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arXiv:2110.03500 · Physical Review Applied

Hydrogenated Amorphous Silicon Carbide: A Low-Loss Deposited Dielectric for Microwave to Submillimeter-Wave Superconducting Circuits

Bruno T. Buijtendorp, Sten Vollebregt, K. Karatsu, David Johannes Thoen, Vignesh Murugesan, Kevin Kouwenhoven, Sebastian Hähnle, J. J. A. Baselmans, Akira Endo

Published Dec 1, 2022• 18 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevapplied.18.064003

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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 (aaa-SiC:H), using superconducting chips with Nb-Ti-N/a\mathrm{N}/aN/a-SiC:H/Nb-Ti-N microstrip resonators. We deposit the aaa-SiC:H by plasma-enhanced chemical vapor deposition at a substrate temperature of 400\ensuremath∘C400{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}400\ensuremath∘C. The aaa-SiC:H has a millimeter-submillimeter loss tangent ranging from 0.9\ifmmode×\else\texttimes\fi10\ensuremath−40.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}0.9\ifmmode×\else\texttimes\fi10\ensuremath−4 at 270 GHz to 1.5\ifmmode×\else\texttimes\fi10\ensuremath−41.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}1.5\ifmmode×\else\texttimes\fi10\ensuremath−4 at 385 GHz. The microwave loss tangent is 3.1\ifmmode×\else\texttimes\fi10\ensuremath−53.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}3.1\ifmmode×\else\texttimes\fi10\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 aaa-SiC:H films are free of blisters and have low stress: \ensuremath−20\ensuremath{-}20\ensuremath−20 MPa compressive at 200-nm thickness to 60 MPa tensile at 1000-nm thickness.
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Dielectric loss in low-temperature superconducting integrated circuits can cause lower overall efficiency, particularly in the 90 to 220 GHz regime. We present a method to tune the dielectric loss for silicon oxide deposited by plasma-enhanced chemical-vapor deposition at ambient temperatures. Deposition in an environment with a higher silane-to-oxygen ratio produces silicon oxide films with a lower loss-tangent and a slightly higher optical index of refraction, while contributing no appreciable change in film stress. We measured the dielectric loss by fabricating a series of Nb-SiOx-Nb microstrip resonators in the frequency range of 6 to 9 GHz and comparing their temperature dependence to a model of parasitic two-level-system fluctuators. The dielectric loss-tangent of silicon oxide was improved from 6 ×10-3for stoichiometric silicon dioxide to 2 ×10-3for a more silicon-rich silicon oxide. We present details of the fabrication process and measurements of microstrip resonators.
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Low-loss dielectric films are an important component of superconducting circuits with applications ranging from quantum information to superconducting detectors for astrophysics. In millimeter-wave detector circuits, dielectric loss can be the limiting factor on efficiency and resonator performance. Some advantageous detector fabrication architectures require dielectric deposition at temperatures substantially lower than those conventionally used to achieve low dielectric loss. In this work, we characterize silicon nitride (SiNx_xx​) films deposited by inductively coupled plasma chemical vapor deposition (ICPCVD) at 150 ∘C150\,^{\circ}\mathrm{C}150∘C for use in temperature-constrained superconducting detector fabrication. We measure the dielectric loss tangent of these films at ∼4−7\sim 4-7∼4−7 GHz using superconducting niobium microstrip resonators and extract the loss by fitting the temperature-dependent resonant frequency to a two-level-system model. We vary the N2_22​/SiH4_44​ precursor-flow ratio between dielectric depositions and find a clear trend of decreasing loss with decreasing N2_22​/SiH4_44​. We additionally find that applying an RF substrate bias during deposition reduces the loss compared to films deposited without bias. These results identify precursor ratio and substrate bias as practical process parameters for improving low-temperature SiNx_xx​ for superconducting detector circuits.
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