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arXiv:0908.2948 · Applied Physics Letters

Reducing quantum-regime dielectric loss of silicon nitride for superconducting quantum circuits

Hanhee Paik, Kevin D. Osborn

Published Feb 15, 2010• 106 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1063/1.3309703

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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.
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.
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.
Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (Tc) and residual resistivity (_0) of 20 nm, 50 nm and 100 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray di_raction. The high process temperature (250_ C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films.
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.
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 $400{\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The aaa-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 aaa-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.
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.