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IEEE Transactions on Applied Superconductivity

Investigation and Optimization of LEKID Coupling Structures and Multi-Pixel Arrays at 4.2 K

S. Wünsch, G. Hammer, Thomas Kappler, F. Geuppert, M. Siegel

Published Dec 9, 2010• 9 citations
Publisher

Abstract

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 factors$Q_{L}$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.

DOI
10.1109/tasc.2010.2090445

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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.
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Future ground-based cosmic microwave background (CMB) experiments will require more than 10510^5105 polarization sensitive pixels covering multiple atmospheric bands. The scientific potential for such an experiment is impressive; however, the technical challenges are daunting: such an instrument will require square meters of focal plane covered in background limited cryogenic detectors and a dramatic increase in readout capability. We are developing novel kinetic inductance detectors (KIDs) optimized for this purpose. These devices use a twin-slot microwave antenna, superconducting Nb transmission line, and a novel coupling scheme that deposits mm-wavelength power onto a high-resistivity meander deposited as the first layer on a bare Si wafer. This architecture allows us to independently adjust the detector and antenna properties and to pursue multi-band designs. We have fabricated superconducting resonators made from atomic layer deposited (ALD) titanium nitride (TiN), with thicknesses ranging from 3 to 40 nm. We find a strong dependence of transition temperature on thickness, from 0.6 to 4.2 K for our thinnest and thickest films, respectively. In dark measurements, we find internal quality factors that range from 10410^4104 to 7×1057\times 10^57×105 depending on film thickness, and kinetic inductance as high as 8 nH/square. The very small volumes and high kinetic inductance make it possible to engineer extremely sensitive detectors with inductor volumes approaching a few cubic microns that operate at readout frequencies of tens to hundreds of MHz. By taking advantage of the large fractional bandwidth available at low frequencies, we expect to achieve multiplexing densities that exceed that of state of the art TES arrays even without further improvements in film quality factor. We will present the characterization of film properties and dark devices, as well as well as initial optical results for antenna coupled single-band and single-pol devices. We will also discuss designs and sensitivity projections for future dual-pol and multi-band arrays ready for deployment in near-future CMB instruments.