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arXiv:2610.10767

Microwave Loss in Low-Temperature ICPCVD Silicon Nitride: Dependence on Precursor Ratio and RF Substrate Bias

H. Athreya, J. Cornelison, M. Mikhailov, Roald J. H. van der Kolk, C. L. Chang, T. Cecil, M. N. Ali, J. A. Zebrowski

Published Oct 7, 2026• 0 citations
arXivHTMLPDF

Abstract

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 (SiN$_x$) films deposited by inductively coupled plasma chemical vapor deposition (ICPCVD) at $150\,^{\circ}\mathrm{C}$ for use in temperature-constrained superconducting detector fabrication. We measure the dielectric loss tangent of these films at $\sim 4-7$ 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 N$_2$/SiH$_4$ precursor-flow ratio between dielectric depositions and find a clear trend of decreasing loss with decreasing N$_2$/SiH$_4$. 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 SiN$_x$ for superconducting detector circuits.

DOI
10.48550/arxiv.2610.10767

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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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Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2 × 107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have deduced that one of the causes of TLS loss is the amorphous silicon layer at the film–substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS–TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.
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