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Fabrication and characterization of low-loss Al/Si/Al parallel plate capacitors for superconducting quantum information applications (vol 11, 11, 2025)

Anthony P McFfadden (21429773), Aranya Goswami (9651754), Tongyu Zhao (11305905), Teun van Schijndel (21429776), Trevyn FQ Larson (21429779), Sudhir Sahu (21429782), Stephen Gill (13096200), Florent Lecocq, Raymond Simmonds (21429785), Chris Palmstrom (21429788)

Published May 30, 2025• 0 citations
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Abstract

Fabrication and characterization of low-loss Al/Si/Al parallel plate capacitors for superconducting quantum information applications (vol 11, 11, 2025)

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Fabrication and characterization of low-loss Al/Si/Al parallel plate capacitors for superconducting quantum information applications

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Abstract Increasing the density of superconducting circuits requires compact components, however, superconductor-based capacitors typically perform worse as dimensions are reduced due to loss at surfaces and interfaces. Here, parallel plate capacitors composed of aluminum-contacted, crystalline silicon fins are shown to be a promising technology for use in superconducting circuits by evaluating the performance of lumped element resonators and transmon qubits. High aspect ratio Si-fin capacitors having widths below 300 nm with an approximate total height of 3 μm are fabricated using anisotropic wet etching of Si(110) substrates followed by aluminum metallization. The single-crystal Si capacitors are incorporated in lumped element resonators and transmons by shunting them with lithographically patterned aluminum inductors and conventional A l/A l O x /A l Josephson junctions respectively. Microwave characterization of these devices suggests state-of-the-art performance for superconducting parallel plate capacitors with low power internal quality factor of lumped element resonators greater than 500 k and qubit T 1 times greater than 25 μs. These results suggest that Si-Fins are a promising technology for applications that require low-loss, compact, superconductor-based capacitors with minimal stray capacitance.

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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.

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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.

Applied Physics Reviews•Oct 15, 2026•0 citations
Abstract Increasing the density of superconducting circuits requires compact components, however, superconductor-based capacitors typically perform worse as dimensions are reduced due to loss at surfaces and interfaces. Here, parallel plate capacitors composed of aluminum-contacted, crystalline silicon fins are shown to be a promising technology for use in superconducting circuits by evaluating the performance of lumped element resonators and transmon qubits. High aspect ratio Si-fin capacitors having widths below 300 nm with an approximate total height of 3 μm are fabricated using anisotropic wet etching of Si(110) substrates followed by aluminum metallization. The single-crystal Si capacitors are incorporated in lumped element resonators and transmons by shunting them with lithographically patterned aluminum inductors and conventional A l/A l O x /A l Josephson junctions respectively. Microwave characterization of these devices suggests state-of-the-art performance for superconducting parallel plate capacitors with low power internal quality factor of lumped element resonators greater than 500 k and qubit T 1 times greater than 25 μs. These results suggest that Si-Fins are a promising technology for applications that require low-loss, compact, superconductor-based capacitors with minimal stray capacitance.
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.
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.