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IEEE Transactions on Applied Superconductivity

Optimized Microwave LEKID Arrays for High-Resolution Applications

S. Wünsch, R. Prinz, Christopher Groetsch, M. Siegel

Published Apr 3, 2013• 8 citations
Publisher

Abstract

High-resolution applications demand a strong increase in number of pixels of detector arrays. In terms of the lumped-element kinetic inductance detector, this requires a detailed knowledge of the microwave properties of each single pixel; moreover, the unwanted interaction (cross-talk) between them has to be examined and eliminated. In this paper, the basic microwave properties of lumped-element kinetic inductance detector structures were investigated. Simulations demonstrate the influence of the embedding design parameters, e.g., orientation and coupling distances, in respect of the microwave behavior. In addition, different array arrangements to increase the number of pixels and to identify possible reasons causing unwanted cross-talk were examined. For this purposes several samples in niobium thin film technology on silicon substrates were designed, fabricated and characterized. Loaded quality factorsQLup to 3 000 at 4.2 K were achieved. The results of the simulations and measurements of small and large arrays will be presented and discussed.

DOI
10.1109/tasc.2013.2251056

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High-resolution applications demand a strong increase in number of pixels of detector arrays. In terms of the lumped-element kinetic inductance detector, this requires a detailed knowledge of the microwave properties of each single pixel; moreover, the unwanted interaction (cross-talk) between them has to be examined and eliminated. In this paper, the basic microwave properties of lumped-element kinetic inductance detector structures were investigated. Simulations demonstrate the influence of the embedding design parameters, e.g., orientation and coupling distances, in respect of the microwave behavior. In addition, different array arrangements to increase the number of pixels and to identify possible reasons causing unwanted cross-talk were examined. For this purposes several samples in niobium thin film technology on silicon substrates were designed, fabricated and characterized. Loaded quality factorsQLup to 3 000 at 4.2 K were achieved. The results of the simulations and measurements of small and large arrays will be presented and discussed.
The development of large multi-pixel detector applications with lumped element kinetic inductance detectors (LEKID) requires a detailed knowledge about the mode of operation in respect of the microwave regime. In this paper the basic properties of LEKID structures for applications at liquid helium temperatures were investigated. In consideration of the operation temperature, the different microwave behavior of LEKID structures coupled to microstrip and coplanar strip readout transmission lines, e.g. loaded and unloaded quality factors, have been examined. For this purpose several test structures, which are suitable for large arrays, in multi-pixel applications, were designed and characterized with frequency around 6 GHz. Loaded quality factorsQLQ_{L}QL​up to 10,000 at 4.2 K were achieved. The devices were fabricated in niobium thin film technology on silicon substrates. LEKID arrays with 3 resonators were designed, fabricated and measured with our self-developed FDM readout system. The results of the measurements will be presented and discussed.
Microwave Kinetic Inductance Detectors (MKIDs), just like a planar resonance cavity resonating at a microwave frequency, are emerging as a kind of high-sensitivity detector suitable for large format arrays at terahertz (THz) wavelengths. There are two types of MKIDs, namely distributed (or antenna-coupled) MKIDs and lumped-element MKIDs (aka LeKIDs). Various superconducting thin films (such as Al, TiN, NbTiN, NbN, and Nb) have been investigated for MKIDs. They do work so long as the detected photon energy exceeds their energy gap, but their response and noise behaviors are yet to be fully understood. Here we report on the design, fabrication, and characterization of distributed and lumped-element MKIDs made of NbTiN superconducting film. Detailed simulation and measurement results will be presented.
Large-scale arrays of microwave kinetic inductance detectors (MKIDs) are attractive candidates for use in imaging instruments for next generation submillimeter-wave telescopes such as CCAT. We have designed and fabricated tightly packed ~250-pixel MKID arrays using lumped-element resonators etched from a thin layer of superconducting TiNxdeposited on a silicon substrate. The high pixel packing density in our initial design resulted in large microwave crosstalk due to electromagnetic coupling between the resonators. Our second design eliminates this problem by adding a grounding shield and using a double-wound geometry for the meander inductor to allow conductors with opposite polarity to be in close proximity. In addition, the resonator frequencies are distributed in a checkerboard pattern across the array. We present details for the two resonator and array designs and describe a circuit model for the full array that predicts the distribution of resonator frequencies and the crosstalk level. We also show results from a new experimental technique that conveniently measures crosstalk without the need for an optical setup. Our results reveal an improvement in crosstalk from 57% in the initial design down to ≤2% in the second design. The general procedure and design guidelines in this work are applicable to future large arrays employing microwave resonators.
Lumped-element superconducting resonators are a promising technology for their use in millimeter-wave observations and quantum computing applications that require large arrays of extremely sensitive detectors. Among them, lumped-element kinetic inductance detectors (LEKIDs) have shown good performance in the submillimeter band in several earth-based telescopes. In this work, LEKIDs for their use as millimeter-wave receivers of astronomical applications are presented. LEKID arrays using a thin bilayer of superconducting titanium/aluminum (Ti/Al), deposited on the silicon substrate, have been designed and fabricated. The design of a dual-polarization LEKID with the goal of detection at the W-band for two orthogonal polarizations is described and a fabricated array has demonstrated absorption at ambient temperature. Also, an approximate design methodology of the coupling parameter for LEKIDs' readout, essential for dynamic range optimization of the detector under millimeter-wave radiation, is proposed. In addition, the resonance characteristics and coupling factor of the fabricated superconducting resonators using high-quality internal factor Qiunder cryogenic temperatures have been analyzed. The design guidelines in this work are applicable to other LEKID arrays, and the presented superconducting Ti/Al thin-film LEKIDs can be used in future receiver arrays in the millimeter bands.
By using LEKIDs the prospect rises to multi-pixel arrays with an extremely small footprint. Moreover, these arrays have the advantage of frequency-division multiplex readout method (FDM). However, the increase of packing density of LEKIDs leads to an increase of crosstalk between adjacent pixels and complicates the readout process significantly. In this paper, we present a newly developed LEKID structure with reduced crosstalk and present their improved performance in terms of equally spaced resonance frequencies within an array. Thus, the array can be realized with a large number of pixels in a small analog readout bandwidth. We present simulation and measurement results of various linear arrays with 5 pixels, which are constructed with the new LEKID structures. When designing the resonance frequencies importance was attached to an equal spacing between the resonance frequencies at around 6 GHz within our available analog readout bandwidth of 400 MHz. The samples were fabricated in niobium thin film technology on silicon and sapphire substrates and then measured at liquid helium temperatures.
We present results from completely lithographic antenna-coupled Microwave Kinetic Inductance detectors (MKIDs). MKIDs are superconducting resonators with resonant frequency and quality-factor which are highly sensitive to changes in the density of the quasiparticle population which occurs when photons above the superconducting gap energy are absorbed. The resonators are coupled to submm light through on-chip phased-array slot antennas. Each planar antenna consists of an array of N long slots which are fed along their length at M points. The resulting N*M feed points are combined in-phase using a binary summing tree made of low-loss superconducting microstrip lines. Due to its large size, the resulting planar antenna produces a narrow beam pattern and can therefore be used without additional optical coupling elements such as feedhorns or substrate lenses. The output of the antenna is a single superconducting thin-film microstrip line which can be efficiently coupled to one (or more) kinetic inductance coplanar waveguide resonators to produce a single (or multi-color) pixel in an imaging focal plane array, using in-line lumped element lithographic band-pass filters. Such highly integrated architectures can be easily fabricated on a single substrate, and many detectors can be frequency multiplexed through coupling to a single feedline. Microwave readout provides a lot of bandwidth per detector, allowing a large number of pixels to be read using a single cryogenic microwave amplifier and warm readout electronics.