Reducing the Susceptibility of Lumped-Element KIDs to Two-Level System Effects

A L Hornsby,E Shirokoff,Q Y Tang,S M Doyle,P S Barry

doi:10.1007/s10909-020-02501-7

Abstract

Arrays of lumped-element kinetic inductance detectors (LEKIDs) optically coupled through an antenna-coupled transmission line are a promising candidate for future cosmic microwave background experiments. However, the dielectric materials used for the microstrip architecture are known to degrade the performance of superconducting resonators. In this paper, we investigate the feasibility of microstrip coupling to a LEKID, focusing on a systematic study of the effect of depositing amorphous silicon nitride on a LEKID. The discrete and spatially separated inductive and capacitive regions of the LEKID allow us to vary the degree of dielectric coverage and determine the limitations of the microstrip coupling architecture. We show that by careful removal of dielectric from regions of high electric field in the capacitor, there is minimal degradation in dielectric loss tangent of a partially covered lumped-element resonator. We present the effects on the resonant frequency and noise power spectral density and, using the dark responsivity, provide an estimate for the resulting detector sensitivity.

Highlights

The temperature and polarization anisotropies contained in the cosmic microwave background (CMB) offer a unique window into how our Universe began
We investigate the effects on the microwave resonator caused by deposition of a silicon nitride (S iNx ) dielectric layer over the lumped-element kinetic inductance detectors (LEKIDs) architecture
We measured S21 of the resonators as a function of base temperature between 110–320 mK using a vector network analyzer (VNA), and extracted the resonant frequency following the procedure outlined in Khalil et al.[15]

Summary

Introduction

The temperature and polarization anisotropies contained in the cosmic microwave background (CMB) offer a unique window into how our Universe began. The number of detectors required by the generation of CMB experiments presents a significant technical challenge. Kinetic inductance detectors (KIDs) are superconducting resonators whose resonant frequency and quality factor are modified with absorbed optical power[6]. Most existing CMB experiments implement an onchip microstrip optical coupling architecture, where radiation is guided onto a thin-film microstrip line and routed to the detector, enabling multi-chroic, polarization-sensitive pixels. The resonator inductor doubles as both a high-Q microwave lumped-element inductor, as well as an absorbing mm-wave microstrip line (Fig. 1, right). The microstrip line from the antenna is galvanically connected to the center of a hairpin-style inductor. This addition of dielectric is necessary for transmission line coupling in antenna-coupled devices. We compare LEKID resonators with varying coverage of SiNx to study the impact on resonant frequency as a function of temperature and noise, as a first look at TLS contributions in a LEKID device architecture

Prototype Device

Results

Dark Response

Dark Detector Noise

Conclusion