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arXiv:2606.21348

Enhanced electron-beam lithography to reduce the frequency scatter of 200-400 GHz superconducting microstrip resonators for on-chip filterbank spectrometers

Leon G. G. Olde Scholtenhuis, David Johannes Thoen, J. J. A. Baselmans, K. Karatsu, L. H. Martin, S. Vollebregt, Akira Endo

Published Jun 19, 2026• 0 citations
arXivHTMLPDF

Abstract

Integrated superconducting spectrometers (ISSs) provide the instantaneous bandwidth, sensitivity, and scalable architecture for large-scale spectroscopic surveys in submillimeter-wave astronomy and cosmology. However, the accuracy with which the resonant frequencies of superconducting microstrip band pass filters can be spaced, has limited the spectral resolution for these spectrometers with continuous spectral coverage to $\frac{F}{ΔF} < 500$. The origin of this frequency scatter has been largely unknown. In this work, we demonstrate a four-fold improvement in the frequency spacing of superconducting microstrip resonators by optimizing electron-beam lithography. We find that reducing the beam step size (BSS) on the nanometer scale reduces the random frequency scatter, and that avoiding main-field stitching across filter patterns can eliminate a systematic frequency shift between groups of resonators, indicating the different origins of these two modes of frequency deviation. These findings demonstrate that nanometer-scale control of lithographic processes is imperative for the realization of the next-generation integrated superconducting spectrometers with higher spectral resolution.

DOI
10.48550/arxiv.2606.21348

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Integrated superconducting spectrometers (ISSs) provide the instantaneous bandwidth, sensitivity, and scalable architecture for large-scale spectroscopic surveys in submillimeter-wave astronomy and cosmology. However, the accuracy with which the resonant frequencies of superconducting microstrip band pass filters can be spaced, has limited the spectral resolution for these spectrometers with continuous spectral coverage to FΔF<500\frac{F}{ΔF} < 500ΔFF​<500. The origin of this frequency scatter has been largely unknown. In this work, we demonstrate a four-fold improvement in the frequency spacing of superconducting microstrip resonators by optimizing electron-beam lithography. We find that reducing the beam step size (BSS) on the nanometer scale reduces the random frequency scatter, and that avoiding main-field stitching across filter patterns can eliminate a systematic frequency shift between groups of resonators, indicating the different origins of these two modes of frequency deviation. These findings demonstrate that nanometer-scale control of lithographic processes is imperative for the realization of the next-generation integrated superconducting spectrometers with higher spectral resolution.
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.
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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.
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.