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∘C for use in temperature-constrained superconducting detector fabrication. We measure the dielectric loss tangent of these films at
∼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.
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-
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.
We present a lab-on-chip experiment to accurately measure losses of superconducting microstrip lines at microwave and sub-mm wavelengths. The microstrips are fabricated from NbTiN, which is deposited using reactive magnetron sputtering, and amorphous silicon which is deposited using plasma-enhanced chemical vapor deposition (PECVD). Sub-mm wave losses are measured using on-chip Fabry-P{é}rot resonators (FPR) operating around $350\ $GHz. Microwave losses are measured using shunted half-wave resonators with an identical geometry and fabricated on the same chip. We measure a loss tangent of the amorphous silicon at single-photon energies of
tanδ=3.7±0.5×10−5 at $6\ $GHz and
tanδ=2.1±0.1×10−4 at $350\ $GHz. These results represent very low losses for deposited dielectrics, but the sub-mm wave losses are significantly higher than the microwave losses, which cannot be understood using the standard two-level system loss model.
An understanding of the origins of power loss in superconducting interconnects is essential for the energy efficiency and scalability of superconducting digital logic. At microwave frequencies, power dissipates in both the superconducting wires and the dielectric and these losses can be of comparable magnitude. We describe an approach to accurately disentangle such losses by exploiting their frequency dependence in a multimode transmission-line resonator. This is supported by the concept of a resonator geometric factor extracted from the Ansys High Frequency Structure Simulator (hfss), a commercial three-dimensional finite-element method (FEM) that we adopt for solving a superconductor interior. Using the technique, we have optimized a planarized fabrication process of reciprocal quantum logic (RQL) for the minimum interconnect loss at 4.2 K and gigahertz frequencies. The microstrip interconnects are composed of niobium (
Nb) insulated by silicon dioxide (
SiO2) made from a tetraethoxysilane (TEOS) precursor. Two process generations use damascene fabrication and the third one uses Cloisonn\'e fabrication. For all three,
SiO2 exhibits a dielectric loss tangent $\mathrm{tan}\phantom{\rule{0.1em}{0ex}}\ensuremath{\delta}=0.0012\ifmmode\pm\else\textpm\fi{}0.0001$, independent of the
Nb wire width over 0.25--4 $\text{\ensuremath{\mu}}\mathrm{m}$. The intrinsic microwave resistance
Rs of
Nb varies with both the process and the wire width. For damascene fabrication, scanning transmission electron microscopy (STEM) and energy-dispersive x-ray spectroscopy (EDS) reveal that plasma oxidation and grain-growth orientation increase
Rs above the Bardeen-Cooper-Schrieffer (BCS) resistance ${R}_{\mathrm{BCS}}\ensuremath{\approx}17\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{\ensuremath{\Omega}}$ at 10 GHz. For Cloisonn\'e fabrication, we demonstrate ${R}_{s}=13\ifmmode\pm\else\textpm\fi{}1.4\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{\ensuremath{\Omega}}$ down to $0.25\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$ wire width, which is below
RBCS and arguably the lowest microwave resistance reported for
Nb at 4.2 K.
Silicon-on-insulator technology is widely used to fabricate silicon based devices, from advanced transistors to photonic circuits or nanomechanical systems. Integrating low loss superconducting quantum circuits with silicon-on-insulator substrates enables to couple the advantages offered by the mature silicon technology to the exquisite sensitivity of superconducting circuits. The natural approach, inherited from research in superconducting microwave devices, is to use a substrate made with highly resistive silicon, known for its low level of microwave losses. In this work, using superconducting microwave resonators, we show that counterintuitively, standard resistivity silicon-on-insulator substrates perform better than high resistivity silicon-on-insulator substrates at cryogenic temperatures. In the latter case, the presence of a parasitic sheet conduction at the interface between bulk silicon and silicon oxide acts as the dominant loss mechanism. This parasitic sheet can be suppressed using substrates with intentionally induced traps. In such substrates, losses are ultimately limited by the dielectric losses of the silicon oxide layer. These substrates offer interesting perspectives for the development of superconducting nanoelectromechanical systems. First, the release, i.e. the removal of the silicon oxide, could be limited to the moving parts, thereby maintaining the mechanical integrity of the rest of the device. Additionally, such structure would enhance heat evacuation into the bulk of the substrate which is an issue in current devices such as microwave-to-optics converters.
We report on the characterization of microwave loss of thin aluminum oxide films at low temperatures using superconducting lumped resonators. The oxide films are fabricated using plasma oxidation of aluminum and have a thickness of 5 nm. We measure the dielectric loss versus microwave power for resonators with frequencies in the GHz range at temperatures from 54 to 303 mK. The power and temperature dependence of the loss are consistent with the tunneling two-level system theory. These results are relevant to understanding decoherence in superconducting quantum devices. The obtained oxide films are thin and robust, making them suitable for capacitors in compact microwave resonators.