Low-Tc Superconducting Detector for Future CMB Missions

Abstract

The next generation of Cosmic Microwave Background (CMB) missions require sensitive detectors to probe the processes that shaped the early Universe. The 50-90GHz frequency range is of particular interest, both for the measurement of the weak B-mode polarization patterns from primordial gravitational waves and for detection of spectral distortions in the CMB.
Since Microwave Kinetic Inductance Detectors (MKIDs) offer great multiplexing capabilities, high sensitivity, easy fabrication, and reduced cost, they would be ideal for CMB missions. MKIDs are superconducting pair-breaking detectors, which makes them capable of detecting photons with a minimum photon energy of twice the superconducting gap energy. The superconducting gap energy is intrinsic to the superconductor embedded in the detector's hybrid microwave resonator structure, and thus poses a direct limit on the frequencies the detector can measure. For example, the conventionally used Aluminum (Tc=1.2-1.4K) would only be able to detect photons with frequencies larger than 90GHz. β-phase Tantalum (β-Ta), a disordered superconductor with Tc=0.6-1.0K, would enable detection of frequencies as low as 45GHz.
In this thesis, we therefore investigate the viability of using β-Ta in designing hybrid MKIDs for frequencies in the 50-90GHz range.

The effect of quasiparticle trapping due to disorder in superconductors is known to reduce the sensitivity of MKIDs. Consequently, the volume of β-Ta within the microwave resonator must be minimized to mitigate this effect and achieve photon-noise-limited sensitivity. We opt for a lens-antenna coupled hybrid MKID design to decouple the frequency sensitive antenna and the active volume of the resonator, such that both can be optimized independently.

The detectors presented in this thesis have been designed for 70GHz radiation, at the center of the 50-90GHz range. The lens antenna features an extended hemispherical lens coupling radiation from a black body source to a twin-slot antenna. The twin-slot antenna couples this pair-breaking radiation to the volume of β-Ta. We consider β-Ta/NbTiN hybrid MKIDs for the quarter-wave resonators. To approximate the properties of the superconducting materials at both readout and pair-breaking frequencies, Mattis-Bardeen theory is used.

Given the large normal state resistivity of β-Ta, the narrow coplanar waveguide in the hybrid MKID has a large characteristic impedance. This makes it difficult for the twin-slot antenna to match in impedance. Therefore, the dimensions of the narrow coplanar waveguide have to be optimized to minimize both its active volume and characteristic impedance, simultaneously. We obtain a trade-off in its dimensions, which is also limited by UV lithography fabrication limitations. The twin-slot antenna design is subsequently optimized for a sufficient impedance match.

We identify radiation losses at readout frequencies due to the addition of the twin-slot antenna structure. These losses are found to exceed the dissipation within the quasiparticle system predicted by Mattis-Bardeen theory at low operating temperatures, and would therefore dominate the internal quality factor of the MKIDs.

Conclusively, this thesis presents a viable methodology to design MKIDs in the range of 50-90GHz using β-Ta. The dimension optimizations of the narrow coplanar waveguide and the twin-slot antenna are not fully decoupled, and their impedance matching poses challenges to the MKID design. For future research, it is important to find an approach that would give more engineering freedom in their separate designs.