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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Microwave crosstalk in lumped element far-IR MKIDs

Omid Noroozian, Peter K. Day, Byeong Ho Eom, Henry George LeDuc, Jonas Stasys Zmuidzinas

Published Sep 1, 2010• 2 citations
Publisher

Abstract

We have made close-packed far-infrared MKID arrays with ~ 250 pixels using TiN on silicon. Measurements show a large scatter in quality factor arising from crosstalk. This is confirmed by pump-probe experiments and EM simulations. Our new shielded resonator designs show very low crosstalk levels.
DOI
10.1109/icimw.2010.5612440

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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.
Microwave crosstalk between rewound-spiral resonators was estimated for the optimal design of a microwave kinetic inductance detectors (MKIDs) array. An electromagnetic simulator (sonnet EM) was used to calculate the frequency responses. First, we analyzed the microwave interactions by changing the geometrical distance, Drr, between the centers of two resonators and the frequency interval, Δf, placed in the horizontal direction and the vertical direction. To analyze the microwave crosstalk, we changed the kinetic inductance, Lk, at one resonator, and we estimated the resonant frequencies. Our simulated results reveal that the microwave crosstalk was less than 0.22% placed on the horizontal and vertical directions with Drr= 1000 μm and Δf = 9 MHz. We then used these elements to design a 25-element spiral-MKIDs array and fabricated it by using an NbN thin film deposited on an r-sapphire substrate. The crosstalk was estimated using a vector network analyzer and a signal generator for pumping to the single element. In the measured results, 25 resonant dips were clearly observed in the frequency range between 2.70 and 2.95 GHz at 3.45 K. Furthermore, we could estimate the crosstalk between the resonators and found it to be less than 10-4% in both horizontal and vertical patterns.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.
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.