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arXiv:2404.11417 · Journal of Low Temperature Physics

Directional Filter Design and Simulation for Superconducting On-Chip Filter-Banks

Louis H. Marting, K. Karatsu, Akira Endo, J. J. A. Baselmans, Alejandro Pascual Laguna

Published May 4, 2024• 3 citations
arXivHTMLPDF

Abstract

Many superconducting on-chip filter-banks suffer from poor coupling to the detectors behind each filter. This is a problem intrinsic to the commonly used half-wavelength filter, which has a maximum theoretical coupling of 50 %. In this paper, we introduce a phase-coherent filter, called a directional filter, which has a theoretical coupling of 100 %. In order to study and compare different types of filter-banks, we first analyze the measured filter frequency scatter, losses, and spectral resolution of a DESHIMA 2.0 filter-bank chip. Based on measured fabrication tolerances and losses, we adapt the input parameters for our circuit simulations, quantitatively reproducing the measurements. We find that the frequency scatter is caused by nanometer-scale line width variations and that variances in the spectral resolution is caused by losses in the dielectric only. Finally, we include these realistic parameters in a full filter-bank model and simulate a wide range of spectral resolutions and oversampling values. For all cases, the directional filter-bank has significantly higher coupling to the detectors than the half-wave resonator filter-bank. The directional filter eliminates the need to use oversampling as a method to improve the total efficiency, instead capturing nearly all the power remaining after dielectric losses.

DOI
10.1007/s10909-024-03118-w

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Many superconducting on-chip filter-banks suffer from poor coupling to the detectors behind each filter. This is a problem intrinsic to the commonly used half-wavelength filter, which has a maximum theoretical coupling of 50 %. In this paper, we introduce a phase-coherent filter, called a directional filter, which has a theoretical coupling of 100 %. In order to study and compare different types of filter-banks, we first analyze the measured filter frequency scatter, losses, and spectral resolution of a DESHIMA 2.0 filter-bank chip. Based on measured fabrication tolerances and losses, we adapt the input parameters for our circuit simulations, quantitatively reproducing the measurements. We find that the frequency scatter is caused by nanometer-scale line width variations and that variances in the spectral resolution is caused by losses in the dielectric only. Finally, we include these realistic parameters in a full filter-bank model and simulate a wide range of spectral resolutions and oversampling values. For all cases, the directional filter-bank has significantly higher coupling to the detectors than the half-wave resonator filter-bank. The directional filter eliminates the need to use oversampling as a method to improve the total efficiency, instead capturing nearly all the power remaining after dielectric losses.
Abstract Superconducting on-chip filter banks provide a scalable, space saving solution to create imaging spectrometers at millimetre and submillimetre wavelengths. We present an easy to realise, lithographed superconducting filter design with a high tolerance to fabrication error. Using a capacitively coupled λ/2\lambda /2λ/2 λ / 2 microstrip resonator to define a narrow ( λ/Δλ=300\lambda /\Delta \lambda = 300λ/Δλ=300 λ / Δ λ = 300 ) spectral pass band, the filtered output of a given spectrometer channel directly connects to a lumped-element kinetic inductance detector. We show the tolerance analysis of our design, demonstrating <11%<11\%<11% < 11 % change in filter quality factor to any one realistic fabrication error and a full filter-bank efficiency forecast to be 50% after accounting for fabrication errors and dielectric loss tangent.
We present a high efficiency, moderate resolution on-chip superconducting filterbank spectrometer designed for line intensity mapping and broadband wave-like dark matter searches. Existing implementations used by the millimeter-wave community rely on resistive feedline termination for standing wave mitigation which caps the per-channel efficiency ηchη_\mathrm{ch}ηch​ at 50\% and utilize microstrip resonators which highly couple the resolving power R=ν/δν\mathcal{R}=ν/δνR=ν/δν and ηchη_\mathrm{ch}ηch​ to thin-film dielectric loss. The architecture presented in this paper addresses these limitations by eliminating the termination resistor and employing niobium coplanar waveguide resonators patterned directly on single-crystal silicon, which fixes the resonator's internal quality factor by the silicon substrate property rather than by the loss tangent of a deposited film. The resolving power is set by the impedance ratio between the resonator and feedline. Electromagnetic and circuit simulations of a ten-channel filterbank near 90~GHz at unity oversampling show R=1170\mathcal{R}=1170R=1170 and ηch>78%η_\mathrm{ch}> 78\%ηch​>78%, with no significant trend in efficiency across channels. A ten-channel prototype, designed for R=92±5\mathcal{R}=92\pm5R=92±5 and ηch=(81±3)%η_\mathrm{ch}=(81\pm3)\%ηch​=(81±3)% over 90--106~GHz with a 560~nm interlayer based on simulations, was fabricated and optically characterized, yielding R=208±74\mathcal{R}=208\pm74R=208±74 and ηch=(77±6)%η_\mathrm{ch}=(77\pm6)\%ηch​=(77±6)%. Measured ηchη_\mathrm{ch}ηch​ well above the 50\% termination limit, together with demonstrated impedance tuning of R\mathcal{R}R, validate the design principles of this filterbank spectrometer and establishes a route to high-efficiency, high-resolution on-chip spectrometer arrays.
We present an on-chip superconducting filter bank spectrometer based on transition-edge sensors (TES) as a technology for realising a microwave atmospheric sounding instrument with several hundred channels and sky- noise limited performance. Each device consists of a wideband feed coupled to a transmission line filter bank, with a TES behind each filter. In the first instance we have targeted atmospheric temperature sounding using the oxygen (O2) absorption line at 60 GHz, however the device is being scaled to 180 GHz for humidity sounding. The technology developed is also generally applicable to astronomical instrumentation. We have fabricated a set of test devices to demonstrate key device technologies, such as channel placement, spectral resolution and sensitivity. We will describe device design, test configuration and results.
We experimentally demonstrate the principle of an on-chip submillimeter wave filter bank spectrometer, using superconducting microresonators as narrow band-separation filters. The filters are made of NbTiN/SiNx/NbTiN microstrip line resonators, which have a resonance frequency in the range of 614-685 GHz, two orders of magnitude higher in frequency than what is currently studied for use in circuit quantum electrodynamics and photodetectors. The frequency resolution of the filters decreases from 350 to 140 with increasing frequency, most likely limited by dissipation of the resonators.
Superconducting microelectronics (SME) technology has the potential of realizing very high speed digital receivers capable of performing direct digitization of radio frequency signals with very low power consumption. The SME receiver is implemented on a single chip using Niobium based low temperature superconductive (LTS) Josephson Junction (JJ) technology by HYPRES. Analogue RF filters are still required at the receiver front end and are key components of the overall superconductor digital receiver. SME receivers usually require two types of RF filters; a wideband bandpass filter and a bandstop filter (a notch filter). The notch filter is required to eliminate interference and unwanted signals in the passband. In this thesis, design of highly miniaturized lumped element wideband and bandstop filters is investigated and some challenges are addressed. The filters are fabricated by the HYPRES process and therefore can be integrated with the SME receiver on the same chip. \nIn a wideband filter, the coupling between the adjacent resonators is high. Achieving such a strong coupling is one of the challenges of designing wideband filters. The wideband filters realized with distributed elements usually suffer from very low spurious frequency. As the bandwidth of the filter becomes wider, the spurious peak of the second harmonic gets closer to the passband of the filter. In the first part of this work, the possibility of realizing lumped element superconducting bandpass filters (BPF) with a relative bandwidth of 80% is investigated. \nIn the second part of the thesis, design and realization of lumped element superconducting bandstop filters (BSF) is discussed. The challenge for designing a bandstop filter is providing a good match over a wide frequency range. So narrowband inverters cannot be used. Instead, usually λ/4 matched transmission lines provide 90° phase shift between the resonators of a notch filter. The possibility of replacing the long transmission line with other means or eliminating the inverters and using both shunt and series resonators are investigated. Having both series and shunt resonators introduces some new challenges that are addressed in the thesis and discussed thoroughly. \nA tunable notch resonator is presented. The tunability is provided by a superconducting MEMS varactor that is realized in our group by doing some post processing on the device fabricated by HYPRES. The tunability range of the device at cryogenic temperatures is investigated. A 3-pole tunable BSF is also designed that uses the same tunable resonators. The tunability of the filter is investigated through simulation.
With improved understanding of loss mechanisms in superconducting resonant circuits (SRC), it is possible to design a variety of devices which take advantage of the high Q. Superconducting surface resistance, trapped-flux loss, and residual losses which are annealable can all be related to the superconductor itself. Dielectric dissipation is well enough understood to permit this source of loss to be minimized. Coupling and radiation losses can be made negligible. SRC's are described with a variety of geometries to fill the range from 10 MHz to 10 GHz. Type-I superconductors provide the highest Q's and are readily fabricated by several techniques. The superconducting resonator can be tuned accurately in frequency by temperature control of the surface reactance as well as the dielectric constant of liquid helium. An external magnetic field can be used to tune the bandwidth as well as the resonant frequency. A quarter-wave reentrant cavity is ideally suited for use as an optically tuned frequency-control element. This is accomplished using the photodielectric effect in high-resistivity semiconductor wafers which terminate the stub. Wide-bandwidth optical communication detectors are described using semiconductors with short recombination lifetime. Oscillators have been built which are optically tuned, and others which have frequency stabilities as good as commercial frequency counters. Use of SRC's to study material properties at low temperature is emphasized.