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Applied Physics Reviews

Minimizing microwave loss of amorphous hydrogenated silicon parallel-plate capacitors for superconducting integrated devices

Daria A. Moskaleva, Dmitry A. Baklykov, Dmitry O. Moskalev, Аnton I. Ivanov, Aleksei R. Matanin, Nikita S. Smirnov, Nikita D. Korshakov, E. V. Zikiy, J. Agafonova, Victor I. Polozov, Margarita V Androschuk, Andrey S. Chernyshov, Julia A. Kurochkina, Igor S. Pilnik, Ivan I. Kargapoltsev, Ilya A. Ryzhikov, Alexander V. Andriyash, Ilya A. Rodionov

Published Oct 15, 2026• 0 citations

Abstract

Amorphous hydrogenated silicon (a-Si:H) based lumped elements are one of the promising ways for large-scale integration of superconducting quantum devices. We present a detailed study devoted to the plasma-enhanced chemical vapor deposition process parameters' influence on the structure and morphology of a-Si:H dielectric thin films. We experimentally demonstrate a microcrystalline-to-amorphous growth transition by changing the H2/SiH4 ratio and establish a relationship between the film's structure, thickness, and dielectric loss tangent (tan δ). It is shown that the best dielectric loss tangents of tan δLP = 4.5 × 10−5 (average 9.2 × 10−5) and tan δHP = 1.0 × 10−6 (average 1.2 × 10−6) for superconducting lumped elements with 350 nm-thick amorphous films can be achieved in single- and multi-photon regimes at sub-20 mK, correspondingly. These values are extracted by characterizing superconducting resonators with integrated parallel-plate capacitors (C = 0.2…6.0 pF) for a frequency range from 4.0 to 7.0 GHz. The presented low-loss a-Si:H films can be useful for various superconducting quantum applications, which is illustrated by the fabrication of lumped-element bandpass filters with a well-predicted amplitude–frequency response characteristic.

DOI: 10.1063/5.0322287

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Amorphous hydrogenated silicon (a-Si:H) based lumped elements are one of the promising ways for large-scale integration of superconducting quantum devices. We present a detailed study devoted to the plasma-enhanced chemical vapor deposition process parameters' influence on the structure and morphology of a-Si:H dielectric thin films. We experimentally demonstrate a microcrystalline-to-amorphous growth transition by changing the H2/SiH4 ratio and establish a relationship between the film's structure, thickness, and dielectric loss tangent (tan δ). It is shown that the best dielectric loss tangents of tan δLP = 4.5 × 10−5 (average 9.2 × 10−5) and tan δHP = 1.0 × 10−6 (average 1.2 × 10−6) for superconducting lumped elements with 350 nm-thick amorphous films can be achieved in single- and multi-photon regimes at sub-20 mK, correspondingly. These values are extracted by characterizing superconducting resonators with integrated parallel-plate capacitors (C = 0.2…6.0 pF) for a frequency range from 4.0 to 7.0 GHz. The presented low-loss a-Si:H films can be useful for various superconducting quantum applications, which is illustrated by the fabrication of lumped-element bandpass filters with a well-predicted amplitude–frequency response characteristic.

The improvement of the coherence times of superconducting qubits depends on the reduction of the dielectric losses in the insulating materials implemented for the device fabrication. These losses depend on the density of spurious dipoles of different nature (two-level systems, TLSs) which couple to phase qubits and, hence, limit their coherence times. Hydrogenated amorphous silicon (a-Si:H), because of its lower loss tangent (tan δ) among conventional dielectrics (a-SiO, a-SiO2, a-SiNx:H), is considered as the best amorphous dielectric for superconducting qubit application. We have developed a reliable method for the direct measurement of dielectric losses in amorphous dielectric thin ﬁlms using a novel design based on four lumped superconducting LC resonators connected in series without coupling capacitors. The losses are obtained as tan δ = 1/Q0, where Q0 is the intrinsic quality factor of the resonator, measured at 3 dB above the resonance frequency without any ﬁtting procedure. The series type LC resonators with a-Si:H as dielectric were fabricated by the Nb technology. The measurements were done at the conditions of a qubit application (0.5 -10 GHz frequency range and low temperatures). The low values of the loss tangent of a-Si:H (up to 2.5 x 10−5 at 4.2 K) have required the development of superconducting housing for the resonators in order to eliminate a spurious dependence of tan δ on the microwave power, by reducing losses which were not originated in the dielectric itself. The results of the simulations agree well with the experiments.

Two-level systems (TLS) are an important, if not dominant, source of loss and noise for superconducting resonators such as those used in kinetic inductance detectors and some quantum information science platforms. They are similarly important for loss in photolithographically fabricated superconducting mm-wave/THz transmission lines. For both lumped-element and transmission-line structures, native amorphous surface oxide films are typically the sites of such TLS in nonmicrostripline geometries, while loss in the (usually amorphous) dielectric film itself usually dominates in microstriplines. We report here on the demonstration of low TLS loss at GHz frequencies in hydrogenated amorphous silicon (a-Si:H) films deposited by plasma-enhanced chemical vapor deposition in superconducting lumped-element resonators using parallel-plate capacitors (PPCs). The values we obtain from two recipes in different deposition machines, $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and $12\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, improve on the best achieved in the literature by a factor of 2--4 for a-Si:H and are comparable to recent measurements of amorphous germanium. Moreover, we have taken care to extract the true zero-temperature, low-field loss tangent of these films, accounting for temperature and field saturation effects that can yield misleading results. Such robustly fabricated and characterized films render the use of PPCs with deposited amorphous films a viable architecture for superconducting resonators and they also promise extremely low loss and high quality factor for photolithographically fabricated superconducting mm-wave/THz transmission lines used in planar antennas and resonant filters.

Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits, and kinetic inductance parametric amplifiers. Compared with planar structures, multilayer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multilayer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measure the subkelvin and low-power microwave and millimeter-submillimeter-wave dielectric loss of hydrogenated amorphous silicon carbide (

a

-SiC:H), using superconducting chips with Nb-Ti-

\mathrm{N}/a

-SiC:H/Nb-Ti-N microstrip resonators. We deposit the

a

-SiC:H by plasma-enhanced chemical vapor deposition at a substrate temperature of $400{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The

a

-SiC:H has a millimeter-submillimeter loss tangent ranging from $0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 270 GHz to $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 385 GHz. The microwave loss tangent is $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. These are the lowest low-power subkelvin loss tangents that have been reported for microstrip resonators at millimeter-submillimeter and microwave frequencies. The

a

-SiC:H films are free of blisters and have low stress: $\ensuremath{-}20$ MPa compressive at 200-nm thickness to 60 MPa tensile at 1000-nm thickness.

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.

We have fabricated superconducting microwave resonators in a lumped element geometry using single crystal silicon dielectric parallel plate capacitors with C>2 pF. Aluminum devices with resonant frequencies between 4.0 and 6.5 GHz exhibited an average internal quality factor Qi of 2×105 in the single photon excitation regime at T=20 mK. Attributing the observed loss solely to the capacitive element, our measurements place an upper bound on the loss tangent of the silicon dielectric layer of tan δi=5×10−6. This level of loss is an order of magnitude lower than is currently observed in structures incorporating amorphous dielectric materials, thus making single crystal silicon capacitors an attractive, robust route for realizing long-lived quantum circuits.

The loss of amorphous hydrogenated silicon nitride (a-SiNx:H) is measured at 30 mK and 5 GHz using a superconducting LC resonator down to energies where a single-photon is stored, and analyzed with an independent two-level system defect model. Each a-SiNx:H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent tan δ0 in a-SiNx:H is strongly correlated with N–H impurities, including NH2. By slightly reducing x we are able to reduce tan δ0 by approximately a factor of 50, where the best films show tan δ0≃3×10−5.