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IEEE Transactions on Applied Superconductivity

Low Loss Superconducting Microstrip Development at Argonne National Lab

C. L. Chang, Peter A. R. Ade, Zeeshan Ahmed, S. W. Allen, Kam Arnold, Jason E. Austermann, A. N. Bender, Lindsey E. Bleem, Bradford A. Benson, J. E. Carlstrom, Hsiao-Mei Cho, Samuel Ciocys, Jean-François Cliche, T. M. Crawford, Ari Jozef Cukierman, Jie Ding, T. de Haan, M. Dobbs, D. Dutcher, Wendeline B. Everett, Adam Gilbert, N. W. Halverson, D. Hanson, N. L. Harrington, K. Hattori, J. W. Henning, Gene C Hilton, Gilbert P Holder, W. L. Holzapfel, Johannes Hubmayr, K. D. Irwin, Ryan Keisler, Lloyd Knox, D. Kubik, Chao-Lin L. Kuo, Adrian Tae-Jin Lee, Erik M. Leitch, D. Li, M. McDonald, S. S. Meyer, J. Montgomery, M. Myers, Tyler J. Natoli, H. Nguyen, V. Novosad, S. Padin, Zhaodi Pan, John E. Pearson, Chrystian M. Posada, Christian L. Reichardt, John E. Ruhl, Benjamin R. B. Saliwanchik, G. Simard, G. Smecher, James T. Sayre, Erik D. Shirokoff, Antony A. Stark, Kyle T. Story, Aritoki Suzuki, Keith Louis Thompson, Carole Tucker, K. Vanderlinde, Joaquin D. Vieira, Alexey A. Vikhlinin, Guangming Wang, Volodymyr G. Yefremenko, K. W. Yoon

Published Nov 20, 2014• 12 citations

Abstract

Low loss superconducting microstrip is an essential component in realizing 100 kilo-pixel multichroic cosmic microwave background detector arrays. We have been developing a low loss microstrip by understanding and controlling the loss mechanisms. We present the fabrication of the superconducting microstrip, the loss measurements at a few GHz frequencies using half-wavelength resonators, and the loss measurements at 220 GHz frequencies with the superconducting microstrip coupled to slot antennas at one end and to TES detectors at the other end. The measured loss tangent of the microstrip made of sputtered Nb and SiOx is 1-2e-3.

DOI: 10.1109/tasc.2014.2369231

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Low loss superconducting microstrip is an essential component in realizing 100 kilo-pixel multichroic cosmic microwave background detector arrays. We have been developing a low loss microstrip by understanding and controlling the loss mechanisms. We present the fabrication of the superconducting microstrip, the loss measurements at a few GHz frequencies using half-wavelength resonators, and the loss measurements at 220 GHz frequencies with the superconducting microstrip coupled to slot antennas at one end and to TES detectors at the other end. The measured loss tangent of the microstrip made of sputtered Nb and SiOx is 1-2e-3.

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\pm0.5\times10^{-5}

at $6\ $GHz and

\tanδ= 2.1\pm 0.1\times10^{-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.

We present our recent development of superconducting detectors for measurements of cosmic microwave background. We have fabricated antenna-coupled superconducting tunnel junctions (STJs). Two different types of STJs have been fabricated: the parallel-connected twin junction and the microstrip. Both types of STJs made of Nb and Al have successfully detected 80 GHz millimeter wave radiation with photon-assisted tunneling. We have also developed microwave kinetic inductance detectors (MKIDs). The MKIDs offer us high multiplexing factors with a single readout line using the frequency-domain readout. We have developed abosrption-type and transmission-type MKIDs whose resonators are formed with either coplanar waveguides (CPW) or microstrips. The quality factor of the CPW MKID made of Nb is measured to be about 105. The microstrip MKID is being developed for the multichroic measurements.

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.

We have developed a new technique for accurate measurement of the loss of superconducting microstrips at mm‐wave frequencies. In this technique, we optically couple power to slot antenna, which is connected to one port of a hybrid coupler. One of the output ports of the hybrid delivers power to a series of mm‐wave microstrip resonators which are capacitively coupled to a feedline followed by an MKID (microwave kinetic inductance detector) that measures the transmitted power. Two other MKIDs are connected to the remaining ports of the hybrid to measure the total incident optical power and the power reflected from the mm‐wave resonators, allowing |S21|2 and |S11|2 to be accurately determined and resonance frequency fr and quality factor Q to be retrieved. We have fabricated such a Nb/SiO2/Nb microstrip loss test device which contains several mm‐ wave resonators with fr∼100 GHz and measured it at 30 mK. All the resonators have shown internal quality factor Qi∼500–2000, suggesting a loss tangent of ∼5×10−4−2×10−3 for the SiO2 in use. For comparison, we have also fabricated a 5 GHz microstrip resonator on the same chip and measured it with a network analyzer. The loss tangent at 5 GHz derived from fitting the f0 and Q data to the two‐level system (TLS) model is 6×10−4, about the same as from the mm‐wave measurement. This suggests that the loss at both microwave and mm‐wave frequencies is probably dominated by the TLS in SiO2. Our results are of direct interest to mm/submm direct detection applications which use microstrip transmission lines (such as antenna‐coupled MKIDs and transition‐edge sensors), and other applications (such as on‐chip filters). Our measurement technique is applicable up to approximately 1 THz and can be used to investigate a range of dielectrics.

Abstract In this paper, we report on the design, fabrication, and characterization of a 220 GHz superconducting transition edge sensor (TES) array developed for ground-based cosmic microwave background (CMB) experiments. Unlike conventional TES arrays adopting thermometers made of bilayer superconducting films, the TES array has thermometers made of simpler single-layer titanium (Ti) film deposited on a suspended silicon nitride (SiN x ) membrane. The thermal weak link is realized by releasing the TES’s thermal island including the Ti thermometer by dry etching with xenon difluoride (XeF2), giving a typical thermal conductance of 25 pW K−1. Its thermal-conductance mechanism is further studied by fitting the measured TES’s complex impedance with a three-block thermal model. The dark and optical noise equivalent power (NEP) of a superconducting TES are both measured. Despite existing lens reflection and dielectric loss in the superconducting microstrip line, its typical optical NEP reaches 100 aW Hz−0.5, which meets the sensitivity requirement for ground-based CMB experiments.

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.