on Si CPW has a 2-8dBc/Hz more noise at all interfaces compared to an Al on Si microstrip; a buried oxide layer has no influence on the loss in terms of the Q-factor for 100nm Al on Si substrate CPWs; and that the noise of an Al on SOI CPW is bulk dominated, and the dielectric bulk contributes 10dBc/Hz more to the noise, compared to an Al on Si CPW. Experimental analysis shows that a gapwidth of wgap ? 10?m for Al on SOI CPWs shows no response, due to high two-level system noise. Low Q-factors in the order of 104 have been measured for Al on SOI CPWs, which is low compared to literature. A high set-up noise of -85dBc/Hz and divergent relationships with the internal power indicate that TLS-noise does not dominate the measurements, and set-up negatively influences the measurements. In conclusion, the resonator geometry is an important factor for the performance of a KID in terms of noise and Q-factors. A buried oxide layer has no consequence for the Q-factor, and negatively affects the noise of a CPW.
Sub-mm astronomy in space calls for an array of photon noise limited detectors, both for imaging and broadband spectroscopy. Microwave Kinetic Inductance Detectors (MKIDs), superconducting resonance circuits, are a suitable candidate for this purpose due to its multiplexing potential, but in literature excess noise in phase readout is encountered and attributed to so-called two-level systems (TLSs). Reduction in TLS induced noise and loss will provide greater flexibility in design and a route towards background limited detector performance. In this thesis, TLSs from surface and bulk sources are modelled, so that their behaviour can be predicted through numerical computations of the field distributions inside the resonators. These calculations not only provide a guide for sensible chip designs, but allow for interpretation of experimental data and determination of dominant TLS sources. It is found that for Al CPW resonators on Si or SiN, the noise is surface dominated but with a non-negligible bulk contribution, while for microstrips on a SiN membrane, the noise is bulk dominated. As the loss in microstrips for narrow microstrips is dominated by the substrate-air interface, the dominant TLS loss and noise sources do not necessarily coincide and should be treated independently. This makes it impossible to determine the dominant CPW surface noise contribution. Additionally, microstrips and CPWs on the same dielectric perform similarly, while Si is better than SiN, both in terms of loss and noise, due to a combination of SiN interface and bulk effects. Finally, material dependent loss and noise parameters have been determined and the importance of thorough Si surface cleaning has been established, yielding the best Al CPW noise ever encountered. For sub-mm astronomy in space, the logical path to improvement would be the use of thorough cleaned Si as a dielectric, overetching and the use of LEKIDS and hybrid resonators, where microstrips are still viable for use. Importantly, having located the important TLS locations for various cases, tackling these problems areas further could provide the step towards background limited performance in space.