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Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI

Three-dimensional microwave kinetic inductance detectors

Nicholas F. Cothard, Thomas R. Stevenson, Christine A. Jhabvala, Jason Glenn

Published Aug 16, 2024• 0 citations
Publisher

Abstract

We present the design and optical characterization of a novel highly compact three-dimensional microwave kinetic inductance detector (3D-MKID). At short wavelengths, such as the far- and mid-infrared, the pixel density of MKID arrays is often limited by the size and geometry of each resonator’s capacitor. Sending the non-optically active component of the resonator into the third dimension minimizes the footprint of each pixel, allowing much higher array densities to be achieved. In our 3D-MKID design, we compactify the resonator by conformally coating deep-etched holes in the silicon substrate with superconducting films formed by atomic layer deposition. The resulting geometry consists of three-dimensional coaxial transmission lines, which are then connected to a meandered absorber on the substrate surface. An array of these resonators are capacitively coupled to a microstrip feedline. We present the characterization of a prototype 3D-MKID array and compare it to simulate resonator properties. We additionally describe the detector sensitivity performance when illuminated by a far-infrared blackbody source.

DOI
10.1117/12.3019177

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We present the design and optical characterization of a novel highly compact three-dimensional microwave kinetic inductance detector (3D-MKID). At short wavelengths, such as the far- and mid-infrared, the pixel density of MKID arrays is often limited by the size and geometry of each resonator’s capacitor. Sending the non-optically active component of the resonator into the third dimension minimizes the footprint of each pixel, allowing much higher array densities to be achieved. In our 3D-MKID design, we compactify the resonator by conformally coating deep-etched holes in the silicon substrate with superconducting films formed by atomic layer deposition. The resulting geometry consists of three-dimensional coaxial transmission lines, which are then connected to a meandered absorber on the substrate surface. An array of these resonators are capacitively coupled to a microstrip feedline. We present the characterization of a prototype 3D-MKID array and compare it to simulate resonator properties. We additionally describe the detector sensitivity performance when illuminated by a far-infrared blackbody source.
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.
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.
A conventional low-noise detector requires a technique to both absorb incident power and convert it to an electrical signal at cryogenic temperatures. This innovation combines low-noise detector and readout functionality into one device while maintaining high absorption, controlled polarization sensitivity, and broadband detection capability. The resulting far-infrared detectors can be read out with a simple approach, which is compact and minimizes thermal loading. The proposed microwave kinetic inductance detector (MKID) consists of three basic elements. The first is the absorptive section in which the incident power is coupled to a superconducting resonator at far-infrared frequency above its superconducting critical frequency (where superconductor becomes normal conductor). This absorber's shape effectively absorbs signals in the desired polarization state and is resonant at the radio frequency (RF) used for readout of the device. Control over the metal film used in the absorber allows realization of structures with either a 50% broadband or 100% resonance absorptance over a 30% fractional bandwidth. The second element is a microwave resonator - which is realized from the thin metal films used to make the absorber as transmission lines - whose resonance frequency changes due to a variation in its kinetic inductance. The resonator's kinetic inductance is a function of the power absorbed by the device. A low-loss dielectric (mono-crystalline silicon) is used in a parallel-plate transmission line structure to realize the desired superconducting resonators. There is negligible coupling among the adjacent elements used to define the polarization sensitivity of each detector. The final component of the device is a microwave transmission line, which is coupled to the resonator, and allows detection of changes in resonance frequency for each detector in the focal plane array. The spiral shape of the detector's absorber allows incident power with two polarizations to couple to the detector equally. A stepped impedance resonator was used that allows the incident power absorbed in the detecting membrane area to be uniformly distributed in the detector's transmission line at the RF readout frequency. This maximizes the sensitivity of the detector. The signal is read out via a frequency multiplexing technique that requires a minimum number of interface transmission lines for readout. This reduces the packaging complexity and coupling to the device's thermal environment.
We designed and fabricated a microwave kinetic inductance detector (MKID) using a niobium nitride (NbN) symmetric spiral resonator array. Previously we revealed that a rewound spiral structure works as not only a high-Q half-wavelength resonator but also as a broadband terahertz antenna. We conducted simulations for a 9 resonator array assuming NbN as the superconducting material and sapphire as the dielectric substrate, and obtained a maximum attenuation of over 30 dB and unloaded quality factors of over 2 × 10 5 for frequencies between 4.4 and 4.9 GHz. We fabricated the 9 resonator array MKID using NbN thin film deposited on an m-sapphire substrate by using dc magnetron sputtering. We observed half-wavelength resonances of around 4.5 GHz at 4 K. We measured the optical response of the MKID. The frequency shift was 0.5 MHz when illuminated with 650 nm photons.
Microwave kinetic inductance detectors (MKID) are superconducting broadband direct detectors. If a superconducting thin film patterned into a resonator is illuminated with photons, Cooper pairs break up and the kinetic inductance of the film changes. This shifts the resonance frequency downwards and the dissipation of the resonator increases, which can be detected in the phase and amplitude of a transmitted signal. \n \nIn this thesis, a waveguide coupled MKID is designed and simulated. 350 GHz radiation is coupled into a copper waveguide via a horn antenna. A waveguide probe leads the signal onto a 9 µm silicon membrane where it is absorbed in a 40 nm superconducting aluminum film which is the inductive part of a superconducting resonator. The capacitor of the resonator is made of niobium. Two versions of the device are developed, a transmission line resonator and a lumped element resonator design. The new design will enable an accurate understanding of the optical sensitivity of MKIDs and allow comparison of different resonator materials. Since two different resonator types are used, the direct comparison of transmission line and lumped resonators will also be possible. \n \nA measurement setup is developed including an ADR cryostat to cool MKID devices down to below 40 mK. An analog readout system using either a vector network analyzer or an IQ mixer is designed and built. The necessary software to control the ADR, the temperature monitoring, the readout signal generation and detection and the signal acquisition of the setup is also developed. \n \nThe waveguide coupled MKID design is fabricated in our in-house microfabrication laboratory and measured in the new setup. In my measurements, the devices do not show a resonance feature. The VNA and DC measurements are analyzed to determine a plausible hypothesis of the failure. The devices are diagnosed for critical issues during fabrication which will be addressed in a future iteration. \n \nTo verify the measurement setup, a test device of the IRAM NIKA 2 MKID camera is measured and the complete analysis pipeline is exercised. The quasi-particle lifetime in the aluminum thin films is measured and a dark and optical noise equivalent power are determined. The results are within the uncertainty comparable to measurements of IRAM themselves. \n \nA test chip designed in my master thesis that contains eight NbTiN resonators is measured in the new setup and non-linear effects under strong readout power are studied. Parametric amplification and oscillation of a resonator are shown qualitatively.
This article is focused on the design and development of superconducting Microwave Kinetic Inductance Detector (MKID) which is used at near, mid and far-infrared, optical and ultraviolet wavelengths for astronomical applications. MKIDs are superconducting micro-resonators which are very much considered, due to high sensitivity, simplicity in fabrication and ease of read out by a coplanar waveguide or microstrip transmission line. Here a new structure for inductor of lumped element microwave resonator is proposed that causes a very high Quality factor in smaller dimensions. Our resonator's intrinsic Q factor is above 8×106in dimensions of 441×487 μm2which is the highest value in such dimensions. due to the small dimensions of this resonator, makes it desirable to use in large arrays spectrometers.